Module I – Genetics According to Mendel

Home , Dihybrid cross, Monohybrid cross, Punnett square

Educators Lesson Module I – Genetics According to Mendel

his teaching license because of a disagreement with Genetics According examiners about the nature of heredity. Ironically, Mendel’s failure to become a teacher is what drove him to Mendel to discover the principal rules of genetics. I Bent on proving his ideas correct, Mendel began his famous pea experiments. In 1865, he presented the Part I results of his experiments to the Brunn Society of Natural History, one of the most prestigious scientific BACKGROUND INFORMATION societies in the world. In 1866, the society published his paper in their proceedings. Even though the journal reached 120 universities and other societies, Introduction Mendel’s paper was mostly ignored by the science It has been known for thousands of years that biological community of the time. It was not until 1900 that traits are passed to offspring by their parents who Mendel’s paper was rediscovered and viewed as a received these traits from past generations. This fact significant scientific achievement. was most successfully applied by farmers who recognized the importance of saving the seeds of the best How did Mendel succeed where earlier scientists plants for the next year’s crop and using the best failed to understand heredity? Mendel kept his animals for breeding. But until recent times, the experiments simple. scientific mechanisms that drive inheritance were not understood. Today, we call the scientific study of Mendel had many years of plant breeding experience. inheritance genetics. As a result, he selected the garden pea because it grew well in small gardens, produced a large number of In 1866, an Augustinian priest named Gregor Mendel seeds, and was easily pollinated. Pea plants are self- proposed the first theory about the units of inheritance, pollinating because the anthers (male, pollen producer) that we call genes. He described two basic rules that and the stigma (female, receives pollen) are enclosed in govern how traits are transmitted from one generation the same flower. This selfing produces strains of to another. Mendel’s work is recognized as one of the pea plants that are identical for many generations, greatest breakthroughs in the history of science and called true-breeding. marked the beginning of the science of genetics we know today. From his past experiences, Mendel knew how to cross- pollinate plants and how the characteristics of peas The science of genetics has served as the foundation for could be used in his studies. For his experiments, today’s advancements in crop and animal production, Mendel selected only true-breeding strains and studied for detecting and treating inherited human diseases, only a few traits at a time. He grew his chosen plants and for the production of medicines that treat human for two years to make sure he had a true-breeding strain and animal afflictions. Since the early part of the of peas. When he began crossing true-breeding strains, 20th century, genetics has been at the forefront of Mendel made sure that the two parents differed only in research in biology and is today the cornerstone of the few traits he had chosen. Scientists working before biological research. Mendel failed in their similar experiments because they did not use true-breeding strains and did not limit the The Nature of Mendel’s Discovery traits they were testing.

In 1842, after completing all the formal education his Mendel was methodical in recording and applying family could afford, 21-year-old Gregor Mendel entered mathematical analysis to his results. His careful a Austrian monastery as a means to continue his numerical analysis had never been done in true- education. In 1851, with the assistance of the monas- breeding experiments before and was an essential factor tery, Mendel entered the University of Vienna. There in Mendel’s success. he studied physics, chemistry, zoology, botany, and mathematics, including the new field of statistics. Mendel’s Rule of Segregation Mendel returned to the monastery and began teaching physics and natural history at the local high school. In In his first series of experiments, Mendel made crosses 1856, Mendel walked out of the oral examination for between true-breeding pea strains that differed in one trait, seed shape. Parents either had round or wrinkled

8 Iowa State University Extension and ISU Office of Biotechnology Lesson Module I – Genetics According to Mendel Educators

seeds. These crosses are referred to as monohybrid Mendel theorized that each parent plant passed one crosses. The plants involved in the original cross are allele for each trait to its offspring through their call the parental or P generation. The progeny (off- gametes (sperm or egg). Based on the fact that reces- spring) seeds from this cross are referred to as the first sive traits reappear in the offspring of heterozygotes,

filial or F1 generation. Mendel proposed that alleles do not mix or blend in heterozygotes. This idea was contrary to the “blending

Mendel found that all the F1 generation seeds resulting theory” that was the thinking of the day. Mendel’s from his original cross of parents with round and results suggested that dominant and recessive alleles do wrinkled seeds were round. (See p. 10-11 to view the not blend, but are passed intact from heterozygotes to results of Mendel’s crosses.) He concluded that the their offspring. wrinkled trait seemed to be masked or “dominated” by

the round trait in the F1 seeds. Mendel called the round Mendel hypothesized that the two alleles segregate trait dominant and the wrinkled trait recessive. Mendel (move apart) from one another during gamete forma-

planted the F1 seeds, raised the plants, and allowed tion. From a heterozygote (Rr), two types of haploid

them to self-pollinate to produce a second filial or F2 gametes formed. One type of gamete contains the generation of seeds. He found that both round and round allele (R), and one contains the wrinkled allele

wrinkled seeds appeared in the F2 generation in a ratio (r). Homozygous individuals produce only one type of of three round (dominant trait) to one wrinkled gamete. Homozygous individuals with round seeds (recessive trait). (RR) would produce only gametes containing the round allele (R), and wrinkled (rr) individuals would produce Mendel continued each cross for another generation by only gametes containing the wrinkled allele (r).

collecting F2 seeds, planting them, rearing the plants, and allowing them to self-pollinate to produce a third From this hypothesis, Mendel formulated his first rule filial or F3 generation. Mendel discovered that F2 of genetics, the Rule of Segregation. This rule states individuals with the recessive trait always produced that pairs of factors (alleles) segregate or separate

progeny with the recessive trait. Some F2 individuals during the formation of gametes. with the dominant trait produced only progeny with

round seeds, while other F2 individuals with the dominant trait had progeny with both round and Mendel’s Rule of Independent wrinkled seeds. From these results, Mendel hypoth- Assortment esized that alternative traits, round or wrinkled seeds, Mendel wanted to understand how two or more traits are determined by “factors.” Today, we call these factors are inherited simultaneously. He hypothesized two genes. Mendel proposed that genes exist in different possibilities. One possibility was that the traits were forms and, consequently, can produce different traits. inherited as a unit. If one parent has round and yellow Today, we call different forms of one gene alleles. seeds and the other parent has wrinkled and green seeds, these pairs of traits might be passed together to Upon completing his monohybrid crosses, Mendel their offspring. The offspring would produce either proposed that each individual pea plant carried two round-yellow seeds or wrinkled-green seeds. The copies (two alleles) of each gene. Each allele is given a second possibility was that traits are inherited indepen- letter. The genetic makeup of an individual symbolized dently. If so, future generations would display a by letters is called its genotype. Uppercase or capital combination of the traits, such as round-green or letters indicate a dominant allele, and lowercase or wrinkled-yellow seeds. small letters indicate a recessive allele. An individual with two identical alleles is said to be homozygous. A Mendel investigated his hypothesis by crossing plants plant that is true-breeding for round seeds is homozy- that differed in two traits, called a dihybrid cross. He gous for the allele controlling round seed (RR), and a crossed plants that were homozygous true-breeding plant that is true-breeding for wrinkled seeds is strains for the dominant (RRYY) and recessive (rryy) homozygous for the allele controlling wrinkled seed alleles of both traits. The F1 generation seeds were all (rr). An individual that possesses two different alleles round and yellow. This agreed with results Mendel had of one gene is called heterozygous (Rr). The observable observed in earlier crosses where round was dominant characteristic (smooth, wrinkled) of an individual is to wrinkled and yellow was dominant to green. called its phenotype. Therefore, the genotype of the F1 generation had to be (RrYy). (See p. 11 to view the crosses made by Mendel.)

Iowa State University Extension and ISU Office of Biotechnology 9 Educators Lesson Module I – Genetics According to Mendel

The F1 plants were allowed to self-pollinate. Mendel combined in the square to produce the possible observed four different seed phenotypes from this cross: genotypes of the progeny (offspring.) round-yellow, round-green, wrinkled-yellow, and wrinkled-green. These phenotypes represent all the possible combinations of the two traits. From these and F2 Seeds from Selfed F1 Plants future experiments, Mendel concluded that seed shape and color were not inherited as a unit. Mendel hypoth- Genotype: Rr esized that in order to produce the four different One homozygous and phenotypes, the alleles from these genes must segregate round RR independently. This means that the heterozygous F1 Two heterozygous and individual would have a 1/2 chance of producing a round Rr RRRrR gamete containing an R or an r allele and a 1/2 chance One homozygous and of producing a gamete containing a Y or a y allele. wrinkled rr

Therefore, a gamete from the F1 generation had a 1/4 (1/2 X 1/2) chance of having any one of the four Phenotype: rrRrr genotypes (RY, Ry, rY, ry). Three round One wrinkled

Mendel proposed that when the F1 generation self- (a dominant to recessive ratio of 3:1) pollinated, each gamete combined randomly with another gamete. Since there are four possible gamete genotypes, there are 16 gamete combinations (4 X 4). From the 16 gamete combinations, there are nine different F2 genotypes and four different phenotypes. F3 Seeds from Selfed F2 Plants RR With the evidence from his crosses, Mendel concluded that the alleles of different genes did segregate indepen- Genotype and Phenotype: RR dently. The independent segregation of alleles of All F3 seeds are different genes is today called the Rule of Independent homozygous Assortment. It states that factors (alleles) for different and round. RR characteristics are distributed to gametes independently. RRRRR

The genotypes and phenotypes of all progeny of mono- and dihybrid crosses are easily analyzed by constructing a Punnett square. Named after the British geneticist RRRRR R.C. Punnett, this chart is composed of either 4 or 16 squares that represent the number of possible genotype combinations of progeny. To use a Punnett square, the possible gamete combinations are placed to the side of the square either on the row or column side. One side of the square represents the male gametes and the other F3 Seeds from Selfed F2 Plants Rr side represents the female gametes. The gametes are Genotype: One homozygous and Rr round RR Two heterozygous and Results of Mendel’s RR RRRrR Monohybrid Crosses round Rr One homozygous and

F1 Seeds from Cross wrinkled rr of Parents rrRrR Phenotype: rrRrr Genotype and Three round Phenotype: One wrinkled All F seeds are 1 rrRrR (a dominant to recessive heterozygous and ratio of 3:1) round. Rr

10 Iowa State University Extension and ISU Office of Biotechnology Lesson Module I – Genetics According to Mendel Educators

F3 Seeds From Selfed F2 Plants rr F2 Seeds from Selfed F1 Plants RrYy

Genotype and Phenotype: rr RyY RYryr

All F3 seeds are homozygous and wrinkled. rr rrrrr RYY RyRY RYRY RyrY RrY

rrrrr Ryy RyRY RyRy RyrY Rry

rYY RyrY RYrY ryrY rrY

Mendel’s Dihybrid Crosses ryy RyrY Ryry ryrY rry F1 Seeds from Cross of Parents RRYY x rryy

Genotype: RYY RYRYR Observe the nine different genotypes in the Punnett square above. RRYY, RRYy, RRyy, RrYY, RrYy, Rryy, rrYY, rrYy, rryy ryy RyrY RyrY RyrY RrY Phenotype: The ratio of the four different phenotypes are 9 (9/16) round-yellow seeds, 3 (3/16) round-green seeds, ryy RyrY RyrY RyrY RrY 3 (3/16) wrinkled-yellow seeds and 1 (1/16) wrinkled- green seeds.

ryy RyrY RyrY RyrY RrY Credit Notes Atherly, Alan G.; Girton, Jack R.; and McDonald, John F. The Science of Genetics. Saunders College Publishing. 1999 ryy RyrY RyrY RyrY RrY Modern Biology. Holt, Rinehart and Winston Publish- ing. Austin, Texas. 2002

Genotype and Phenotype: Biology: The Dynamics of Life. Glencoe/McGraw-Hill All F seeds are heterozygous and round-yellow. RrYy 1 Publishing. Columbus, Ohio. 2002

Iowa State University Extension and ISU Office of Biotechnology 11 Educators Lesson Module I – Genetics According to Mendel

Materials Make copies of the Learning More About student Genetics According handout for “Genetics According to Mendel” (Mendel- 1) on p. 17-21. Also, make copies of the See for Yourself to Mendel student handouts Mendel-2 and Mendel- 3 on p. 23 and I 25. Students will need one copy each of Mendel-1 and Mendel-3. Students will need two copies each of the Part I handout Mendel-2. If using an overhead projector, teachers can make transparencies from the overhead TEACHING RESOURCES masters Mendel-a through Mendel-i on p. 27-43. Doing the Activity Lesson Plan: Punnett Squares During the first class period, use the Learning More About student handout (Mendel-1) and the overhead In this activity, students will learn how to analyze the transparencies to explain monohybids. During the genotypes and phenotypes of all progeny of mono- and second class session, explain dihybrids. Teachers may dihybrid crosses. want to use the Mendel-1 handout as a reading assign- ment before the first class period. Assign the problems Science Education Standards at the end of the Mendel-1 handout as classwork or homework. Tell students to write their answers for Science as Inquiry: Content Standard A problems 1 on the monohybrid student handout – Abilities necessary to do scientific inquiry (p.175) (Mendel-2) and to write their answers for problem 2 on – Understanding about scientific inquiry (p.176) the dihybrid student handout (Mendel-3). Life Science: Content Standard C During the third class period, allow time for students to – The cell (p.184) ask questions or finish their assignments during the – Molecular basis of heredity (p. 185) first half of the class period. Collect the assignments. – Matter, energy, and organization in living systems Use the overhead transparencies with blank Punnett (p.186) squares (Mendel a-i on p. 27-43) to discuss the correct answers. History and Nature of Science: Content Standard G – Science as a human endeavor (p. 200) – Nature of scientific knowledge (p. 201) Reflect and Apply – Historical perspectives (p. 201) These problems are included in the Learning More About Genetics According to Mendel student handout on p.17- Science Process Skills 21. The answers are on p. 14 and 15. • Observing Punnett Square Problems • Ordering • Categorizing 1. In a certain breed of rabbits, black fur (B) is • Relating dominant to white (b). • Applying a. If a homozygous black male is crossed with a Life Skills heterozygous female, what are the possible phenotypes and genotypes of their offspring? • Science processing • Problem solving b. What would be the genotypes and phenotypes of • Decision making offspring from a cross between two heterozygous rabbits? Time 2. In a special breed of cat, the short hair allele (H) is Preparation: 30 minutes to copy handouts dominant to long (h) and the color tiger-striped Activity: Three 45-minute blocks of class time, with an allele (T) is dominant to white (t). What would be optional Day 4 for Reebop activity

12 Iowa State University Extension and ISU Office of Biotechnology Lesson Module I – Genetics According to Mendel Educators

the possible phenotypes and genotypes of the Credit Notes offspring from a mating of a heterozygous short- Atherly, Alan G. Girton, Jack R. McDonald, John F. haired tiger-striped male cat with a long-haired The Science of Genetics. Saunders College Publishing. white female cat? 1999

Enhancement Activity Modern Biology. Holt, Rinehart and Winston Publish- The Modern Genetics for All Students curriculum is ing. Austin, Texas. 2002 available on a Web site at www.so.wustl.edu/science_ outreach/curriculum/genetics.html that is maintained Biology: The Dynamics of Life. Glencoe/McGraw-Hill by Washington University in St. Louis. This is a very Publishing. Columbus, Ohio. 2002 good curriculum for studying Mendelian genetics.

After completing a free online registration form, educators may download the curriculum as a portable document format (pdf) file. In Chapter 2 under Section C titled “If all the Kids Have Mom and Dad’s Genes, Why Don’t They All Look Alike?”, the program will download a pdf file with four very good activities. The first activity in that section, “Really Relating to Reebops” is highly recommended as an enhancement activity for this module.

Iowa State University Extension and ISU Office of Biotechnology 13 Educators Lesson Module II – Marker Assisted Selection

Punnett Squares – Answers for Problem 1, Monohybrids

Key to phenotypes Name ______BB= black Class/Section ______Bb= black Problem No. 1 bb= white

Parents’ phenotype black black

Parents’ genotype BB Bb

Possible gametes B B Bb

F1 BB F1 Results Phenotype ratio: B BB BB All black

Genotype ratio: b Bb Bb 1/2 BB 1/2 Bb

PART B (if needed)

Parents’ phenotype black black

Parents’ genotype Bb Bb

Possible gametes B b Bb

Bb F Results F1 1 Phenotype ratio: B BB Bb 3/4 black 1/4 white

b Bb bb Genotype ratio:

1/4 BB, 1/2 Bb, 1/4 bb

14 Iowa State University Extension and ISU Office of Biotechnology Lesson Module I – Genetics According to Mendel

Punnett Squares – Answers for Problem 2, Dihybrids

Key to phenotypes Name ______HH or Hh short-haired = Class/Section ______hh= long-haired Problem No. 2 TT or Tt= tiger-striped

tt= white

Parents’ phenotype short-haired tiger-striped long-haired white

Parents’ genotype HhTt hhtt

Possible gametes HT Ht hT ht

ht ht ht ht

F1 Results HT Ht hT ht Phenotype ratio: ht HhTt Hhtt hhTt hhtt 1/4 short-haired tiger-striped 1/4 short-haired white 1/4 long-haired tiger-striped ht HhTt Hhtt hhTt hhtt 1/4 long-haired white

ht HhTt Hhtt hhTt hhtt Genotype ratio: 1/4 HhTt 1/4 Hhtt ht HhTt Hhtt hhTt hhtt 1/4 hhTt 1/4 hhtt

Iowa State University Extension and ISU Office of Biotechnology 15 Lesson Module I – Genetics According to Mendel Student Handout Mendel-1 Learning more about . . . Genetics According to Mendel

Introduction Bent on proving his ideas correct, Mendel began his It has been known for thousands of years that biological famous pea experiments. In 1865, he presented the traits are passed to offspring by their parents who results of his experiments to the Brunn Society of received these traits from past generations. This fact Natural History, one of the most prestigious scientific was most successfully applied by farmers who recog- societies in the world. In 1866, the society published nized the importance of saving the seeds of the best his paper in their proceedings. Even though the plants for the next year’s crop and using the best journal reached 120 universities and other societies, animals for breeding. But until recent times, the Mendel’s paper was mostly ignored by the science scientific mechanisms that drive inheritance were not community of the time. It was not until 1900 that understood. Today, we call the scientific study of Mendel’s paper was rediscovered and viewed as a inheritance genetics. significant scientific achievement.

In 1866, an Augustinian priest named Gregor Mendel How did Mendel succeed where earlier scientists proposed the first theory about the units of inheritance, failed to understand heredity? Mendel kept his that we call genes. He described two basic rules that experiments simple. govern how traits are transmitted from one generation to another. Mendel’s work is recognized as one of the Mendel had many years of plant breeding experience. greatest breakthroughs in the history of science and As a result, he selected the garden pea because it grew marked the beginning of the science of genetics we well in small gardens, produced a large number of know today. seeds, and was easily pollinated. Pea plants are self- pollinating because the anthers (male, pollen producer) The science of genetics has served as the foundation for and the stigma (female, receives pollen) are enclosed in today’s advancements in crop and animal production, the same flower. This selfing produces strains of for detecting and treating inherited human diseases, pea plants that are identical for many generations, and for the production of medicines that treat human called true-breeding. and animal afflictions. Since the early part of the 20th century, genetics has been at the forefront of From his past experiences, Mendel knew how to cross- research in biology and is today the cornerstone of pollinate plants and how the characteristics of peas biological research. could be used in his studies. For his experiments, Mendel selected only true-breeding strains and studied The Nature of Mendel’s Discovery only a few traits at a time. He grew his chosen plants for two years to make sure he had a true-breeding strain In 1842, after completing all the formal education his of peas. When he began crossing true-breeding strains, family could afford, 21-year-old Gregor Mendel entered Mendel made sure that the two parents differed only in a Austrian monastery as a means to continue his the few traits he had chosen. Scientists working before education. In 1851, with the assistance of the monas- Mendel failed in their similar experiments because they tery, Mendel entered the University of Vienna. There did not use true-breeding strains and did not limit the he studied physics, chemistry, zoology, botany, and traits they were testing. mathematics, including the new field of statistics. Mendel returned to the monastery and began teaching Mendel was methodical in recording and applying physics and natural history at the local high school. In mathematical analysis to his results. His careful 1856, Mendel walked out of the oral examination for numerical analysis had never been done in true- his teaching license because of a disagreement with breeding experiments before and was an essential factor examiners about the nature of heredity. Ironically, in Mendel’s success. Mendel’s failure to become a teacher is what drove him to discover the principal rules of genetics.

Iowa State University Extension and ISU Office of Biotechnology 17 Student Handout Lesson Module I – Genetics According to Mendel Mendel-1

Mendel’s Rule of Segregation characteristic (smooth, wrinkled) of an individual is In his first series of experiments, Mendel made crosses called its phenotype. between true-breeding pea strains that differed in one trait, seed shape. Parents either had round or Mendel theorized that each parent plant passed one wrinkled seeds. These crosses are referred to as allele for each trait to its offspring through their monohybrid crosses. The plants involved in the gametes (sperm or egg). Based on the fact that reces- original cross are call the parental or P generation. The sive traits reappear in the offspring of heterozygotes, offspring or progeny seeds from this cross are referred Mendel proposed that alleles do not mix or blend in heterozygotes. This idea was contrary to the “blending to as the first filial or F1 generation. theory” that was the thinking of the day. Mendel’s results suggested that dominant and recessive alleles do Mendel found that all the F1 generation seeds resulting from his original cross of parents with round and not blend, but are passed intact from heterozygotes to wrinkled seeds were round. (See p. 19-20 to view the their offspring. results of Mendel’s crosses.) He concluded that the wrinkled trait seemed to be masked or “dominated” by Mendel hypothesized that the two alleles segregate (move apart) from one another during gamete forma- the round trait in the F1 seeds. Mendel called the round trait dominant and the wrinkled trait recessive. Mendel tion. From a heterozygote (Rr), two types of haploid gametes formed. One type of gamete contains the planted the F1 seeds, raised the plants, and allowed round allele (R), and one contains the wrinkled allele them to self-pollinate to produce a second filial or F2 generation of seeds. He found that both round and (r). Homozygous individuals produce only one type of gamete. Homozygous individuals with round seeds wrinkled seeds appeared in the F2 generation in a ratio of three round (dominant trait) to one wrinkled (RR) would produce only gametes containing the round (recessive trait). allele (R), and wrinkled (rr) individuals would produce only gametes containing the wrinkled allele (r). Mendel continued each cross for another generation by From this hypothesis, Mendel formulated his first rule collecting F2 seeds, planting them, rearing the plants, and allowing them to self-pollinate to produce a third of genetics, the Rule of Segregation. This rule states that pairs of factors (alleles) segregate or separate filial or F3 generation. Mendel discovered that F2 individuals with the recessive trait always produced during the formation of gametes.

progeny with the recessive trait. Some F2 individuals with the dominant trait produced only progeny with Mendel’s Rule of Independent round seeds, while other F2 individuals with the dominant trait had progeny with both round and Assortment wrinkled seeds. From these results, Mendel hypoth- Mendel wanted to understand how two or more traits esized that alternative traits, round or wrinkled seeds, are inherited simultaneously. He hypothesized two are determined by “factors.” Today, we call these factors possibilities. One possibility was that the traits were genes. Mendel proposed that genes exist in different inherited as a unit. If one parent has round and yellow forms and, consequently, can produce different traits. seeds and the other parent has wrinkled and green Today, we call different forms of one gene alleles. seeds, these pairs of traits might be passed together to their offspring. The offspring would produce either Upon completing his monohybrid crosses, Mendel round-yellow seeds or wrinkled-green seeds. The proposed that each individual pea plant carried two second possibility was that traits are inherited indepen- copies (two alleles) of each gene. Each allele is given a dently. If so, future generations would display a letter. The genetic makeup of an individual symbolized combination of the traits, such as round-green or by letters is called its genotype. Uppercase or capital wrinkled-yellow seeds. letters indicate a dominant allele, and lowercase or small letters indicate a recessive allele. An individual Mendel investigated his hypothesis by crossing plants with two identical alleles is said to be homozygous. A that differed in two traits, called a dihybrid cross. He plant that is true-breeding for round seeds is homozy- crossed plants that were homozygous true-breeding gous for the allele controlling round seed (RR), and a strains for the dominant (RRYY) and recessive (rryy) plant that is true-breeding for wrinkled seeds is alleles of both traits. The F1 generation seeds were all homozygous for the allele controlling wrinkled seed round and yellow. This agreed with results Mendel had (rr). An individual that possesses two different alleles observed in earlier crosses where round was dominant of one gene is called heterozygous (Rr). The observable to wrinkled and yellow was dominant to green.

18 Iowa State University Extension and ISU Office of Biotechnology Lesson Module I – Genetics According to Mendel Student Handout Mendel-1

Therefore, the genotype of the F generation had to 1 Results of Mendel’s be (RrYy). (See p. 20 to view the crosses made Monohybrid Crosses by Mendel.)

F1 Seeds from Cross RR

The F1 plants were allowed to self-pollinate. Mendel of Parents observed four different seed phenotypes from this cross: round-yellow, round-green, wrinkled-yellow, and Genotype and rrRrR wrinkled-green. These phenotypes represent all the Phenotype: possible combinations of the two traits. From these and All F1 seeds are future experiments, Mendel concluded that seed shape heterozygous and and color were not inherited as a unit. Mendel hypoth- round. Rr esized that in order to produce the four different rrRrR phenotypes, the alleles from these genes must segregate independently. This means that the heterozygous F1 individual would have a 1/2 chance of producing a gamete containing an R or an r allele, and a 1/2 chance of producing a gamete containing a Y allele or a y allele. F2 Seeds from Selfed F1 Plants

Therefore, a gamete from the F1 generation had a 1/4 (1/2 X 1/2) chance of having any one of the four Genotype: Rr genotypes (RY, Ry, rY, ry). One homozygous and round RR Mendel proposed that when the F generation self- Two heterozygous and 1 RRRrR pollinated, each gamete combined randomly with round Rr another gamete. Since there are four possible gamete One homozygous and genotypes, there are 16 gamete combinations (4 X 4). wrinkled rr From the 16 gamete combinations, there are nine rrRrr different F2 genotypes and four different phenotypes. Phenotype: With the evidence from his crosses, Mendel concluded Three round that the alleles of different genes did segregate indepen- One wrinkled dently. The independent segregation of alleles of (a dominant to recessive ratio of 3:1) different genes is today called the Rule of Independent Assortment. It states that factors (alleles) for different characteristics are distributed to gametes independently.

The genotypes and phenotypes of all progeny of mono- F3 Seeds from Selfed F2 Plants RR and dihybrid crosses are easily analyzed by constructing a Punnett square. Named after the British geneticist Genotype and Phenotype: RR

R.C. Punnett, this chart is composed of either 4 or 16 All F3 seeds are squares that represent the number of possible genotype homozygous combinations of progeny. To use a Punnett square, the and round. RR RRRRR possible gamete combinations are placed to the side of the square either on the row or column side. One side of the square represents the male gametes and the other side represents the female gametes. The gametes are combined in the square to produce the possible RRRRR genotypes of the progeny (offspring.)

Iowa State University Extension and ISU Office of Biotechnology 19 Student Handout Lesson Module I – Genetics According to Mendel Mendel-1

F3 Seeds from Selfed F2 Plants Rr F2 Seeds from Selfed F1 Plants RrYy

Genotype: Rr RyY RYryr One homozygous and round RR Two heterozygous and RRRrR RYY RyRY RYRY RyrY RrY round Rr One homozygous and wrinkled rr rrRrr Ryy RyRY RyRy RyrY Rry Phenotype: Three round One wrinkled (a dominant to recessive rYY RyrY RYrY ryrY rrY ratio of 3:1)

F Seeds From Selfed F Plants rr 3 2 ryy RyrY Ryry ryrY rry Genotype and Phenotype: rr All F3 seeds are homozygous and Genotype: wrinkled. rr rrrrr Observe the nine different genotypes in the Punnett square above. RRYY, RRYy, RRyy, RrYY, RrYy, Rryy, rrYY, rrYy, rryy

Phenotype: rrrrr The ratio of the four different phenotypes are 9 (9/16) round-yellow seeds, 3 (3/16) round-green seeds, 3 (3/16) wrinkled-yellow seeds and 1 (1/16) wrinkled- Mendel’s Dihybrid Crosses green seeds.

F Seeds from Cross of Parents RRYY x rryy 1 Reflect and Apply RYY RYRYR Punnett Square Problems

ryy RyrY RyrY RyrY RrY Your teacher will give you worksheets where you can write your answers for these problems.

1. In a certain breed of rabbits, black fur (B) is dominant to white (b). ryy RyrY RyrY RyrY RrY a. If a homozygous black male is crossed with a heterozygous female, what are the possible phenotypes and genotypes of their offspring? ryy RyrY RyrY RyrY RrY b. What would be the genotypes and phenotypes of offspring from a cross between two heterozygous rabbits? ryy RyrY RyrY RyrY RrY 2. In a special breed of cat, the short hair allele (H) is dominant to long (h) and the color tiger-striped Genotype and Phenotype: allele (T) is dominant to white (t). What would be All F1 seeds are heterozygous and round-yellow. RrYy

20 Iowa State University Extension and ISU Office of Biotechnology Lesson Module I – Genetics According to Mendel Student Handout Mendel-1

the possible phenotypes and genotypes of the Phenotype offspring from a mating of a heterozygous short- Description of an observable trait haired tiger-striped male cat with a long-haired white female cat? Punnett square A chart of squares that represents the possible genotypes of offspring from two parents Learn the Language Recessive Alleles An allele (r) that expresses itself in a phenotype Different forms of the same gene only in homozygous individuals (rr)

Dihybrid cross Rule of independent assortment Cross made between two parents that differ in Rule of Gregor Mendel that alleles for different two traits characteristics are independently distributed to gametes Dominant An allele (R) that expresses itself in the phenotype Rule of segregation of homozygous (RR) or heterozygous (Rr) indi- Rule of Gregor Mendel that pairs of alleles separate viduals because it is dominant to the recessive when gametes are formed allele (r)

Second filial (F2) generation First filial (F1) generation The second generation of offspring from The first generation of offspring from two parents two parents

Gamete Third filial (F3) generation The male and female sex cells that come together The third generation of offspring from two parents during the reproduction process Trait Gene A genetic characteristic of an organism Basic unit of inheritance True-breeding Genetics Plants that are identical genetically for many The scientific study of inheritance generations because they are homozygous

Genotype The genetic makeup of an individual, typically Credit Notes expressed in alphabetical letters Atherly, Alan G. Girton, Jack R. McDonald, John F. The Science of Genetics. Saunders College Publishing. Haploid 1999 Having only one set of chromosomes Modern Biology. Holt, Rinehart and Winston Publish- Heterozygous ing. Austin, Texas. 2002 Having two different alleles of a gene (Rr) Biology: The Dynamics of Life. Glencoe/McGraw-Hill Homozygous Publishing. Columbus, Ohio. 2002 Having two identical alleles of a gene (RR or rr) … and justice for all The U.S. Department of Agriculture (USDA) prohibits discrimination in all its programs and activities on the basis of race, color, national origin, gender, religion, age, disability, political beliefs, sexual orientation, and marital or family status. (Not Monohybrid cross all prohibited bases apply to all programs.) Many materials can be made available in alternative formats for ADA clients. To file a complaint of discrimination, write Cross made between parents that differ in one trait USDA, Office of Civil Rights, Room 326-W, Whitten Building, 14th and Independence Avenue, SW, Washington, DC 20250-9410 or call 202-720-5964.

Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, Parental (P) generation 1914 in cooperation with the U.S. Department of Agriculture. Stanley R. Johnson, director, Cooperative Extension Service, Iowa State University of Science and Parents involved in the original cross Technology, Ames, Iowa.

Iowa State University Extension and ISU Office of Biotechnology 21 Lesson Module I – Genetics According to Mendel Student Handout Mendel-2 See for yourself . . . Punnett Squares – Monohybrids

Name ______Key to phenotypes = Class/Section ______= Problem No. ______=

PART A Parents’ phenotype

Parents’ genotype

Possible gametes

F1 F1 Results Phenotype ratio:

Genotype ratio:

PART B (if needed)

Parents’ phenotype

Parents’ genotype

Possible gametes

F1 F1 Results Phenotype ratio:

Genotype ratio:

… and justice for all The U.S. Department of Agriculture (USDA) prohibits discrimination in all its programs and activities on the basis of race, color, national origin, gender, religion, age, disability, political beliefs, sexual orientation, and marital or family status. (Not all prohibited bases apply to all programs.) Many materials can be made available in alternative formats for ADA clients. To file a complaint of discrimination, write USDA, Office of Civil Rights, Room 326-W, Whitten Building, 14th and Independence Avenue, SW, Washington, DC 20250-9410 or call 202-720-5964.

Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914 in cooperation with the U.S. Department of Agriculture. Stanley R. Johnson, director, Cooperative Extension Service, Iowa State University of Science and Technology, Ames, Iowa.

Iowa State University Extension and ISU Office of Biotechnology 23 Lesson Module I – Genetics According to Mendel Student Handout Mendel-3 See for yourself . . . Punnett Squares – Dihybrids

Key to phenotypes Name ______= Class/Section ______= Problem No. ______=

Parents’ phenotype

Parents’ genotype

Possible gametes

F 1 Results

Phenotype ratio:

Genotype ratio:

Iowa State University Extension and ISU Office of Biotechnology 25 Mendel’s Monohybrid

F1 Generation Seeds from cross of RR x rr parents

rrRrR

Genotype and phenotype ratio:

All F1 seeds are heterozygous and round Rr.

Lesson Module I – Genetics According to Mendel Overhead Master: Mendel-a

Iowa State University Extension and ISU Office of Biotechnology 27 Mendel’s Monohybrid

F2 Generation

Seeds from selfed F1 plants Rr Rr

RRRrR

rrRrr

Genotype ratio: 1/4 homozygous and round RR 1/2 heterozygous and round Rr 1/4 homozygous and wrinkled rr

Phenotype ratio: 3/4 round 1/4 wrinkled

Lesson Module I – Genetics According to Mendel Overhead Master: Mendel-b

Iowa State University Extension and ISU Office of Biotechnology 29 Mendel’s Monohybrid

F3 Generation

Seeds from selfed F2 plants RR

RRRRR

Genotype and phenotype ratio:

All F3 seeds are homozygous and round RR.

Lesson Module I – Genetics According to Mendel Overhead Master: Mendel-c

Iowa State University Extension and ISU Office of Biotechnology 31 Mendel’s Monohybrid

F3 Generation

Seeds from selfed F2 plants Rr

RRRrR

rrRrr

Genotype ratio: 1/4 homozygous and round RR 1/2 heterozygous and round Rr 1/4 homozygous and wrinkled rr

Phenotype ratio: 3/4 round 1/4 wrinkled

Lesson Module I – Genetics According to Mendel Overhead Master: Mendel-d

Iowa State University Extension and ISU Office of Biotechnology 33 Mendel’s Monohybrid

F3 Generation

Seeds from selfed F2 plants rr

rrrrr

Genotype and phenotype ratio:

All F3 seeds are homozygous and wrinkled rr.

Lesson Module I – Genetics According to Mendel Overhead Master: Mendel-e

Iowa State University Extension and ISU Office of Biotechnology 35 Key to phenotypes Monohybrid = = Punnett Squares =

PART A Parents’ phenotype

Parents’ genotype

Possible gametes

F1 Results F 1 Phenotype ratio:

Genotype ratio:

PART B (if needed)

Parents’ phenotype

Parents’ genotype

Possible gametes

F1 Results F1 Phenotype ratio:

Genotype ratio:

Lesson Module I – Genetics According to Mendel Overhead Master: Mendel-f

Iowa State University Extension and ISU Office of Biotechnology 37 Mendel’s Dihybrid

F1 Generation Seeds from cross of parents RRYY x rryy

RYY RYRYR

ryy RyrY RyrY RyrY RrY

Genotype and Phenotype:

All F1 seeds are heterozygous and round-yellow RrYy.

Lesson Module I – Genetics According to Mendel Overhead Master: Mendel-g

Iowa State University Extension and ISU Office of Biotechnology 39 Mendel’s Dihybrid

F2 Generation

Seeds from selfed F1 plants RrYy RyY RYryr

RYY RyRY RYRY RyrY RrY

Ryy RyRY RyRy RyrY Rry

rYY RyrY RYrY ryrY rrY

ryy RyrY Ryry ryrY rry

Nine genotype ratios: Four phenotype ratios: RRYY Rryy 9 (9/16) round-yellow seeds RRYy rrYY 3 (3/16) round-green seeds RRyy rrYy 3 (3/16) wrinkled-yellow seeds RrYY rryy 1 (1/16) wrinkled-green seeds RrYy

Lesson Module I – Genetics According to Mendel Overhead Master: Mendel-h

Iowa State University Extension and ISU Office of Biotechnology 41 Key to phenotypes Dihybrid Punnett = = Squares =

Parents’ phenotype

Parents’ genotype

Possible gametes

Results F 1 Phenotype ratio:

Genotype ratio:

Lesson Module I – Genetics According to Mendel Overhead Master: Mendel-i

Iowa State University Extension and ISU Office of Biotechnology 43 Lesson Module I – Genetics According to Mendel

The presence of markers for a trait of interest gives researchers a way to determine the presence or absence Genetics According of desired alleles. to Mendel Module II will investigate the relationships among I pedigrees, DNA analysis, and marker assisted selection.

Part II Pedigree Analysis PEDIGREES In a pedigree analysis, information about family members is summarized in a special kind of diagram. What is a Pedigree? Each individual is represented by a symbol, a square for a male and a circle for a female. Individuals who have To understand the process of marker assisted selection the trait of interest are represented by black squares (MAS), we first must understand a pedigree. A ( ) or circles ( ). Empty squares ( ) or circles pedigree is the family history of matings and the ( ) represent individuals that do not have the trait. offspring they produce with reference to specific genetic The symbols for individuals are arranged in horizontal traits. Animal and plant breeders have long used rows by generation, with each generation denoted by a pedigrees to help select individuals they believed to Roman numeral. Each individual within the generation have desirable traits. In the case of selective breeding, is numbered. In this manner, it is easy to refer to an the researchers are interested in the presence or absence individual, such as II-2, which means the second of a trait. generation, second individual. (See Figure 1.)

An organism’s pedigree helps researchers determine the When two individuals mate, they are connected by a inheritance of a trait. Using modern analysis tech- horizontal line. Their offspring are arranged together niques, researchers search for the DNA sequence of the under the connecting parental line. Siblings are gene(s) that controls the trait. In the process, they may arranged in birth order from left to right. With this discover other related DNA sequences near to or information, the use of Mendelian rules, and the rules present in the gene that always accompany the gene of of probability, it is possible to generate a hypothesis interest. These accompanying sequences, called (educated guess) about the alleles that are controlling molecular markers, are usually shorter and easier to the trait. The hypothesis from pedigree data usually identify than the larger, more complex gene sequence. includes whether an allele is dominant or recessive, as

Figure 1

Symbols: Pedigree:

Female I

Male

Mating

II Individuals with trait of interest 123

Individuals without trait of interest

Iowa State University Extension and ISU Office of Biotechnology 45 Educators Lesson Module I – Genetics According to Mendel well as predictions about the genotype of the individu- In Pedigree 1, an original parent (I-2) has the trait of als in the pedigree. This hypothesis can be used to interest. The absence of individuals in generations II make predictions about the probable genotypes and and III with the trait of interest indicates that the trait is phenotypes of future offspring. controlled by a recessive allele.

Traits controlled by recessive or dominant alleles show If the individuals with the trait of interest are homozy- a definite pattern of inheritance in the pedigree dia- gous recessive, you can make predictions of the gram. Traits controlled by a dominant allele appear in genotypes of the other individuals. I-2 and IV-1 have all individuals that possess one or more copies of the trait, so their genotypes would be aa. the allele. The recessive allele has traveled from the first genera- Individuals with homozygous dominant alleles (AA) or tion to the fourth without being expressed. In order for heterozygous alleles (Aa) will express the dominant an individual to have the trait, it must have been phenotype. A recessive allele cannot be detected in a inherited through a recessive allele from each parent. heterozygous individual and may stay hidden in a An individual who is a carrier (heterozygous) would family for many generations until a homozygous not have the trait. This is the case of the parents of recessive (aa) individual appears. The parents of an IV-1. Individuals III-1 and III-2 have to be heterozy- offspring that expresses a recessive trait (aa) must both gous (Aa) to have the progeny IV-1 with the trait. Since be heterozygous (Aa) if they did not express the individual I-2 is homozygous recessive (aa), his recessive trait themselves. offspring II-2 and II-3 must be heterozygous. The exact genotypes of I-1, II-1, II-4, III-3 through III-6, and IV-2 Another question that often can be answered using through IV-4 are impossible to determine, except that pedigree analysis is the probability that future offspring they must have either the homozygous dominant (AA) may be heterozygous (carriers) or may be homozygous or heterozygous (Aa) genotype. The pedigree of and have the recessive trait. Pedigree information is additional offspring would be needed to determine used to determine the chance that individuals who both their genotypes. have a family history of a trait may produce offspring with the trait. Credit Notes We will use Pedigree 1 below as an example of how a Atherly, Alan G.; Girton, Jack R.; and McDonald, John F. pedigree can be used to determine if a trait is dominant The Science of Genetics. Saunders College Publishing. or recessive and predict the individuals’ genotype and 1999 phenotype. The use of Punnett squares also can be very helpful in determining the genotype of the parents Olson, Tim. New Genes: Good and Bad. Dept. of and offspring. Animal Sciences, University of Florida.

Suszkiw, Jan. Mapping the Way to Bovine Bounty. ARS National Program Publication.

III

In Pedigree 1, an original parent (I-2) has the trait of interest. The absence of individuals in generations II and III with the trait of interest indicates that the trait is controlled by a recessive allele.

46 Iowa State University Extension and ISU Office of Biotechnology Lesson Module I – Genetics According to Mendel Educators

Materials Genetics According Teachers may want to use student handout Mendel-4 on p. 49-51 and the overhead transparency master to Mendel Mendel-j on p. 55 to help explain pedigrees. The I enrichment problem handout Mendel-5 is on p. 53-54. The problem and answers begin at the bottom of this page. Part II TEACHING RESOURCES Enrichment Problem John and Sue are planning to start a family. They visit a genetic counselor seeking advice about a genetic disease Lesson Plan: Pedigrees that both John’s and Sue’s families have suffered from in the past. There is no genetic test for this recessive trait. In this activity, students will learn how to analyze the Its onset, which can be gradual, occurs after age 40. pedigrees of organisms and determine an inheritance Both John and Sue are in their early 30’s. They would pattern for some common genetic diseases. like to determine the chances of their children inheriting the disease. Science Education Standards A review of John’s family history of the disease shows Science as Inquiry: Content Standard A that John’s paternal grandfather had the disease, but – Abilities necessary to do scientific inquiry John’s paternal grandmother and John’s father do not. (p.175) One of the brothers of John’s father has the disease. – Understanding about scientific inquiry (p.176) John’s mother has the disease.

Life Science: Content Standard C Sue’s family can only trace the occurrence of the disease – The cell (p.184) back to her maternal grandparents, neither of whom – Molecular basis of heredity (p. 185) had the disease. Sue’s mother, father, sister, and – Matter, energy, and organization in living systems brothers have the disease. (p.186) Construct a pedigree diagram and use Punnett squares History and Nature of Science: Content Standard G to help answer the following questions. – Science as a human endeavor (p. 200) – Nature of scientific knowledge (p. 201) – Historical perspectives (p. 201) Reflect and Apply These questions are included in the See for Yourself, Science Process Skills Mendel-5, student handout on p.53-54. Pedigree diagram and Punnett square answers are below. • Observing • Ordering Enrichment Problem Questions • Categorizing 1. What are the genotypes of John’s and Sue’s • Relating parents and grandparents? • Applying

John’s parents: Rr rr Life Skills John’s paternal grandparents: rr Rr • Science processing Sue’s parents: rr rr • Problem solving Sue’s maternal grandparents: Rr Rr • Decision making

2. What are the possible genotypes of John and Sue? Time John rr or Rr Preparation: 10 minutes to copy handouts Sue rr Activity: 45 minutes for activity

Iowa State University Extension and ISU Office of Biotechnology 47 Educators Lesson Module I – Genetics According to Mendel

3. If you are John and Sue’s genetic counselor, and Credit Notes given their family histories, how would you explain the chances of their children inheriting the disease? Atherly, Alan G.; Girton, Jack R.; and McDonald, John F. The Science of Genetics. Saunders College Publishing. If John is rr, there is a 100% chance that their children 1999 will have the disease. If John is heterozygous Rr, there is a 50% chance of each child having the disease. Olson, Tim. New Genes: Good and Bad. Dept. of Because of the late onset of the disease, it is impossible Animal Sciences, University of Florida. to determine if John’s genotype is heterozygous or homozygous for the disease. However, because Sue’s Suszkiw, Jan. Mapping the Way to Bovine Bounty. ARS parents and siblings have the disease, there is a 100% National Program Publication. chance that Sue will experience disease symptoms later in life.

Possible offspring of John and Sue, which depends on John’s genotype

rr Rr

r rr rr r Rr rr

r rr rr rrRrr

48 Iowa State University Extension and ISU Office of Biotechnology Lesson Module I – Genetics According to Mendel Student Handout Mendel-4 Learning more about . . . Pedigrees

What is a Pedigree? Module II will investigate the relationships among To understand the process of marker assisted selection pedigrees, DNA analysis, and marker assisted selection. (MAS), we first must understand a pedigree. A pedigree is the family history of matings and the Pedigree Analysis offspring they produce with reference to specific genetic traits. Animal and plant breeders have long used In a pedigree analysis, information about family pedigrees to help select individuals they believed to members is summarized in a special kind of diagram. have desirable traits. In the case of selective breeding, Each individual is represented by a symbol, a square for the researchers are interested in the presence or absence a male and a circle for a female. Individuals who have of a trait. the trait of interest are represented by black squares ( ) or circles ( ). Empty squares ( ) or circles An organism’s pedigree helps researchers determine the ( ) represent individuals that do not have the trait. inheritance of a trait. Using modern analysis tech- The symbols for individuals are arranged in horizontal niques, researchers search for the DNA sequence of the rows by generation, with each generation denoted by a gene(s) that controls the trait. In the process, they may Roman numeral. Each individual within the generation discover other related DNA sequences near to or is numbered. In this manner, it is easy to refer to an present in the gene that always accompany the gene of individual, such as II-2, which means the second interest. These accompanying sequences, called generation, second individual. (See Figure 1.) molecular markers, are usually shorter and easier to identify than the larger, more complex gene sequence. When two individuals mate, they are connected by a The presence of markers for a trait of interest gives horizontal line. Their offspring are arranged together researchers a way to determine the presence or absence under the connecting parental line. Siblings are of desired alleles. arranged in birth order from left to right. With this information, the use of Mendelian rules, and the rules of probability, it is possible to generate a hypothesis Figure 1

Symbols: Pedigree:

Female I

Male

Mating

II Individuals with trait of interest 123

Individuals without trait of interest

Iowa State University Extension and ISU Office of Biotechnology 49 Student Handout Lesson Module I – Genetics According to Mendel Mendel-4

(educated guess) about the alleles that are controlling very helpful in determining the genotype of the parents the trait. The hypothesis from pedigree data usually and offspring. includes whether an allele is dominant or recessive, as well as predictions about the genotype of the individu- In Pedigree 1, an original parent (I-2) has the trait of als in the pedigree. This hypothesis can be used to interest. The absence of individuals in generations II make predictions about the probable genotypes and and III with the trait of interest indicates that the trait is phenotypes of future offspring. controlled by a recessive allele.

Traits controlled by recessive or dominant alleles show If the individuals with the trait of interest are homozy- a definite pattern of inheritance in the pedigree dia- gous recessive, you can make predictions of the gram. Traits controlled by a dominant allele appear in genotypes of the other individuals. I-2 and IV-1 have all individuals that possess one or more copies of the trait, so their genotypes would be aa. the allele. The recessive allele has traveled from the first genera- Individuals with homozygous dominant alleles (AA) or tion to the fourth without being expressed. In order for heterozygous alleles (Aa) will express the dominant an individual to have the trait, it must have been phenotype. A recessive allele cannot be detected in a inherited through a recessive allele from each parent. heterozygous individual and may stay hidden in a An individual who is a carrier (heterozygous) would family for many generations until a homozygous not have the trait. This is the case of the parents of recessive (aa) individual appears. The parents of an IV-1. Individuals III-1 and III-2 have to be heterozy- offspring that expresses a recessive trait (aa) must both gous (Aa) to have the progeny IV-1 with the trait. Since be heterozygous (Aa) if they did not express the individual I-2 is homozygous recessive (aa), his recessive trait themselves. offspring II-2 and II-3 must be heterozygous. The exact genotypes of I-1, II-1, II-4, III-3 through III-6, and IV-2 Another question that often can be answered using through IV-4 are impossible to determine, except that pedigree analysis is the probability that future offspring they must have either the homozygous dominant (AA) may be heterozygous (carriers) or may be homozygous or heterozygous (Aa) genotype. The pedigree of and have the recessive trait. Pedigree information is additional offspring would be needed to determine used to determine the chance that individuals who both their genotypes. have a family history of a trait may produce offspring with the trait. Learn the Language We will use Pedigree 1 below as an example of how a pedigree can be used to determine if a trait is dominant Dominant or recessive and predict the individuals’ genotype and An allele (R) that expresses itself in the phenotype phenotype. The use of Punnett squares also can be of homozygous (RR) or heterozygous (Rr) individuals because it is dominant to the recessive allele (r)

III

IV In Pedigree 1, an original parent (I-2) has the trait of interest. The absence of individuals in generations II and III with the trait of interest indicates that the trait is controlled by a recessive allele.

50 Iowa State University Extension and ISU Office of Biotechnology Lesson Module I – Genetics According to Mendel Student Handout Mendel-4 Molecular marker Credit Notes A piece of DNA linked to or part of a gene of interest Atherly, Alan G.; Girton, Jack R.; and McDonald, John F. Pedigree The Science of Genetics. Saunders College Publish- Family history of matings and the offspring they ing. 1999 produce with reference to specific genetic traits Olson, Tim. New Genes: Good and Bad. Dept. of Animal Sciences, University of Florida. Recessive An allele (r) that expresses itself in a phenotype Suszkiw, Jan. Mapping the Way to Bovine Bounty. ARS only in homozygous individuals (rr) National Program Publication.

Iowa State University Extension and ISU Office of Biotechnology 51 Lesson Module I – Genetics According to Mendel Student Handout Mendel-5 See for yourself . . . What Are the Chances?

In this activity, you will use a Punnett square and a Name ______pedigree diagram to predict the chances of children inheriting a family disease. Class/Section ______

Family History John and Sue are planning to start a family. They visit a 3. If you are John and Sue’s genetic counselor and, genetic counselor seeking advice about a genetic disease given their family histories, how would you explain that both John’s and Sue’s families have suffered from in the chances of their children inheriting the disease? the past. There is no genetic test for this recessive trait. Draw a Punnett square and pedigree diagram on Its onset, which can be gradual, occurs after age 40. the back of this page to help you explain the Both John and Sue are in their early 30’s. They would chances. like to determine the chances of their children inheriting the disease.

A review of John’s family history of the disease shows that John’s paternal grandfather (on his father’s side) had the disease, but John’s paternal grandmother and John’s father do not. One of the brothers of John’s father has the disease. John’s mother has the disease.

Sue’s family can only trace the occurrence of the disease back to her maternal grandparents (on her mother’s side), neither of whom had the disease. Sue’s mother, father, sister, and brothers have the disease.

Construct a pedigree diagram and use Punnett squares to answer questions 1-3. Use the space on the back of this page.

1. What are the genotypes of John’s and Sue’s parents and grandparents?

2. What are the possible genotypes of John and Sue?

Iowa State University Extension and ISU Office of Biotechnology 53 Student Handout Lesson Module I – Genetics According to Mendel Mendel-5

Pedigree Diagram for Questions 1 and 2

Punnett Square for Question 3

54 Iowa State University Extension and ISU Office of Biotechnology Pedigree 1

Pedigree 1

? AA or Aa aa

? ? AA or AaAa Aa AA or Aa

? ??? Aa Aa

? ?? aa

Lesson Module I – Genetics According to Mendel Overhead Master: Mendel-j

Iowa State University Extension and ISU Office of Biotechnology 55