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Introduction to Genetics
Chapter 11
Chapter 11 Section 1 THE WORK OF GREGOR MENDEL
The Work of Gregor Mendel
• Some Definitions: – Genetics – the study of biological inheritance and variation – Chromosomes – hereditary units of an organism – Gene – segment of a chromosome that determines traits – Alleles – different forms of a gene associated with a specific trait – Hybrids – offspring that result from crosses between 2 parents with different traits – Dominant & Recessive – some alleles (dominant) will completely prevent the expression of others (recessive)
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The Work of Gregor Mendel
• Gregor Mendel’s Peas – Used ordinary garden peas from the monastery garden – Each type used was self-pollinating (each flower has both male and female parts) – Used several types of pea plants that were each true-breeding for particular characteristics • If allowed to self-pollinate, true-breeding plants will produce offspring identical to themselves – Tall plants produce only tall plants; short plants produce only short plants – Wanted to produce crosses of different types • Therefore had to prevent self-pollination
The Work of Gregor Mendel
• Genes and Dominance – Mendel studied seven different pea plant traits – Each of the seven traits had two contrasting characters • Green seed color or yellow seed color • Tall plant or short plant – Each original plant was called the P generation (parental generation)
– The offspring were called the F1 generation (first filial generation) – The offspring of parents with different traits are
called hybrids (in other words, the F1 generation)
The Work of Gregor Mendel
• Genes and Dominance (continued)
– Mendel expected the F1 offspring to have a “blend” of characteristics of both parental generation plants • But ALL the offspring had the characters of just ONE of the parental types! • In each cross, the character of the other parent seemed to have disappeared!
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The Principles of Dominance
P Generation F1 Generation F2 Generation
Tall ShortTall Tall Tall Tall Tall Short
The Principles of Dominance
P Generation F1 Generation F2 Generation
Tall ShortTall Tall Tall Tall Tall Short
The Work of Gregor Mendel
• Genes and Dominance (continued)
– Mendel expected the F1 offspring to have a “blend” of characteristics of both parental generation plants • But ALL the offspring had the characters of just ONE of the parental types! • In each cross, the character of the other parent seemed to have disappeared! – Mendel’s conclusion was that biological factors are passed from one generation to the next • Today those factors are called genes
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The Work of Gregor Mendel
• Genes and Dominance (continued) – Each of the traits Mendel studied was produced by one gene that had two versions • Each “version” of a gene is called an allele – Mendel’s second conclusion is called the “Principle of Dominance” • Some alleles are dominant and some are recessive • An organism with the dominant allele will always exhibit that trait – A plant that has an allele for a tall plant will be tall; if does not have the allele for a tall plant, the plant will be short – The following table shows the seven traits Mendel studied and the dominant allele for each
The Work of Gregor Mendel
• Mendel’s Seven F1 Crosses on Pea Plants
Seed Seed Seed Coat Pod Pod Flower Plant Shape Color Color Shape Color Position Height Round Yellow Gray Smooth Green Axial Tall
Wrinkled Green White Constricted Yellow Terminal Short
Round Yellow Gray Smooth Green Axial Tall
The Work of Gregor Mendel
• Segregation
– The F1 Cross • Had the recessive alleles disappeared or were they just masked?
• Mendel let the F1 plants produce offspring of their own, an F2 generation • The results were surprising – The recessive allele returned!
– Roughly 25% of the F2 offspring showed the “missing” trait from the P generation
– Explaining the F1 Cross
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The Principles of Dominance
P Generation F1 Generation F2 Generation
Tall ShortTall Tall Tall Tall Tall Short
The Principles of Dominance
P Generation F1 Generation F2 Generation
Tall ShortTall Tall Tall Tall Tall Short
The Principles of Dominance
P Generation F1 Generation F2 Generation
Tall ShortTall Tall Tall Tall Tall Short
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The Work of Gregor Mendel
• Segregation (continued)
– Explaining the F1 Cross
• The F1 plants had to have a characteristic from each of the P generation plants
• The recessive allele in the F1 plants was masked by the dominant allele for each specific trait
• During the F1 plant’s own reproduction the alleles for tallness had to be separated from each other, or segregated – He concluded that during the formation of the sex cells, or gametes, there had to be segregation of the alleles
The Work of Gregor Mendel
• Each gamete carries only a single copy of each gene
• Each F1 plant produces two kinds of gametes – Half have the allele for tall plants and have the allele for short plants – In the example (left), Red
is dominant to White so F1 is all red and F2 has both red and white (only 25% white)
Real World Example
• In horses, the black gene uses the designation “E” and a black coat is dominant to a chestnut coat. – A black horse may be “EE” or “Ee” – A chestnut horse is “ee” – An “EE” black horse crossed with an “ee” chestnut horse will always produce an “Ee” black horse.
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Real World Example
• Definitions (again) – Genotype – the genetic makeup of an organism (which alleles it has) – Phenotype – the physical characteristic of an organism (what it looks like) •Examples: – Top & Bottom • Genotype: EE or Ee • Phenotype: “black” – Middle • Genotype: ee • Phenotype: “chestnut”
Another Real World Example
• The white color pattern Tobiano in horses (left) is dominant to non- patterned horses. • The Tobiano gene uses the designation “T” if the gene is present (and “n” if not). • A tobiano horse may be “TT” or “Tn” – A “TT” horse (top left) crossed with a solid color horse will always have a tobiano patterned offspring – The adult horses pictured are the actual parents of the baby horse (called a foal).
Real World Example • Many diseases and medical conditions in humans follow the principles of Mendelian genetics – They are caused by a single gene and are recessive • Sickle-cell anemia • Tay-Sachs disease – Babies appear normal at birth then nerves begin to be damaged. The baby becomes blind, then deaf, then paralyzed and usually dies before age 4. • Cystic fibrosis • Albinism
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The Work of Gregor Mendel
• Analyzing Inheritance – Offspring resemble their parents. Offspring inherit genes for characteristics from their parents. To learn about inheritance, scientists have experimented with breeding various plants and animals. – In each experiment shown in the table on the next slide, two pea plants with different characteristics were bred. Then, the offspring produced were bred to produce a second generation of offspring. Consider the data and answer the questions that follow.
The Work of Gregor Mendel
Parents First Generation Second Generation Long stems short stems All long 787 long: 277 short Red flowers white flowers All red 705 red: 224 white Green pods yellow pods All green 428 green: 152 yellow Round seeds wrinkled seeds All round 5474 round: 1850 wrinkled Yellow seeds green seeds All yellow 6022 yellow: 2001 green
1. In the first generation of each experiment, how do the characteristics of the offspring compare to the parents’ characteristics? 2. How do the characteristics of the second generation compare to the characteristics of the first generation?
Chapter 11 Section 2 PROBABILITY AND PUNNETT SQUARES
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Probability and Punnett Squares
• Genetics and Probability – Since Mendel’s results were repeated regardless of the trait he was looking at (and he grew some 29,000 pea plants over the course of his studies), he concluded that probability could be used to explain the results – The likelihood that event will occur is called probability • If you toss a coin, what is the probability of getting heads? Tails? If you toss a coin 10 times, how many heads and how many tails would you expect to get? Working with a partner, have one person toss a coin
Probability and Punnett Squares
• Genetics and Probability – Activity – If you toss a coin, what is the probability of getting heads? Tails? If you toss a coin 10 times, how many heads and how many tails would you expect to get? – Working with a partner, have one person toss a coin ten times while the other person tallies the results on a sheet of paper. Then, switch tasks to produce a separate tally of the second set of 10 tosses. – Now answer the questions on the following slide.
Probability and Punnett Squares
• Genetics and Probability – Activity 1. Assuming that you expect 5 heads and 5 tails in 10 tosses, how do the results of your tosses compare? How about the results of your partner’s tosses? How close was each set of results to what was expected? 2. Add your results to those of your partner to produce a total of 20 tosses. Assuming that you expect 10 heads and 10 tails in 20 tosses, how close are these results to what was expected? 3. If you compiled the results for the whole class, what results would you expect? 4. How do the expected results differ from the observed results?
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Probability and Punnett Squares
• Genetics and Probability – Flip a coin, you have a 1 out of 2 chance of it coming up heads – Each flip is an independent event so the chance of flipping heads 3 times in a row is: • ½ x ½ x ½ = 1/8 – How does this relate to genetics? • The way in which alleles segregate is completely random so probability can be used to predict the outcome of genetic crosses
Probability and Punnett Squares
• Punnett Squares – A diagram that shows gene combinations that might result from a particular cross
Quick Lab - Making Connections
• Find a partner. Each pair of students will receive a paper bag with 4 paper clips inside. They are identical except for color. Three are blue and one is red. – What is the probability of picking a red item? – Of picking a red item two times in a row? • Without looking, pick an item from the bag (replacing it each time) 20 times, then 60 times. Keep track of your results. – How many red did you expect for 20? For 60? Did your results equal your calculated probabilities for 20? For 60?
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Probability and Punnett Squares
• Punnett Squares – A diagram that shows gene combinations that might result from a particular cross – Organisms that have two identical alleles for a trait are said to be homozygous for that trait – Homozygous organisms are “true-breeding” • BB flowers always are purple • bb flowers are always white – Each combination of genes is the organisms genotype • This square shows 3 genotypes: BB, Bb, and bb
Probability and Punnett Squares
• Punnett Squares – Organisms that have two different alleles for a trait are heterozygous – For a trait that exhibits complete dominance, a homozygous dominant and a heterozygous organism will physically appear the same – they will have the same phenotype – You can tell which is which only by studying their offspring
Probability and Punnett Squares
• Probability and Segregation – Since there is an allele for “tall” in 3 of the combinations you expect a 3:1 ratio of tall to short plants – But 1/3 of the tall plants are homozygous dominant and 2/3 are heterozygous – Their phenotype ratio is 3:1 – Their genotype ratio is 1:2:1 – Is this what we’ve found?
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Probability and Punnett Squares
• Probabilities Predict Averages – Probability predicts average outcome of a large number of events – It cannot predict the outcome of a specific event • Not all families with 4 children have exactly 2 boys and 2 girls • Not all families with 2 children have 1 girl and 1 boy – The samples are two small! – Choose 1000 families with 4 children each and you’d find pretty close to 2000 boys and 2000 girls among all of the children – The larger the sample, the closer to the expected outcome
Quick Lab – How are dimples inherited?
I. Title: How are Dimples Inherited (page 268) II. Purpose: By using random number to assign a genotype, students will be able to conclude how dimples are inherited III. Safety: none IV. Procedure: V. Data/Observations: 1. Write the last four digits of any 1. phone number. Odd digits represent the dominant allele; even digits represent the recessive allele 2. Use the first two digits to represent 2. the father’s genotype. Write his genotype.
Quick Lab – How are dimples inherited?
IV. Procedure: V. Data/Observations: 3. Use the last two digits to 3. represent the mother’s genotype. Write her genotype. 4. Construct a Punnett square for 4. the cross of these parents. Then, using that Punnett square, determine the probability that their child will have dimples. (remember the allele for dimples is dominant) 5. Determine the class average of 5. the percent of children with dimples.
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Quick Lab – How are dimples inherited?
VI. Calculations/Questions: 1. How does the class average compare with your results? 2. How does the class average compare with the result of a cross of two heterozygous parents? 3. What percentage of the children will be expected to have dimples if one parent is homozygous for dimples (DD) and the other is heterzygous (Dd)? VII.Conclusion: – Summarize your findings. Do you see how a dominant trait, such as dimples, is inherited?
Chapter 11 Section 3 EXPLORING MENDELIAN GENETICS
Exploring Mendelian Genetics
• Independent Assortment – Mendel knew from his studies of pea plants in the
F2 generation that alleles segregated during reproduction but do they segregate independently? • Does a round seed always have to be yellow? – Mendel designed an experiment to follow two genes as they passed from one generation to another • This is called a two factor or dihybrid cross
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Exploring Mendelian Genetics
• Independent Assortment – The Two-factor Cross • Each allele segregates independently – half of all gametes will have each allele – For Smooth Yellow seeds, half will be smooth, half not smooth – Then half will be yellow, half will be not yellow
Exploring Mendelian Genetics
• The F1 plants are heterozygous for both characteristics • Proving that genes independently assort can be seen in the results shown using a 4x4 Punnett square – Four different phenotypes observed – Nine genotypes observed
Exploring Mendelian Genetics
• Mendel’s F2 plants produced 556 seeds as he studied round vs. wrinkled AND yellow vs. green – 315 plants were round and yellow – 32 were wrinkled and green – 209 were “mixed” • 101 yellow and wrinkled • 108 green and round • Therefore, the genes had to sort independently of one another!
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Exploring Mendelian Genetics
Exploring Mendelian Genetics
• Conclusions led to the Principle of Independent Assortment – When two traits are considered in the same cross, the segregation of one pair of alleles is independent of the segregation of the other pair of alleles. – There could be four possible gametes • The Punnett square works on this assumption that each gamete occurs about 1/4 of the time – This is why the predicted ratio is 9:3:3:1. – NOTE: Independent assortment is not always true but since many do, this accounts for the tremendous variation in living things!
Exploring Mendelian Genetics
• A Summary of Mendel’s Principles – The inheritance of biological characteristics is determined by genes • Genes are passed from parents to their offspring during reproduction – When two or more alleles for a gene exist, some forms may be dominant and others may be recessive – Generally, each organism has two copies of every gene – one from each parent – The alleles for different genes usually segregate independently of one another
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Exploring Mendelian Genetics
• Beyond Dominant and Recessive Alleles – There are exceptions to Mendel’s principles! • Most genes have more than one allele • Many important traits are controlled by more than one gene • Just because an allele is dominant doesn’t mean it is common • Some alleles are neither dominant nor recessive! – Incomplete Dominance • One allele is not completely dominant over another • The heterozygous phenotype is somewhere in between the two homozygous phenotypes – Example – Four O’clock flowers; snapdragons, coat colors in many animals, etc.
Exploring Mendelian Genetics
• The F1 generation shows a phenotype that is different than either P generation parents – Red and white snapdragons produce all pink flower in F1 • F2 generation shows the F1 generation phenotype as well as both P generation phenotypes
Exploring Mendelian Genetics
A C
• Example A: CCRCCR – double dose of cream gene • Example B: CCRC – single dose of cream gene B • Example C: CC – fully pigmented; no cream gene
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Exploring Mendelian Genetics
• Co-dominance – Both alleles will contribute to the phenotype • Example – in some chickens there is an allele for black feathers and an allele for white feathers. Chickens with both alleles will NOT have gray feathers but will have both black and white feathers (called erminette) • Example – blood type genes from both parents combine to establish your blood type – A type A father and a type B mother can have offspring with type AB blood
Exploring Mendelian Genetics
• Multiple Alleles – Most genes have more than two alleles • An individual can still have only two alleles though! • Some of those alleles can be dominant to others, co- dominant, incomplete dominant or recessive! • Example – Blood type – there are 3 alleles – IA, IB, and i IA and IB are dominant to i but are co-dominant to each other • Example – (page 273 in text) – rabbit coat colors – 4 alleles – c has no color, producing an albino, recessive to all others; ch restricts color to certain areas of the body (making Himalayan), dominant to c and recessive to all others; cch shows a partial color change (called chinchilla), partially dominant to c and ch, recessive to C; C is full color and dominant to all others
Exploring Mendelian Genetics
• Polygenic Traits – Traits controlled by two or more genes – Most traits fall under this category! – Top right horse is a homozygous black horse (like top left) with TWO modifying genes – Bottom right horse is a bay color (normally like lower left color) with 3 alleles of two color modifying genes!
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Exploring Mendelian Genetics
• Applying Mendel’s Principles – Law of Segregation – Law of Independent Assortment – Both apply to more than just plants! • Humans, plants, insects, animals, etc. • Genetics and the Environment – Characteristics determined by genes inherited from parents – Those same characteristics may be affected by the environment as well – Genes provide a “plan” but how the plan develops is often determined by environment • You may have genes for being tall but poor nutrition doesn’t let you grow as tall as your genes would allow
Chapter 11 Section 4 MEIOSIS
General Information about Meiosis
• Mendel did not know exactly where genes were located but it was fairly quickly determined to be located on the chromosomes in the nucleus of a cell. • Mendel’s principles of genetics requires – Each organism must inherit a single copy of every gene from both its parents – When an organism produces its own gametes, those two sets of genes must separate from each other so that each gamete contains only one set of genes
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Meiosis
• Chromosome Number – The two sets of chromosomes that every organism has are called homologous chromosomes • Chromosomes appear in pairs; one of each pair from each parent • Gametes will have half the number of chromosomes – The number that represents the number in the gametes is N and is called haploid. • All other cells have both sets of chromosomes are so are called 2N or diploid. – 2N=46 in humans; fruit flies 2N=8
Meiosis
• Phases of Meiosis – During Meiosis the haploid gamete cells are produced from diploid cells – It involves two distinct divisions called meiosis I and meiosis II. • By the end of meiosis II, the diploid cell that entered meiosis has become 4 haploid cells • Meiosis I – Each chromosome is replicated and the cells begin to divide almost like they do in mitosis • Instead of lining up individually in prophase
Meiosis
• Overview of Meiosis
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Meiosis
Meiosis I
• As in mitosis, cells grow and develop during Interphase I, then duplicate their DNA!
Meiosis
Meiosis I
• Unlike mitosis, in Prophase I of meiosis, each chromosome pairs with its analogous chromosome to form a tetrad • Crossing over occurs here!
Meiosis
• Crossing over – Occurs during Prophase I of Meiosis I • (1) Chromosomes line up in analogous pairs • (2) They cross over one another • (3) The crossed sections are exchanged
– This creates recombinant DNA • It is different from either parent’s chromosomes
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Meiosis
•Overview of the process of Crossing over
Meiosis
Meiosis I
•In Metaphase I, spindle fibers attach to the chromosomes at the centromere • They are still lined up in pairs, unlike in mitosis.
Meiosis
Meiosis I
•In Anaphase I, the spindle fibers pull the homologous chromosomes to opposite ends of cell. • Still diploid but starting to become haploid!
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Meiosis
Meiosis I
•In Telophase I and Cytokinesis, the nuclear envelope reforms and the cell divides in two. • But neither daughter cell has two of each chromosome! They have become haploid!
Meiosis Figure 11-17 Meiosis II
• Meiosis I – Took a diploid cell and created two haploid daughter cells – Two haploid daughter cells now enter a second phase of meiotic division called Meiosis II • Meiosis II – Takes each of the Meiosis I daughter cells and produces two more haploid daughter cells – Entire process of Meiosis I and Meiosis II takes one diploid cell and makes four haploid daughter cells
Meiosis Meiosis II
Prophase II Metaphase II Anaphase II Telophase II Meiosis I results in two The chromosomes line up in a The sister chromatids Meiosis II results in four haploid (N) daughter cells, similar way to the metaphase separate and move toward haploid (N) daughter cells. each with half the number of stage of mitosis. opposite ends of the cell. chromosomes as the original. •In Prophase II, there is no chromosome replication this time but nuclear envelope disappears and spindles form.
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Meiosis Meiosis II
Prophase II Metaphase II Anaphase II Telophase II Meiosis I results in two The chromosomes line up in a The sister chromatids Meiosis II results in four haploid (N) daughter cells, similar way to the metaphase separate and move toward haploid (N) daughter cells. each with half the number of stage of mitosis. opposite ends of the cell. chromosomes as the original. •In Metaphase II, chromosomes line up individually like in mitosis
Meiosis Meiosis II
Prophase II Metaphase II Anaphase II Telophase II Meiosis I results in two The chromosomes line up in a The sister chromatids Meiosis II results in four haploid (N) daughter cells, similar way to the metaphase separate and move toward haploid (N) daughter cells. each with half the number of stage of mitosis. opposite ends of the cell. chromosomes as the original. •In Anaphase II, sister chromatids are pulled apart and move to opposite ends of the cell
Meiosis Meiosis II
Prophase II Metaphase II Anaphase II Telophase II Meiosis I results in two The chromosomes line up in a The sister chromatids Meiosis II results in four haploid (N) daughter cells, similar way to the metaphase separate and move toward haploid (N) daughter cells. each with half the number of stage of mitosis. opposite ends of the cell. chromosomes as the original. •In Telophase II and Cyntokinesis, four total daughter cells are formed; nuclear envelope reappears.
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Meiosis
• Gamete Formation –In males • The four haploid daughter cells at the end of meiosis II are four equally sized sperm –In females • Generally only one of the four haploid daughter cells at the end of meiosis II forms an egg – The egg takes nearly all of the cytoplasm – The remaining 3 form polar bodies which are not used in reproduction » The polar bodies pick up the extra sets of chromosomes so that too many are not contained in the egg
Meiosis
Comparing Mitosis and Meiosis Mitosis Meiosis • A diploid cell produces two • A diploid cell produces four diploid daughter cells haploid daughter cells • Asexual reproduction • Sexual reproduction • Allows an organism’s body • Produces gametes only to grow and replace cells – Does not occur in organisms that do not reproduce through sexual reproduction!
Chapter 11 Section 5 LINKAGE AND GENE MAPS
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Linkage and Gene Maps
• Gene Linkage – When genes for two different traits are always (or nearly always) inherited together – Linked genes are close together on the same chromosome • Rarely assort independently so they are inherited together • Still can be separated but not often • Genes that are close together tend to stay together, genes that are far apart tend to separate – The chromosomes sort independently but not individual genes!
Linkage and Gene Maps
• Gene Maps – Crossing over separates genes on the same chromosome – Can sometimes separate linked genes – How far apart genes were could be seen in how frequently they were linked • The farther apart, the less often they’d be linked – The rate that linked genes were separated could be used to produce a “map” showing where on a chromosome a gene is located
Linkage and Gene Maps
• Gene map of chromosome 2 of the fruit fly (as estimated in 1911) – Genes are named after the abnormal problem they cause
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Challenge Question
• The allele that causes galactosemia (g) is recessive to the allele for normal lactose metabolism (G). A normal woman whose father had galactosemia marries a man with galactosemia who had normal parents. They have three children, two normal and one with galactosemia. • What are the genotypes of each: The woman, her father, her husband, the husbands parents, and their 3 children?
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