Each Trait That Mendel Studied Existed in 2 Discrete Forms. for Example

Module 3C – Patterns of Inheritance Objective # 16

In this module, we will examine how Define the term “monohybrid cross”. studying the appearance of traits over Describe one of the monohybrid several generations has allowed crosses carried out by Mendel and scientists to discover the basic laws explain how the results of these that govern the transmission of genetic crosses led him to formulate the Law information from one generation to of Segregation and the Law of the next. Dominance.

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Objective 16 Objective 16

So far, we have focused on how chromosomes However, over 50 years before scientists get passed from cell to cell during cell division, and from one generation to the next during understood the role that chromosomes eukaryotic life cycles. play in transmitting genetic information, an Austrian monk named Gregor Since chromosomes contain the genetic information, the study of chromosomes Mendel discovered the basic principles provides a key to understanding how the of transmission genetics by studying the genetic information is transmitted from one pattern of inheritance for 7 different generation to the next. traits in pea plants.

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Objective 16 Mendel’s Seven Traits Each trait that Dominant Recessive Dominant Recessive 1. Flower Color 5. Pod Shape Mendel studied X X existed in 2 Purple White Inflated Constricted 2. Seed Color discrete forms. 6. Flower Position X Yellow Green For example, seed X 3. Seed Texture Axial Terminal shape could be 7. Plant Height X wrinkled or Round Wrinkled 4. Pod Color X round: Tall Short X Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 5 Green Yellow 6

1 Objective 16 Objective 16

Mendel was successful where many before him had failed because: ¾ He used statistics to mathematically ¾ He simplified the problem by first analyze the results of his crosses and examining just one trait at a time. help determine the general patterns of his results. ¾ He limited his study to traits that existed in 2 discrete forms. ¾ He kept quantitative data over several generations, and used large sample sizes. 7 8

Objective 16 Objective 16

Mendel’s success was also due to a Mendel began with crosses where he certain amount of luck. Each trait that followed the inheritance of one trait. he followed happened to be controlled Although he didn’t know it, each trait by a single gene locus. This produces he studied was controlled by one gene the simplest possible pattern of locus (a single pair of alleles.) This type inheritance. of cross is called a monohybrid cross. For example, Mendel followed the inheritance of flower color, which exists in 2 discrete forms: purple and white. 9 10

Objective 16 Objective 16

When he crossed some pure-breeding Next, he allowed the F1 plants to self- purple flowered plants with some fertilize in order to produce the F2 pure-breeding white flowered plants generation. Again, the results were (this is called the parental or P unexpected: generation) he got some surprising results: ¾ Out of 929 F2 plants, he found 705 ¾ all of the offspring (called the F1 with purple flowers and 224 with white generation) had purple flowers! flowers, a ratio of 3.15 to 1.

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2 One of Mendel’s Monohybrid Crosses Objective 16 Purple White Mendel explained his results by proposing P generation the following 4-part hypothesis: Cross- fertilize 1)Each individual has two “hereditary factors” controlling a given trait. The F generation All Purple 1 pure-breeding purple parents have 2 Self-fertilize hereditary factors for purple flowers (AA), and the pure-breeding white plants have 2 hereditary factors for white F generation : 2 705 Purple 224 White 13 flowers (aa). 14 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Objective 16 Objective 16

Today we call Mendel’s “hereditary From chromosome studies we know factors” alleles. that the parents are diploid, so there The appearance of an organism is called are 2 sets of chromosomes in each cell, its phenotype (e.g. purple flowers), and and 2 alleles at each gene locus. the alleles the organism or gamete has is its genotype (e.g. AA). Because flower color is controlled by one gene locus, each parent must have 2 alleles controlling this trait.

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Objective 16 Objective 16

Homologous Chromosomes If the 2 alleles are identical, the individual is homozygous for that trait, White flowers Normal Cdk Normal Ras and if the 2 alleles are different, the individual is heterozygous for that trait:

White flowers Normal Cdk Defective Ras 17 18

3 Objective 16 Objective 16 2) When the parents produce gametes, Today we know that homologous the 2 hereditary factors separate, and chromosomes separate during meiosis I, each gamete receives one of the 2 leading to formation of haploid gametes. factors. Therefore, all gametes Because gametes are haploid, each produced by the purple parent (AA) gamete has 1 set of chromosomes, and 1 have one purple allele (A), and all allele at every gene locus. gametes produced by the white parent Because flower color is controlled by one (aa) have 1 white allele (a). This is gene locus, each gamete must have 1 called Mendel’s Law of Segregation. 19 allele controlling this trait. 20

Objective 16 Objective 16 3) The offspring are formed when a Today we know the offspring are gamete from one parent joins with a diploid, so there are 2 sets of gamete from the other parent. chromosomes in each cell, and 2 alleles

Therefore, each F1 offspring receives at each gene locus (one inherited from one purple hereditary factor (A) each parent). from the purple parent (AA) and one white hereditary factor (a) from the white parent (aa).

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Objective 16 Objective 16 4) When an individual is heterozygous, Follow the procedure on the class only one of the 2 alleles is expressed. handout, “Predicting the Outcome of Mendel called the expressed allele Monohybrid and Dihybrid Crosses”, dominant, and the non-expressed to determine if Mendel’s 4-part allele recessive. Because purple is hypothesis can accurately predict the

dominant to white, all the F1 plants outcome of the F2 generation. (Aa) have purple flowers. This is known as Mendel’s Law of Dominance. 23 24

4 Objective # 17 One of Mendel’s Monohybrid Crosses

White Purple (aa) (Aa) Explain how a testcross can be Gametes Gametes aa A a used to determine whether an

A Purple A individual with a dominant Gametes AaAa (Aa) Gametes AA Aa A a phenotype is homozygous or Aa Aa Aa aa Purple F generation (AA) F1 generation 2 heterozygous. All purple ¾ purple, ¼ white

Objective 17 Testcross Dominant Phenotype Alternative 1 – Plant with To determine whether an individual dominant phenotype is ? AA with a dominant phenotype is homozygous Gametes Recessive homozygous for the dominant allele or AA heterozygous, Mendel crossed the phenotype aa a Aa Aa individual in question with an Gametes individual that had the recessive a Aa Aa

phenotype: If all offspring are purple; the unknown plant is homozygous.

Testcross Alternative 2 – Plant with Objective # 18 dominant phenotype is Dominant homozygous Phenotype ? Define the term “dihybrid cross”. Aa Gametes Describe one of the dihybrid Aa crosses carried out by Mendel and Recessive phenotype a Aa aa explain how the results of these Gametes aa a Aa aa crosses led him to formulate the

If half of offspring are Law of Independent Assortment. white; the unknown plant is heterozygous.

5 Objective 18 Objective 18

Based on his monohybrid crosses with In a dihybrid cross: pea plants, Mendel proposed the Law of ¾ the parents are diploid, so there are 2 Segregation and the Law of Dominance. alleles at each gene locus = 4 alleles total Next, he conducted a series of dihybrid ¾ the gametes are haploid, so there is 1 crosses. allele at each gene locus = 2 alleles total A dihybrid cross is a cross where we ¾ the offspring are diploid, so there are 2 follow the inheritance of alleles at 2 alleles at each gene locus = 4 alleles total different gene loci simultaneously.

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Objective 18 Objective 18

For example, in pea plants seed shape is In the F1 generation, Mendel found controlled by one gene locus where that all the offspring had round, yellow round (R) is dominant to wrinkled (r) seeds. while seed color is controlled by a In the F2 generation, the approximate different gene locus where yellow (Y) is proportions were as follows:

dominant to green (y). 9/16 round, yellow

Mendel crossed 2 pure-breeding plants: 3/16 round, green

one with round yellow seeds and the 3/16 wrinkled, yellow other with green wrinkled seeds. 33 1/16 wrinkled, green 34 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Objective 18 Objective 18 R R r r In order to explain his results, Mendel Parents Y Y y y proposed the Law of Independent Assortment. Parental Gametes R r Y y Independent Assortment means alleles at F Offspring R r the 2 gene loci segregate independently of 1 Y y each other and are NOT transmitted as a F1 Offspring’s R R r r unit. Therefore, each plant can produce Gametes Y y Y y gametes with allele combinations that were not present in the gametes inherited from If the allele pairs assort independently, all 4 its parents: 35 types of gametes are equally likely. 36

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F2 with independent assortment: All of Mendel’s dihybrid crosses Genotypes RY Ry rY ry showed an approximate 9:3:3:1 of gametes RR RR Rr Rr phenotypic ratio for the F generation. produced RY YY Yy YY Yy 2 by F1 RR RR Rr Rr Therefore, his results are in agreement Ry generation Yy yy Yy yy with the prediction made based on the Rr Rr rr rr rY Law of Independent Assortment. YY Yy YY Yy Rr Rr rr rr ry Yy yy Yy yy Expected phenotypic ratio is 9 : 3 : 3 : 1 37 38

Objective # 19 Objective 19a Explain how each of the following patterns a) Incomplete dominance: of inheritance represents an exception to Mendel’s original principles: ¾ neither allele is dominant and heterozygous individuals have an a) incomplete d) polygenic traits intermediate phenotype dominance e) epistasis ¾ for example, in Japanese four b) codominance f) pleiotropy o’clock, plants with one red allele c) multiple alleles g) environmental and one white allele have pink effects on gene flowers: expression 39 40

Incomplete Dominance Objective 19b Gametes CR CW b) Codominance: CRCR ¾ neither allele is dominant and both CR alleles are expressed in heterozygous CRCR CRCW individuals Gametes ¾ we will examine an example of CW F1 generation codominance when we discuss R W R W W W All C C C C C C human ABO blood types F2 generation W W C C 1 : 2 : 1

7 Objective 19c Objective 19c c) Multiple alleles: ¾ This gene (designated I) codes for an ¾ when there are more than 2 possible enzyme that adds sugar molecules to alleles at a given gene locus (even lipids on the surface of red blood cells. though each diploid individual has ¾ There are 3 possible alleles at this gene only 2). locus: ¾ the human gene locus that controls IA adds galactosamine ABO blood type involves multiple IB adds galactose alleles and codominance. i adds neither sugar 43 44

Objective 19c Objective 19c Antigens present Blood Possible genotypes ¾ These sugars act as recognition Type markers (antigens) for the immune galactosamine A IA IA IA i system. only ¾ The immune system will produce galactose B IB IB IBi antibodies against cells with foreign only antigens and mark them for both AB IAIB destruction. neither O ii

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Objective 19c Objective 19c

Parents IAIB ii ¾ If a person with type AB blood Parental IA IB i i marries a person with type O, what Gametes blood types are possible among the offspring? i i F1 Offspring: IA IAi IAi

IB IBi IBi 47 48

8 Objective 19d d) Polygenic traits: ¾ most traits are not controlled by a single gene locus, but by the combined interaction of many gene loci. These are called polygenic traits. ¾ polygenic traits, such as height in humans, often show continuous variation, rather then a few discrete forms: 49 50

Objective 19e Objective 19e e) Epistasis: ¾ a dominant allele (B) produces a black ¾ this is a type of polygenic inheritance coat while the recessive allele (b) where the alleles at one gene locus can produces a brown coat hide or prevent the expression of ¾ however, a second gene locus controls alleles at a second gene locus. whether any eumelanin at all is ¾ for example, in Labrador retrievers one deposited in the fur. Dogs that are gene locus affect coat color by homozygous recessive at this locus (ee) controlling how densely the pigment will have yellow fur no matter which alleles are at the first locus: eumelanin is deposited in the fur. 51 52

Epistasis Objective 19f E_ ee Dark pigment in fur f) Pleiotropy: No dark pigment in fur ¾ this is when a single gene locus E_bb E_B_ eebb eeB_ affects more than one trait.

Yellow fur Yellow fur Brown fur Black fur ¾ for example, in Labrador retrievers the gene locus that controls how dark the pigment in the hair will be also affects the color of the nose, lips, and eye rims.

9 Objective 19g g) Environmental effects on gene expression: ¾ the phenotype of an organism depends not only on which genes it has (genotype), but also on the environment under which it develops.

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Objective 19g

¾ Although scientists agree that phenotype depends on a complex interaction between genotype and environment, there is a lot of debate and controversy about the relative importance of these 2 factors, particularly for complex human traits.