Module 6

Module 6—Mendelian Genetics: The Transmission of Traits to the Next Generation

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Many traits are actually regulated by more than one gene. The result is a range of phenotypes, from one extreme to the other. The range in phenotypes is termed continuous traits. Eye colour in Drosophila; milk production in cows; and skin colour, eye colour, and height in humans are a few examples of traits determined by more than one gene. Traits that are determined by the interaction of two or more genes are termed polygenic traits. Although the phenotype expression of these traits can be influenced by the environment, as you learned in the last lesson, these traits also have many genes interacting to form the final phenotype.

 

This lesson will focus on traits that result from the interaction of only two genes. Traits can be affected by many more than two genes, but this becomes very complicated. Traits affected by multiple genes produce much more variability than a trait affected by one gene. Polygenic genes can involve incomplete dominance, involve multiple alleles, and be affected by the environment.

 

Another source of variation is epistasis. In epistasis, one set of genes will interfere with or affect the expression of another set of genes. The following example of coat colour in mice involves a gene set, B and b, producing colour, while another gene set, C and c, affects pigment production. You will discover how Bbcc phenotype is black, but BbCc phenotype results in no colour, or a white coat. 

 

Study the following examples of polygenic traits.

 

Example 1

 

In the example of chicken combs in the Get Focused section, two genes combine to form a phenotype that neither gene is capable of producing by itself. This is a continuous trait. One of the genes, the rose gene, has two alleles; an R for a rose comb that is dominant over an r allele that leads to a single comb. The other gene, the pea gene, also has two alleles; a P for a pea comb that is dominant over a p allele that will lead to a single comb. Remembering that a chicken will have both of these genes at the same time, here are the possible genotypes for the given phenotypes so far:

• Rose comb: Rrpp or RRpp
• Pea comb: rrPp or rrPP
• Single comb: rrpp

The forth phenotype is the walnut comb. A walnut comb results from the presence of at least one dominant allele in the two different genes. The possible genotypes for this are as follows:

• Walnut comb: RRPP, RrPP, RRPp, or RrPp

When analyzing polygenic traits, the movement of alleles follows the same patterns as in dihybrid crosses, however, the resulting genotypes must be interpreted for only one trait instead of two. For example, if a true breeding rose chicken (RRpp) were crossed with a true breeding pea chicken (rrPP), the F1 would be all walnut (RrPp). Continuing on to the F2, there would be a 9 walnut : 3 rose : 3 pea : 1 single comb phenotypic ratio. This looks just like Mendel’s work, until you remember that those ratios are for four different phenotypes of one trait only.

 

Example 2

 

Epistasis involves gene interaction where one gene masks the expression of another. A common example is coat colour in mice. Here, one gene determines if pigment is created at all, while another gene determines the type of pigment, like black or brown. In this example, the gene that controls the creation of pigment has two alleles. The allele to produce pigment C is dominant over the allele that will lead to no pigment or white, c. The other gene that controls the colour of the pigment also has two alleles. B is dominant and produces a black pigment, while b is recessive and will lead to brown pigment. Here are the possible phenotypes and their genotypes for this trait.

• Black: BBCC, BbCC, BBCc, BbCc
• Brown: bbCC, bbCc
• White: BBcc, Bbcc, bbcc

When trying to determine phenotype in epistasis, it is often helpful to consider a flow chart:

 

 

For an example cross, start with a black mouse (BBCC) and a white mouse (bbcc). The F1 will all be black (BbCc). Continuing on to the F2, there will be 9 black : 3 brown : 4 white mice. This is an unusual ratio and not the typical 9 : 3 : 3 : 1 ratio you might have been expecting. Unusual ratios are a characteristic of epistasis.

 

Polygenic traits can involve the interaction of more than just two genes. These would be too complex for you to predict and analyze; but, in a general sense, they are not too difficult to understand. Basically, the more genes there are involved in creating a single trait, the greater the variety of possible phenotypes. Read about polygenic traits and continuous phenotypes in your textbook on pages 605 to 607, “Polygenic Inheritance.”

 

Watch and Listen

 

Note the questions in the following Try This activity. Then return to the video “Alternate Patterns of Inheritance: the Potential for Diversity” and watch the following sections. Remember that you can ask your teacher for a username and password to access the video.

• “Bio Quest: Rabbit Breeding”
• “Bio Discovery: Other Inheritance Patterns”
• “Bio Review: Patterns of Inheritance”

Try This

 

To ensure your understanding of the concepts of polygenic inheritance, answer the following questions. Ask your teacher for assistance if necessary, and file the results in your course folder.

 

TR 1. How many phenotypes are there for rabbit fur colouration?

 

TR 2. How many genes are involved in determining rabbit fur colours?

 

TR 3. How many genes are involved in determining human skin colour? What else affects human skin colour?

 

TR 4. What is the phenotypic ratio of purple to white flowers in the case of epistasis?

 

TR 5. Define or explain the following patterns of inheritance.

1. incomplete dominance
2. multiple alleles
3. pleiotropy
4. epistasis
5. co-dominance
6. polygenic inheritance
7. the effect of the environment (is this really a pattern of inheritance?)

Self-Check

Use the following questions to confirm your understanding of polygenetic inheritance.

SC 1. In chickens, most birds do not have feathers on their legs. This phenotype is the result of two genes interacting and having only recessive alleles for both genes. The presence of a dominant allele for either gene or for both genes causes feathers. What is the feathered leg : unfeathered leg ratio in the offspring of chickens that are heterozygous for both genes?

1. 9 : 7
2. 12 : 4
3. 13 : 3
4. 15 : 1

SC 2. In corn plants, a dominant allele (I) inhibits kernel colour, while the recessive allele (i) permits colour when homozygous. At a different locus, the dominant gene P causes purple kernel colour, while the homozygous recessive genotype pp causes red kernels. If plants that are heterozygous at both loci are crossed, what will be the phenotypic ratio of the F1 generation?

 

 

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