Module 6 Mendelian Genetics

3.6.8 page 2

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Many traits are actually regulated by more than one gene. Skin colour, eye colour, and height are just a few examples seen in humans. For each of these traits many genes will interact to form the final phenotype. To make things a bit easier to understand, we will focus on traits that result from the interaction of only two genes. There are two main means of causing polygenetic traits: complementary interaction and suppression epistasis. An epistatic gene is a gene that interferes with the expression of another gene.

 

Complementary Interaction

The example of chicken combs from the introduction follows this type of interaction. In this case, two genes combine to form a phenotype that neither is capable of producing itself alone. 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 also 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. This results from the presence of both dominant alleles in the two different genes. The possible genotypes for this are

• Walnut comb: RRPP, RrPP, RRPp or RrPp

When analyzing polygenetic traits, the movement of alleles follows the same patterns as in Di-hybrid crosses, however, the resulting genotypes must be interpreted for only one trait instead of two. For example, if a true breeding Rose chicken (RRpp) was 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.

 

Supression Epistasis

 

This type of gene interaction takes place when one gene masks the expression of another gene. A good 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 our 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, “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 the case of epistasis, it is often helpful to consider a flow chart like the one below.

 

 

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 you might have been expecting, but it is characteristic for epistasis.

 

Continuous Traits

 

Polygenetic traits can involve the interaction of more than just two genes. These would be too complex for us 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 polygenetic traits and continuous phenotypes in your text on pages 605-607.

 

Watch and Listen

 

Let’s finish the rest of the video on Alternate Patterns of Inheritance: the Potential for Diversity (about 10 minutes). Watch the following sections:

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

 

Answer the following questions for your own understanding and save the results in your course folder.

1. How many phenotypes are there for rabbit fur colouration?
2. How many genes are involved determining rabbit fur colours?
3. How many genes are involved in determining human skin colour? What else affects human skin colour?
4. What is the phenotypic ratio of purple to white flowers in the case of epistasis.
5. Define or explain the following patterns of inheritance:
   
   
   1. incomplete dominance
   2. multiple alleles
   3. pleiotropy
   4. epistasis
   5. codominance
   6. polygenic inheritance
   7. the effect of the environment (Is this really a pattern of inheritance?)

Self-Check

Work through the following problems to solidify your understanding of polygenetic inheritance.

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
2. In corn plants, a dominant allele (I) inhibits kernel color, while the recessive allele (i) permits color when homozygous. At a different locus, the dominant gene P causes purple kernel color, 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?