Module 6 Lesson 6 - 2

Polygenic Traits

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Many traits are regulated by more than one gene. Skin colour, eye colour, and height are a few examples in humans. For each of these traits, many genes interact to form the final phenotype. To make things a bit easier to understand, this lesson focuses on traits that result from the interaction of only two genes. Traits that are controlled by more than one gene area called polygenic traits.

Polygenic traits are caused mainly by two means: complementary interaction and suppression epistasis. A complementary interaction occurs when two genes must be present to yield a specific phenotype. An epistatic gene interferes with the expression of another gene. 

Complementary Interaction

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The example of chicken combs from the introduction follows the complementary interaction type. In this case, two genes combine to form a phenotype that is not 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 also leads to a single comb. Remember that a chicken has both of these genes at the same time. The possible genotypes for the given phenotypes are given in this table:

Rose comb	RRpp or Rrpp	One dominant 'R' allele is required to express the rose comb phenotype.
Pea comb	rrPP or rrPp	One dominant 'P' allele is required to express the pea comb phenotype.
Single comb	rrpp	All recessive alleles result in a single comb.

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 in the following table:

Walnut comb

RRPP  RrPP  RRPp  or RrPp

Two dominant alleles (R and P) are required to express the walnut comb phenotype.

When analyzing polygenic traits, realize that 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) was crossed with a true breeding pea chicken (rrPP), the F1 would be all walnut (RrPp).

	Rp
rP	RrPp

In 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.

Suppression Epistasis

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For an example cross, start with a black mouse (BBCC) and a white mouse (bbcc). The F1 generation will all be black (BbCc).

	BC
bc	BbCc

In the F2 generation (BbCc X BbCc), 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 expected, but it is characteristic for epistasis.