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NEL The Basis of Heredity 617

Section18.5

Examples of polygenic traits in humans include skin colour, height, and intelligence.

In other animals and plants, many desirable traits, such as milk production in cows or

yield in canola, are also determined by more than one gene pair. This makes breeding for

these traits very difficult.

In some cases, two different genotypes interact to produce a phenotype that neither

is capable of producing by itself. In other cases, one of the genes will interfere with the

expression of the other, masking its effect. Genes that interfere with the expression of other

genes are said to be epistatic.

Observed phenotypic ratios of polygenic traits vary significantly from the phenotypic

ratios predicted by Punnett square analysis of non-interacting genes. Coat colour in

dogs provides an examp1e of epistatic genes. As shown in the Punnet square in

Figure 8, the allele Bproduces black coat-colour, while the recessive allele bproduces

brown coat-colour. However, a second gene also affects coat-colour. The allele Wof this

second gene prevents the formation of pigment, thereby preventing colour. The reces-

sive allele wdoes not prevent colour. The genotype wwBbwould be black, but the geno-

type WwBbwould appear white. The Wallele masks the effect of the Bcolour gene. In

humans, the gene responsible for albinism is epistatic. This gene interferes with the

expression of genes that determine pigment formation in the skin, hair, and eyes.

Figure 7
Hybridization can be used to produce a more vigorous strain of corn.

Figure 8

Punnett square of a cross between

a white dog (WwBb) and a black

dog (wwBb)

WwBb 
 wwBb

WwBB WwBb

WwBb Wwbb

wB wb

WB

Wb

wwBB wwBb

wwBb wwbb

wB

wb

brown

white

4 8 3 8 3 8

black

plant A plant B

hybrid AB seed

hybrid AB

plant C plant D

hybrid CD seed

hybrid CD

hybrid ABCD

seed

epistatic genea gene that masks

the expression of another gene or

genes

Coat Colour in Labrador

Retrievers

Coat colour variations in this

breed of dog is determined by two

interacting genes. In this

simulation, choose a genotype for

each gene and observe the

phenotype.

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