10.3 – Polygenic Inheritance
10.3.1 – Define polygenic inheritance
Inheritance of phenotypic characters (such as height, eye colour in humans) that are determined by the collective effects of several genes. A single characteristic that is controlled by two or more genes.
10.3.2 – Explain that polygenic inheritance can contribute to continuous variation using two examples, one of which must be human skin colour
Since a single characteristic may be influenced by more than one gene, it may exhibit continuous variation within a population. These genes are collectively called polygenes. Each allele of a polygenic character often contributes only a small amount to the overall phenotype, making study of individual alleles difficult. Phenotypic variation is the result of genotypic variation coupled with environmental variation. Environmental effects smooth out the genotypic variation, giving continuous distribution curves.
Skin Colour
Skin colour is actually determined by more than two genes. However, this example shows only two. It is represented by nine possible genotypes, which form five phenotypes. There are two genes, each with two alleles that control the amount of melanin
There are two genes, each with two alleles that control the amount of melanin A and B code to add melanin
A and B code to add melanin a and b code for no added melanin
a and b code for no added melanin
The amount of pigment produced is directly proportional to the number of dominant alleles for either gene. Having no dominant alleles results in an albino.
However, the phenotype is also influenced by environmental factors. In the case of skin colour, the exposure of the individual to sunlight will slightly alter the amount of melanin produced in their skin. This smooths out the distribution of skin colour into one continuous curve.
Finch Beak Depth
Finches eat seeds, breaking them open with their beaks. The depth of a finch’s beak is controlled by a number of genes; here, we only look at two: A and B code to add
A and B code to add depth
a and b code for no added depth.
Heterozygous cross: AaBb x AaBb
Darker orange indicates greater beak depth. This will form a similar distribution to that of skin colour seen above. Once again, environmental factors will smooth out the distribution.