Topic 10.3: Polygenic inheritance

10.3.1 Define polygenic inheritance

Polygenic inheritance refers to a single characteristic that is controlled by more than two genes (also called multifactorial inheritance)

Polygenic inheritance patterns normally (bell-shaped) distribution curve - it shows continuous variation

By increasing the number of genes controlling a trait, the number of phenotype combinations also increase, until the number of phenotypes to which an individual can be assigned are no longer discrete, but continuous

10.3.2 Explain that polygenic inheritance can contribute to continuous variation using two examples, one of which must be human skin colour

Human skin colour

• The colour of human skin is determined by the amount of dark pigment (melanin) it contains
• At least four (possibly more) genes are involved in melanin production; for each gene one allele codes for melanin production, the other does not
• The combination of melanin producing alleles determines the degree of pigmentation, leading to continuous variation

Grain colour in wheat

• Wheat grains vary in colour from white to dark red, depending on the amount of red pigment they contain
• Three genes control the colour and each gene has two alleles (one coding for red pigment, the other coding for no pigment)
• The most frequent combinations have an equal number of "pigment producing" and "no pigment" alleles, whereas combinations of one extreme or the other are relatively rare
• The overall pattern of inheritance shows continuous variation