Topic 10.3 Polygenic inheritance

Question 1

10.3.1 Define polygenic inheritance.

Answer 1

Polygenic inheritance: trait that is expressed due to the interaction of more than one gene

e.g height, skin colour, eye colour

Question 2

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

Answer 2

• Polygenic inheritance patterns normally follow a normal (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

Skin colour

• 2 genes are involved in melanin production
• The amount of pigment produced is directly proportional to the number of dominant alleles for either gene
  
  • least common: no dominant alleles (aabb - albino) and 4 dominant alleles (AABB - black)
  • more common: light (1 dom. allele) and dark (3 dom.)
  • most common: medium (2 dom.)
    
    • forms bell-curve
  • large number of possible phenotypes as result of number of dominant alleles leads 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