Polygenic Inheritance
Polygenic inheritance
Single phenotypic character that is affected by multiple genes
Quantitative characters
Characters that exist along a spectrum
To understand this, we draw two average sized people, each with a genotype: big A, little a, big B, little b, big C, little c.
How many potential genotypes can we have?
- For each gene there are 2 possible alleles, and we are exploring a trait governed by three genes:
- So 2^3 (8) independently assorted gamete possibilities from each parent so when crossed, there are a total of 64 possible genotypes in the offspring (8 times 8).
- However, for our purposes when discussing the contribution to height, functionally all that matters is the total number of dominant alleles in the genotype (not which alleles are dominant), so we will reduce the number of genotypes to 7.
- Write seven possible genotype combinations of their offspring (these aren't all of the possible combinations, but for our purposes they are enough).
- So big A, big A, little b, little b, little c, little c gives the same amount of "height" as little a, little a, big B, little b, big C, little c.
- As a simplification of how height is controlled in humans, consider that each dominant allele gives an equivalent amount of "height" to an offspring.
- Draw the range of height that the offspring have.
- The tallest is big A, big A, big B, big B, big C, big C.
- The shortest is little a, little a, little b, little b, little c, little c.
We apply these principles to our analysis of polygenic inheritance to simplify complex possibilities.
- Finally, a clearer way to visualize this is to draw a box with six circles in it in line with each of the genotypes of the offspring.
- Fill in one circle in each box per dominant allele in the offspring. So those with the most filled in circles end up being the tallest.
Lastly, it is important to note that genes alone do not explain all of a phenotype. The environment, including socioeconomic status, plays a significant role in many phenotypes. For example, prenatal and childhood nutrition and stress influence height.