March 1, 2021

allele flow

An allele is a possible variation of a gene. For example, if “height” were a gene, “tall” or “short” would be alleles of the gene. (Of course, genetic information carries much, much more detail and cannot be so succintly described. In reality, the observed phenotype of an individual’s height is the result of many interacting genes.)

Genes and their variants – alleles – are not fixed. Interesting patterns emerge when looking at the distribution of alleles in a population. When you look at the change in this distribution between generations, you’re studying allele flow.

Let’s say we have a gene with 2 alleles, simply $A$ and $B$. We start with a balanced distribution. Let $p$ be hte proportion of $A$s and $q$ be the proportion of $B$s.

$$ p = 0.5 \
q = 0.5 $$

AA  AB
AA  AB
AB  AB
AB  BB
AB  BB

In this population, there are 2 individuals with $AA$, 6 individuals with balanced $AB$, and 2 individuals with $BB$.

Assume the left hand column is male and the right hand is female – we’ll allow breeding across the rows, with two offspring to maintain the population size.

$$ p = 0.45 \
q = 0.55 $$

AA  AB
AA  AA
AB  BB
AB  BB
BB  BB

In this population, there are 3 individuals with $AA$, 3 individuals with balanced $AB$, and 4 individuals with $BB$.

The number of individuals with balanced phenotypes has reduced from 6 to 3, but the overall frequency of alleles has only changed by 5%.

We can study these changes statistically to determine how the alleles flow from one generation to the next. This analysis will determine whether changes are random “drift” or selected.

Content by © Jared Davis 2019-2020

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