6 - Population Genetics

Quantitative vs binary trait

• A gene that impacts something on a range(how tall you grow, as an example) vs a gene that decides whether or not you have something(eg. cystic fibrosis, yes or no)

Pleiotropic effect

• When one gene has effects on multiple, seemingly unrelated things

In this locus on two homologous chromosomes, what is the SNP, and what is this person’s genotype?

acgCtaga

acgGtaga

The SNP is “single nucleotide polymorphism, aka a spot where the nucleotide isn’t the same in all people. This person’s genotype would be C/G

What is genotype vs allele frequency? Use the former acgCtaga example, and say 72 people were surveyed, with 20 people being C/C, 35 people being C/G, and 17 people being G/G

• Genotype frequency is how many people have that specific combination: So in our example, genotype frequency would be C/C 20/72 or 0.28, C/G being 35/72 or 0.48, and G/G being 17/72 or 0.24.

• Allele frequency is how often each allele appears, so not as a pair, just individually. So in this example, the 20 people with C/C would give 40 C alleles, and the 35 people with C/G also give another 35, ending up with 75. Meanwhile, the 17 people who are G/G give 34 G alleles and the 35 people with C/G give 35 G alleles, so 69 alleles in total. Frequencies are then 0.52 for C, and 0.48 for G.

In that last example, which one is the major allele, and why?

• The major allele is the one that’s more frequently found, so in that case, it would be C.

In terms of math, what notations can we give these alleles, use the same example.

• We can use p and q, so for example, freq(C) = 0.52 and freq(G) = 0.48, and so p+q=1

Hardy Weinberg Equation

• The equation

• The meaning

• The purpose

• p² + 2pq + q² = 1

• It’s saying that in a population, the genotype frequency should be based on three possibilities. Getting p and p, getting one p and one q, or getting q and q.

• It serves as a baseline for what a population should be if there are no mutations, no migration, no natural selection, and there is random mating and a large population size