Calculating frequencies from count data:

Your neighborhood is home to 300 dogs, 520 cats, 100 goldfish, and 80 guinea pigs.

What is the frequency of dogs as pets in your neighborhood?

frequency of dogs = 300 / (300+520+100+80) = 300 / 1000 = 30%.

 

You are observing a population of pink dolphins in which, at a locus of interest, 50 individuals have a BB genotype, 350 individuals have a Bb genotype, and 100 individuals have a bb genotype.

What is the frequency of BB individuals in this population?

PBB = 50 / (50+350+100) = 50 / 500 = 10%.

What is the frequency of Bb individuals in this population?

PBb = 350 / (50+350+100) = 350 / 500 = 70%.

What is the frequency of bb individuals in this population?

Pbb = 100 / 500 = 20%.

Of course, 10% + 70% + 20% = 100%. No assumptions about population dynamics are needed!

 

What is the frequency of Xs in the following collection?

X X X X X O X O O O

How about in this collection? (What is the frequency of Xs here?)

XX XX XO XO OO

What is the frequency of the B allele in the following (very small) population?

BB BB Bb Bb bb

When counting individual items, it doesn't matter how they're grouped. Frequencies are calculated by counting the number of occurances, divided by the total number of items.

In both collections, there are 6 Xs out of 10 items. The frequency of Xs is 6/10 = 60%.

In the small population, there are 6 B alleles out of 10 total. The frequency of the B allele is 6/10 = 60%.

A shortcut to counting the number of B alleles is to count how many BB genotypes there are, and multiply this by 2 (since each BB genotype has 2 B alleles), then add the number of Bb genotypes (since each Bb genotype has 1 B allele).

The total number of alleles = the total number of individuals x2.

In the small population, there are 2 BB genotypes and 2 Bb genotypes, so 2x2 + 2 = 6 B alleles. There are 5 individuals, so 2x5=10 alleles in total. 6/10 = 60%.

 

Calculating allele frequencies from genotype frequencies:

You are observing a population of pink dolphins in which, at a locus of interest, the frequency of the BB genotype is 10%, the frequency of the Bb genotype is 70%, and the frequency of the bb genotype is 20%.

What is the frequency of the B allele in this population?

The allele frequency is always the homozygote genotype frequency + ½ the frequencies of all the heterozygous genotypes that contain that allele (summed).

The frequency of the B allele in the pink dolphin population is 10% + ½(70%) = 45%.

The frequency of the b allele in the pink dolphin population is 20% + ½(70%) = 55%.

Of course, since there are only two alleles, these two frequencies add up to 100%. Again, no assumptions about population dynamics are needed!

 

What if the population genotype frequencies were   B1B1:10%   B1B2:18%   B1B3:8%   B2B2:25%   B2B3:2%   B3B3:37%

What is the frequency of the B1 allele?

 

This frequency is 10% + ½(18%) + ½(8%) = 23%.

 

The other two allele frequencies (for B2 and B3) are 35% and 42%, respectively.