Name: __________________________________________ Period: ________________
Allelic Frequency and Evolution
Example 1: let's consider a gene with only two alleles. In rabbits, Black fur color (BB or Bb) is dominant to white fur color (bb). In a population of 100 rabbits, 36 rabbits are homozygous dominant (BB), 48 rabbits are heterozygous (Bb) and 16 are white (bb).
Calculating the frequency of the B allele for Generation 1:
Frequency of B= # of B alleles in the population
# of total alleles in the population
Frequency of B= (36 x 2) + (48 x 1)= _____________ = _________________%
(100 x 2) = 200
Calculating the frequency of the b allele for Generation 1:
Frequency of b= # of b allele in the population
# of total alleles in the population
Frequency of b= (16 x 2) + (48 x 1) = ____________= __________________%
(100 x 2) = 200
Note that the two allele frequencies add up to 1. This is a law of population genetics: The sum of all allele frequencies will always be equal to 1. This is because 1 represents the frequency of all possible alleles within the population.
0.6 + 0.4 =1.0
The population of 100 rabbits have offspring. In Generation 2, there are 300 rabbits, 100 rabbits are homozygous dominant (BB), 160 rabbits are heterozygous (Bb) and 40 are white (bb).
Frequency of the B=
Frequency of the b=
Example 2: Let’s consider a gene with multiple alleles. In humans, I^{A} (Type A blood) and I^{B} (Type B blood) are codominant and i (Type O blood) is recessive. In a population of 100 people for Generation 1:
Blood Type |
Genotype |
Number of people in the population |
Type A |
I^{A}I^{A} |
10 |
Type A |
I^{A}i |
15 |
Type B |
I^{B}I^{B} |
10 |
Type B |
I^{B}i |
15 |
Type AB |
I^{A}I^{B} |
14 |
Type O |
ii |
36 |
Frequency of the I^{A} allele= 10+10+15+14 = _____________ = ____________%
200
Frequency of the I^{B} allele=
Frequency of the i allele=
Note that the three allele frequencies should add up to 1.
In a population of 100 people for Generation 2:
Blood Type |
Genotype |
Number of people in the population |
Type A |
I^{A}I^{A} |
5 |
Type A |
I^{A}i |
7 |
Type B |
I^{B}I^{B} |
15 |
Type B |
I^{B}i |
15 |
Type AB |
I^{A}I^{B} |
14 |
Type O |
ii |
44 |
Frequency of the I^{A} allele=
Frequency of the I^{B} allele=
Frequency of the i allele=
Note that the three allele frequencies should add up to 1.