Module 8 Population Genetics

4.8.1 page 2

Explore

When we’re interested in the genetic composition of a population, we’re interested in its gene pool.

 

What's in a gene pool? Imagine if all individuals in the human species threw their two alleles for each of their 30,000 genes into a large basket. That basket represents the human gene pool—it is an inventory of all the genes in that population. How many alleles would be in the gene pool of the entire human population?

 

            30,000 genes  X  2 alleles/gene  X  7 billion people = __?___alleles

 

Fortunately, in Population Genetics we generally only deal with one trait or gene at a time, and with much smaller populations!

Describing Gene Pools

Consider a population of only 10 field mice in a specific region of the southern Alberta prairies. Mice have two different alleles for coat color, black (B) and white (b). The mice are pictured below with their genotypes:

 

 

Imagine that each mouse can throw their two alleles for coat color into a basket.  This is the gene pool.  The gene pool can be described in three ways:

• by its genotype frequencies
• by its phenotype frequencies
• by its allele frequencies

Frequency is measured by dividing the number of a particular subgroup by the total group.

 

Gene Pool of a Field Mouse Population described three ways:

A:

 

Genotype Frequencies of the mouse gene pool:

 

Note: The short hand f(BB) will be used to replace the long-hand "frequency of the BB genotype.”

 

f(BB) = 7 mice    = 0.7 or 70%

          10 mice

 

 f(Bb) = 1 mouse  = 0.1 or 10%

           10 mice

 

f(bb) = 2 mice      = 0.2 or 20%

          10 mice

 

B.

 

Phenotype frequencies of the mouse gene pool:

 

Note: The short hand f(Black) will be used to replace the long-hand "frequency of the black phenotype.”

 

f(Black) =  8 mice      = 0.8 or 80%

              10 mice

 

 f(White) = 1 mouse    = 0.1 or 10%

                10 mice

 

C.

 

Allele frequencies of the mouse gene pool:

 

Note: The short hand f(B) will be used to replace the long-hand "frequency of the B allele.”

 

f(B) = 15 B alleles      = 0.75 or 75%

         20 alleles

 

f(b) = 5 b alleles    = 0.25 or 25%

          20 alleles

 

(Note: there are 10 individuals, so there are 20 alleles in total.)

 

For Population Geneticists, the most useful way to describe a gene pool is by its allele frequencies (as in diagram C above). If you’re only given the genotype frequencies, can you still determine the allele frequencies in the gene pool? Yes!

 

For example, in a population of 10 mice suppose 3 are homozygous dominant, 2 are heterozygotes and 5 are homozygous recessive. What is the frequency of the dominant and recessive alleles in the population? 

 

Solve:

10 mice have 2 alleles each so there are a total of 20 alleles in the gene pool.

 

3 mice are AA, so they have 6 A alleles

 

2 mice are Aa, so they have 2 A alleles and 2 a alleles

 

5 mice are aa so they have 10 a alleles. 

 

Answer:   f(A)  = 8 A alleles = 0.4 or 40% of the alleles are dominant

                        20 alleles

 

             f(a)  =  12 a alleles = 0.6 or 60% of the alleles are recessive

                        20 alleles

 

Self-Check 

SC 1. Out of a population of 100 individuals, if 40% of the alleles are dominant, what is the frequency of the recessive allele in the gene pool?

SC 2. If out of a population of 20 fish, 2 are homozygous dominant, 5 are heterozygous, and 13 are homozygous recessive, what is the frequency of the recessive allele in the population?