Module 8 Lesson 1 - 2

Genetic Diversity

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Interest in the genetic composition of a population is interest in its gene pool.

What is in a gene pool? Imagine if every individual in the human species threw their two alleles for each of their 25,000 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?

           «math xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mn»25000«/mn»«mo»§#160;«/mo»«mi»g«/mi»«mi»e«/mi»«mi»n«/mi»«mi»e«/mi»«mi»s«/mi»«mo»§#160;«/mo»«mo»§#215;«/mo»«mo»§#160;«/mo»«mo»(«/mo»«mn»2«/mn»«mo»§#160;«/mo»«mi»a«/mi»«mi»l«/mi»«mi»l«/mi»«mi»e«/mi»«mi»l«/mi»«mi»e«/mi»«mi»s«/mi»«mo»§#160;«/mo»«mi»p«/mi»«mi»e«/mi»«mi»r«/mi»«mo»§#160;«/mo»«mi»g«/mi»«mi»e«/mi»«mi»n«/mi»«mi»e«/mi»«mo»)«/mo»«mo»§#160;«/mo»«mo»§#215;«/mo»«mo»§#160;«/mo»«mn»7«/mn»«mo»§#160;«/mo»«mi»b«/mi»«mi»i«/mi»«mi»l«/mi»«mi»l«/mi»«mi»i«/mi»«mi»o«/mi»«mi»n«/mi»«mo»§#160;«/mo»«mi»p«/mi»«mi»e«/mi»«mi»o«/mi»«mi»p«/mi»«mi»l«/mi»«mi»e«/mi»«mo»§#160;«/mo»«mo»=«/mo»«mo»§#160;«/mo»«mn»3«/mn»«mo».«/mo»«mn»5«/mn»«mo»§#160;«/mo»«mo»§#215;«/mo»«mo»§#160;«/mo»«msup»«mn»10«/mn»«mn»14«/mn»«/msup»«mo»§#160;«/mo»«mi»a«/mi»«mi»l«/mi»«mi»l«/mi»«mi»e«/mi»«mi»l«/mi»«mi»e«/mi»«mi»s«/mi»«/math»

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

Describing Gene Pools

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The gene pool can be described by

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

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

Example 1

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

Imagine that each mouse can throw its two alleles for coat color into a basket. This is the gene pool.

Gene Pool of a Field Mouse Population 

A) Genotype frequencies of the mouse gene pool Frequency of the BB genotype «math xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mfrac»«mrow»«mn»7«/mn»«mo»§#160;«/mo»«mi»m«/mi»«mi»i«/mi»«mi»c«/mi»«mi»e«/mi»«/mrow»«mrow»«mn»10«/mn»«mo»§#160;«/mo»«mi»m«/mi»«mi»i«/mi»«mi»c«/mi»«mi»e«/mi»«/mrow»«/mfrac»«mo»=«/mo»«mn»0«/mn»«mo».«/mo»«mn»7«/mn»«mo»§#160;«/mo»«mi»o«/mi»«mi»r«/mi»«mo»§#160;«/mo»«mn»70«/mn»«mo»%«/mo»«/math» Frequency of the Bb genotype «math xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mfrac»«mrow»«mn»1«/mn»«mo»§#160;«/mo»«mi»m«/mi»«mi»o«/mi»«mi»u«/mi»«mi»s«/mi»«mi»e«/mi»«/mrow»«mrow»«mn»10«/mn»«mo»§#160;«/mo»«mi»m«/mi»«mi»i«/mi»«mi»c«/mi»«mi»e«/mi»«/mrow»«/mfrac»«mo»=«/mo»«mo»§#160;«/mo»«mn»0«/mn»«mo».«/mo»«mn»1«/mn»«mo»§#160;«/mo»«mi»o«/mi»«mi»r«/mi»«mo»§#160;«/mo»«mn»10«/mn»«mo»%«/mo»«/math» Frequency of the bb genotype «math xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mfrac»«mrow»«mn»2«/mn»«mo»§#160;«/mo»«mi»m«/mi»«mi»i«/mi»«mi»c«/mi»«mi»e«/mi»«/mrow»«mrow»«mn»10«/mn»«mo»§#160;«/mo»«mi»m«/mi»«mi»i«/mi»«mi»c«/mi»«mi»e«/mi»«/mrow»«/mfrac»«mo»=«/mo»«mo»§#160;«/mo»«mn»0«/mn»«mo».«/mo»«mn»2«/mn»«mo»§#160;«/mo»«mi»o«/mi»«mi»r«/mi»«mo»§#160;«/mo»«mn»20«/mn»«mo»%«/mo»«/math»

B) Phenotype frequencies of the mouse gene pool Frequency of the black phenotype «math xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mfrac»«mrow»«mn»8«/mn»«mo»§#160;«/mo»«mi»m«/mi»«mi»i«/mi»«mi»c«/mi»«mi»e«/mi»«/mrow»«mrow»«mn»10«/mn»«mo»§#160;«/mo»«mi»m«/mi»«mi»i«/mi»«mi»c«/mi»«mi»e«/mi»«/mrow»«/mfrac»«mo»=«/mo»«mo»§#160;«/mo»«mn»0«/mn»«mo».«/mo»«mn»8«/mn»«mo»§#160;«/mo»«mi»o«/mi»«mi»r«/mi»«mo»§#160;«/mo»«mn»80«/mn»«mo»%«/mo»«/math» Frequency of the white phenotype «math xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mfrac»«mrow»«mn»2«/mn»«mo»§#160;«/mo»«mi»m«/mi»«mi»i«/mi»«mi»c«/mi»«mi»e«/mi»«/mrow»«mrow»«mn»10«/mn»«mo»§#160;«/mo»«mi»m«/mi»«mi»i«/mi»«mi»c«/mi»«mi»e«/mi»«/mrow»«/mfrac»«mo»=«/mo»«mo»§#160;«/mo»«mn»0«/mn»«mo».«/mo»«mn»2«/mn»«mo»§#160;«/mo»«mi»o«/mi»«mi»r«/mi»«mo»§#160;«/mo»«mn»20«/mn»«mo»%«/mo»«/math»

C) Allele frequencies of the mouse gene pool Because the population is 10 individuals, the total alleles is 20. (Remember that every trait has two alleles.) Frequency of the B allele «math xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mfrac»«mrow»«mn»15«/mn»«mo»§#160;«/mo»«mi»B«/mi»«mo»§#160;«/mo»«mi»a«/mi»«mi»l«/mi»«mi»l«/mi»«mi»e«/mi»«mi»l«/mi»«mi»e«/mi»«mi»s«/mi»«/mrow»«mrow»«mn»20«/mn»«mo»§#160;«/mo»«mi»a«/mi»«mi»l«/mi»«mi»l«/mi»«mi»e«/mi»«mi»l«/mi»«mi»e«/mi»«mi»s«/mi»«/mrow»«/mfrac»«mo»=«/mo»«mo»§#160;«/mo»«mn»0«/mn»«mo».«/mo»«mn»75«/mn»«mo»§#160;«/mo»«mi»o«/mi»«mi»r«/mi»«mo»§#160;«/mo»«mn»75«/mn»«mo»%«/mo»«/math» Frequency of the b allele «math xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mfrac»«mrow»«mn»5«/mn»«mo»§#160;«/mo»«mi»b«/mi»«mo»§#160;«/mo»«mi»a«/mi»«mi»l«/mi»«mi»l«/mi»«mi»e«/mi»«mi»l«/mi»«mi»e«/mi»«mi»s«/mi»«/mrow»«mrow»«mn»20«/mn»«mo»§#160;«/mo»«mi»a«/mi»«mi»l«/mi»«mi»l«/mi»«mi»e«/mi»«mi»l«/mi»«mi»e«/mi»«mi»s«/mi»«/mrow»«/mfrac»«mo»=«/mo»«mo»§#160;«/mo»«mn»0«/mn»«mo».«/mo»«mn»25«/mn»«mo»§#160;«/mo»«mi»o«/mi»«mi»r«/mi»«mo»§#160;«/mo»«mn»25«/mn»«mo»%«/mo»«/math»

For population geneticists, the most useful way to describe a gene pool is by its allele frequencies (as in diagram C above). If you are given only the genotype frequencies, you can still determine the allele frequencies in the gene pool. 

Example 2

In a population of 10 mice, 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? 

Solution:

10 mice have 2 alleles each, making a total of 20 alleles in the gene pool.

3 mice are AA (homozygous dominant) =  6 A alleles.

2 mice are Aa (heterozygous dominant) = 2 A alleles + 2 a alleles.

5 mice are aa  (homozygous recessive) = 10 a alleles.

«math xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mi»f«/mi»«mi»r«/mi»«mi»e«/mi»«mi»q«/mi»«mi»u«/mi»«mi»e«/mi»«mi»n«/mi»«mi»c«/mi»«mi»y«/mi»«mo»§#160;«/mo»«mi»o«/mi»«mi»f«/mi»«mo»§#160;«/mo»«mi»A«/mi»«mo»§#160;«/mo»«mi»a«/mi»«mi»l«/mi»«mi»l«/mi»«mi»e«/mi»«mi»l«/mi»«mi»e«/mi»«mo»§#160;«/mo»«mo»=«/mo»«mo»§#160;«/mo»«mfrac»«mrow»«mn»8«/mn»«mo»§#160;«/mo»«mi»A«/mi»«mo»§#160;«/mo»«mi»a«/mi»«mi»l«/mi»«mi»l«/mi»«mi»e«/mi»«mi»l«/mi»«mi»e«/mi»«mi»s«/mi»«/mrow»«mrow»«mn»20«/mn»«mo»§#160;«/mo»«mi»a«/mi»«mi»l«/mi»«mi»l«/mi»«mi»e«/mi»«mi»l«/mi»«mi»e«/mi»«mi»s«/mi»«/mrow»«/mfrac»«mo»=«/mo»«mo»§#160;«/mo»«mn»0«/mn»«mo».«/mo»«mn»4«/mn»«mo»§#160;«/mo»«mi»o«/mi»«mi»r«/mi»«mo»§#160;«/mo»«mn»40«/mn»«mo»%«/mo»«mo»§#160;«/mo»«mi»o«/mi»«mi»f«/mi»«mo»§#160;«/mo»«mi»t«/mi»«mi»h«/mi»«mi»e«/mi»«mo»§#160;«/mo»«mi»a«/mi»«mi»l«/mi»«mi»l«/mi»«mi»e«/mi»«mi»l«/mi»«mi»e«/mi»«mi»s«/mi»«mo»§#160;«/mo»«mi»a«/mi»«mi»r«/mi»«mi»e«/mi»«mo»§#160;«/mo»«mi»d«/mi»«mi»o«/mi»«mi»m«/mi»«mi»i«/mi»«mi»n«/mi»«mi»a«/mi»«mi»n«/mi»«mi»t«/mi»«/math»

«math xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mi»f«/mi»«mi»r«/mi»«mi»e«/mi»«mi»q«/mi»«mi»u«/mi»«mi»e«/mi»«mi»n«/mi»«mi»c«/mi»«mi»y«/mi»«mo»§#160;«/mo»«mi»o«/mi»«mi»f«/mi»«mo»§#160;«/mo»«mi»a«/mi»«mo»§#160;«/mo»«mi»a«/mi»«mi»l«/mi»«mi»l«/mi»«mi»e«/mi»«mi»l«/mi»«mi»e«/mi»«mo»§#160;«/mo»«mo»=«/mo»«mo»§#160;«/mo»«mfrac»«mrow»«mn»12«/mn»«mo»§#160;«/mo»«mi»a«/mi»«mo»§#160;«/mo»«mi»a«/mi»«mi»l«/mi»«mi»l«/mi»«mi»e«/mi»«mi»l«/mi»«mi»e«/mi»«mi»s«/mi»«/mrow»«mrow»«mn»20«/mn»«mo»§#160;«/mo»«mi»a«/mi»«mi»l«/mi»«mi»l«/mi»«mi»e«/mi»«mi»l«/mi»«mi»e«/mi»«mi»s«/mi»«/mrow»«/mfrac»«mo»=«/mo»«mo»§#160;«/mo»«mn»0«/mn»«mo».«/mo»«mn»6«/mn»«mo»§#160;«/mo»«mi»o«/mi»«mi»r«/mi»«mo»§#160;«/mo»«mn»60«/mn»«mo»%«/mo»«mo»§#160;«/mo»«mi»o«/mi»«mi»f«/mi»«mo»§#160;«/mo»«mi»t«/mi»«mi»h«/mi»«mi»e«/mi»«mo»§#160;«/mo»«mi»a«/mi»«mi»l«/mi»«mi»l«/mi»«mi»e«/mi»«mi»l«/mi»«mi»e«/mi»«mi»s«/mi»«mo»§#160;«/mo»«mi»a«/mi»«mi»r«/mi»«mi»e«/mi»«mo»§#160;«/mo»«mi»r«/mi»«mi»e«/mi»«mi»c«/mi»«mi»e«/mi»«mi»s«/mi»«mi»s«/mi»«mi»i«/mi»«mi»v«/mi»«mi»e«/mi»«/math»

Self-Check

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1. In a population of 100 individuals, 40% of the alleles are dominant. What is the frequency of the recessive allele in the gene pool?
   
   
2. In 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? Show your answer to two significant digits.