4.2.6 Genetic Diversity

Question 1

What is genetic diversity?

Answer 1

The genetic diversity within a species is the genetic variation that exists within a species

Although individuals of the same species will have the same genes at the same loci they will not necessarily have the same alleles for each gene

The gene pool is comprised of all the alleles of all the genes within a species

There can be genetic differences or diversity between populations of the same species which increases the size of the gene pool
This may be because the two populations occupy slightly different ranges in their habitat and so are subject to slightly different selection pressures that affect the allele frequencies in their populations

Genetic diversity within a single population can also be observed

Diversity in a species is important as it creates a larger gene pool which can help the population adapt, and survive changes in the environment
The changes could be biotic factors such as new predators, pathogens and competition with other species or they could be abiotic factors like temperature, humidity and rainfall

Genetic diversity can be assessed using several different measurements:
The proportion of polymorphic gene loci
The number of loci that have two or more alleles
The proportion of the population that is heterozygous for any specific gene locus
Allele richness
The number of different alleles that exist for specific genes

All three measurements involve determining whether there are multiple alleles at a locus. Phenotypes can sometimes be used to identify the presence of multiple alleles

For some genes, when each different allele is expressed in the phenotype of an individual they produce observable differences

For other genes, different alleles do not always produce an observable change in the phenotype of individuals
In this situation, the DNA sequences or the protein products of the alleles must be examined and compared
Note that some of the differences discovered might not be of major importance

Question 2

How do you calculate the proportion of polymorphic gene loci?

Answer 2

Genetic polymorphism occurs when there are two or more alleles present at a single loci
The rarest allele will have a frequency greater than 1% or greater than 5%
These numbers are of no particular significance, they have been randomly chosen by scientists

A monomorphic locus is one that does not have multiple alleles
Sometimes tables of data will refer to monomorphic loci as having one allele

A polymorphic locus is one that has multiple alleles
The most common allele must have a frequency less than 95% or 99%
If the most common allele has a frequency greater than 99% then the other allele(s) are extremely rare and likely to disappear

In order to assess the genetic diversity of a species population, scientists must identify a number of gene loci to investigate
They identify how many of these gene loci are polymorphic
The number of polymorphic gene loci is then divided by the total number of loci being investigated

The equation for calculating the proportion of polymorphic gene loci (P) is:
P = number of polymorphic gene loci ÷ total number of loci investigated

Question 3

What are the limitations of P (the proportion of polymorphic genetic loci)?

Answer 3

The proportion of polymorphic genetic loci (P) does not illustrate the allele richness of a breed or species
A study that looked at different blood proteins in dogs found that all genetic loci were polymorphic, P = 1. However, the number of alleles for each gene locus was not the same, it varied from 2 to 11

Due to the limitations of P other methods can be used to assess genetic diversity
Comparing the amino acid sequences of proteins
This is a useful method when investigating allozymes
Allozymes or alloenzymes are slightly different forms of the same enzyme. Each allozyme is coded for by a different allele and they function in a slightly different manner
Comparing DNA sequences
Due to the fact that the genetic code is degenerate, the amino acid sequence of two alleles could be the same but their DNA base sequence could be different
Nearly all of the genetic diversity assessment is now done at the level of base sequences
Scientists usually focus on specific sequences in nuclear DNA and mitochondrial DNA (mtDNA)