OCR AS Biology

Revision Notes

4.2.6 Genetic Diversity

Assessing Genetic Diversity

  • The genetic diversity within a species is the diversity of alleles and genes in the genome of a species
    • In other words, it 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 has also been observed
    • This diversity in a species is important as it creates a larger gene pool which can help the population adapt to, and survive, changes in the environment
    • The changes could be in biotic factors such as new predators, pathogens and competition with other species
    • Or the changes could be through 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 looking at whether there are different alleles at a locus
  • 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

Calculating the proportion of polymorphic gene loci

  • Genetic polymorphism is present 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 had 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
    • Then 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

Worked Example

There are many different registered pedigree dog breeds that exist. A conservationist stated that the degree of inbreeding that has occurred in some of the older breeds could be problematic as it likely reduces genetic diversity.

Scientists investigated 100 gene loci in 2 different breeds. For breed A (the older breed), 59 out of 100 gene loci were found to be polymorphic at the 0.95 level. For breed B, 87 out of 100 gene loci were found to be polymorphic at the 0.95 level. Calculate P for both breeds and evaluate whether the results support the conservationist’s statement.

Step 1: Calculate P for breed A

P = number of polymorphic gene loci ÷ total number of loci investigated

P = 59 ÷ 100

P = 0.59

Step 2: Calculate P for breed B

P = 87 ÷ 100

P = 0.87

Step 3: Determine if results support the statement

The older breed A has a lower P value than breed B. This suggests that it has lower genetic diversity which could be caused by inbreeding. More studies would need to be carried out on a larger number of breeds in order to prove this statement true.

Limitations of P

  • 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)

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