Human Population Genetics


Resources: Protocol; Presentation; Wikipedia.

General Terms:

Population = group of individuals of the same species, living in the same habitat.
Note: Two organisms are said to belong to the same species if they can interbreed to produce fertile offspring.

Allele Frequency = percentage of a certain allele within the population

Genotype Frequency = percentage of organism with a genotype within the population

Hardy-Weinberg Equilibrium:

There are four basic types of mating patterns in various species:

  • autogamy (self-fertilisation)
  • inbreeding (mating between individuals with the same/very similar genotype)
  • outbreeding (mating between individuals with different genotypes)
  • panmixia (random mating)

The mating pattern in the human population approximates panmixia from the genetic point of view. In panmictic populations, the genetic composition does not change down the generations (the genotype frequency remains the same).

This stautus is called the Hardy-Weinberg Equilibrium.

The total of the frequencies of all alleles of one gene is 100%. If we mark the frequency of the dominant allele, A, as ‘p,’  and the frequency of the recessive allele, a, as ‘q,’ in a gene with two alleles then the following is true:

p + q = 1

Accordingly, the total of the frequencies of all possible genotypes relating to one gene is 100%. Using a mating square we can deduce the genotype frequencies form the allele frequencies. For a gene with two alleles these will be as follows:

  • the frequency of the dominant homozygotes, AA, is p²
  • the frequency of the recessive homozygotes, aa, is q²
  • the frequency of the heterozygotes, Aa, is 2pq.

For a gene with two alleles there are three possible genotypes, for the frequencies of which it is true that:

p² + q² +2pq = 1

Men:
– genotype XAY (p)
– genotype XaY (q)

Women:
– genotype XAXA ( )
– genotype XAXa (2pq)
– genotype XaXa ()

In order to maintain the Hardy-Weinberg equilibrium the following conditions have to be met:

  • The population is panmicticIn a non-random mating, individuals tend to choose partners similar to themselves, which then leads to increased frequency of homozygotes in the next generation.
  • Selection does not occurIf selection occurs, alleles beneficial for current conditions will be transmitted to the next generation leading to a rise in allele frequency.
    Note: Positive Selection is the selection for a particular advantageous allele.
  • Mutations do not occurIf a new, adgantageous, allele is created via mutation, the frequency of this allele may increase over generations.
  • Migration is absentBoth immigration and emigration can alter allele frequencies. For this to be avoided, no new alleles should come into the population.
  • The population is large enoughA large breeding population decreases the impact of chance alone on disrupting the genetic equilibrium.
    A genetic drift refers to the change in the frequency of an allele in a population due to selection advantages/disadvantages.

Factors affecting the gene pool:

  • Geographical barriers can keep an allele within a population – regionally specific hereditary disorders.
  • Cultural habits can limit the partner selection – enables occurrence and maintenance of rare hereditary disorders
  • A decline in frequency of some alleles caused by negative selection
  • A decline in frequency of some alleles due to selection pressures of today’s medicine and genetics (E.g. IVF).
  • Fixing of some alleles by a selection pressures (see below).

Balanced Polymorphism:

Balanced polymorphism refers to a state in which the allele frequencies are in equilibrium. It is cause by balance between beneficial effects of an allele in heterozygotes and the disadvantageous effect of the very same allele in homozygotes.

Example of polymorphism:

Disease: Sickle-Cell Anaemia
Type: Autosomal Recessive Hereditary
Cause: Point Mutation of β-globin chain
Effect: Altered haemoglobin molecule, causing the formation of sickle-cell shape erythrocytes under certain conditions.

Heterozygotes for the allele coding for the mutated β-globin chain are a good example of balanced polymorphism. In areas where malaria is frequent, it has been found that the frequency of the recessive allele is much higher than in areas where malaria is not present. This is because heterozygotes are protected from malaria (Plasmodium malarie does not reproduce in sickle-cells) and they do not experience the symptoms of the Sickle-cell anaemia that homozygotes do.

Extra:

Disease: Tay-Sachs Disease
Type: Autosomal Recessive Hereditary
Cause: HEXA gene (encoding Beta-N-acetylhexosaminidase, a lysosomal enzyme) mutation
Effect: Certain glycolipids cannot be digested. This leads to accumulation within the cell, followed by apoptosis.
Symptoms:

Children: Blindness; Deafness; Muscle Atrophy; Premature Death
Adults: Speech and Cognitive difficulties; Psychiatric disorders; Motility problems.

 

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