23. Phenotype, genotype and allele frequencies. Hardy – Weinberg law.

population

Population: A population consists of all individuals of the same biological species that live in the same geographical area at the same time and are capable of interbreeding. This concept is fundamental in understanding how species interact and evolve.

Population Biology and Genetics: Population biology and genetics study the interactions of alleles and genes within a population, focusing on the distribution and frequency of alleles and how these frequencies change over time due to evolutionary processes such as natural selection, mutation, gene flow, and genetic drift.

key concepts

• Gene Pool: The total collection of alleles at all genetic loci in all individuals within a population.

• Natural Selection: The process by which individuals with favorable traits have higher survival and reproductive success, leading to changes in allele frequencies over time.

• Genetic Drift: Random changes in allele frequencies within a population's gene pool, often having a more significant effect in small populations.

  • Bottleneck Effect: A sharp reduction in the size of a population due to events such as natural disasters, which can cause significant genetic drift.

  • Founder Effect: When a small group of individuals becomes isolated from the main population, leading to reduced genetic diversity and potential genetic drift.

• Gene Flow: The movement of genes between populations through migration, resulting in the gain or loss of alleles in the population.

• Allele Fixation: A situation in which one allele of a gene becomes the only allele present in a population, while alternative alleles are eliminated.

Phenotype, Genotype, and Allele Frequencies

• Phenotype Frequency: The proportion of individuals in a population that exhibit a specific phenotype. It is calculated by dividing the number of individuals with a given phenotype by the total number of individuals in the population.

• Genotype Frequency: The proportion of individuals in a population that possess a specific genotype. It is calculated by dividing the number of individuals with a given genotype by the total number of individuals in the population.

• Allele (Gene) Frequency: The relative frequency of a particular allele at a genetic locus within a population. It is calculated by dividing the number of copies of a specific allele by the total number of alleles for that gene in the population.

Hardy-Weinberg

The Hardy-Weinberg principle states that allele and genotype frequencies in a population will remain constant from generation to generation in the absence of evolutionary forces. For a population to be in Hardy-Weinberg equilibrium, several conditions must be met:

1. The population must be very large to minimize the effects of genetic drift.

2. There must be no genetic drift.

3. Mating must be random.

4. There must be no mutations altering the gene pool.

5. There must be no migration (gene flow) in or out of the population.

6. The population must not be isolated from other populations.

7. There must be no natural selection acting on the traits.

If any of these conditions are violated, the population may not be in Hardy-Weinberg equilibrium, leading to changes in allele frequencies.

• Let A represent a dominant allele and aa represent a recessive allele.

• p is the frequency of the A allele, and qq is the frequency of the aa allele, with the relationship p+q=1

Equilibrium Equation:

• The Hardy-Weinberg equilibrium is expressed by the equation p2+2pq+q2=1, where:

  • p2 represents the frequency of the AA genotype.

  • 2pq represents the frequency of the Aa genotype.

  • q2 represents the frequency of the aa genotype.

This equation predicts the expected distribution of genotypes in a population under Hardy-Weinberg equilibrium conditions.