Population Genetics: Principles, Applications, and Disease Allele Frequencies

Population Genetics

Introduction to Population Genetics

Population genetics is the study of genetic variation within populations and how gene and allele frequencies change over time due to evolutionary forces. It provides a framework for understanding the distribution of genetic traits and the mechanisms that drive genetic diversity.

• Gene Pool: The collection of all alleles at all loci in a freely interbreeding population.

• Genetic Variation: Includes SNPs, CNVs, and mutations that result in different alleles among individuals and families.

• Population Genetics Focus: Examines differences in allele frequencies between populations and the factors influencing these differences.

Alleles vs Genotypes

Understanding the distinction between alleles and genotypes is fundamental in population genetics.

• Allele: Different forms of a gene (e.g., A or a).

• Genotype: The combination of alleles an individual has at a locus (e.g., AA, Aa, or aa).

• Phenotype: The observable expression of the genotype (e.g., affected or unaffected).

• Genotype Frequency: Proportion of each genotype in the population.

• Allele Frequency: Proportion of each allele in the gene pool.

Determining Allele and Gene Frequencies

Allele and gene frequencies are essential for understanding genetic structure and disease risk in populations.

• For autosomal recessive diseases, genotype frequencies can be determined in two cases:

  • When wild-type (AA) and heterozygous (Aa) individuals can be distinguished phenotypically or biochemically.

  • When wild-type (AA) and heterozygous (Aa) individuals cannot be distinguished phenotypically.

Example: ΔCCR5 Cell Receptor Mutation

The frequency of the ΔCCR5 mutation is a classic example of allele and genotype frequency analysis in human populations.

• CCR5: A cell surface cytokine receptor, coded by a gene on chromosome 3p21, expressed on T cells, macrophages, dendritic cells, and microglia.

• ΔCCR5 Mutation: A 32-base pair deletion resulting in a nonfunctional receptor, conferring resistance to HIV infection.

• Frequency varies among populations and can be determined using PCR and gel electrophoresis.

Hardy-Weinberg Principles

The Hardy-Weinberg equilibrium provides a mathematical model for predicting genotype frequencies from allele frequencies in a population under ideal conditions.

• Equation:

$p^2 + 2pq + q^2 = 1$

• Where p = frequency of one allele, q = frequency of the other allele.

• Genotype Frequencies: AA = $p^2$, Aa = $2pq$, aa = $q^2$

• Allele frequencies remain constant from generation to generation if Hardy-Weinberg conditions are met.

Hardy-Weinberg Conditions

For a population to be in Hardy-Weinberg equilibrium, several conditions must be satisfied:

• Random mating

• Infinitely large population

• No new mutations

• No migration in or out of the population

• No selection; all genotypes are equally viable and fertile

• Generations are discrete

• Allele frequencies are equal in both sexes

Importance and Applications of Hardy-Weinberg Law

The Hardy-Weinberg law is crucial for estimating gene and genotype frequencies, especially when heterozygotes and homozygotes are indistinguishable.

• Allows estimation of carrier frequency for recessive diseases (e.g., cystic fibrosis, PKU).

• Explains why mutant genes do not disappear from a population even if the major phenotype is rare.

• Useful for estimating gene frequencies from disease prevalence data.

Use of Hardy-Weinberg to Determine Frequency of Autosomal Recessive Genes

Carrier frequency can be calculated using Hardy-Weinberg principles.

• For a recessive condition with frequency $q^2$, carrier frequency is $2pq$.

Example: If frequency of affected individuals is 1/10,000 ($q^2$), then $q = 1/100$ and $p ext{ (approx)} = 1$. Carrier frequency $2pq = 2 imes 1 imes 1/100 = 1/50$.

Applications of Hardy-Weinberg Law

Carrier risk calculations for genetic counseling:

• If both parents are carriers, the chance of having an affected child is 1/4.

• If one parent is a carrier and the other is not known, the risk is calculated by multiplying carrier frequencies and chance of inheritance.

Selected Examples of Disease Alleles with Different Frequencies in Populations

Disease	Population Variation
Sickle Cell Anemia	High in Africa, Less Common Elsewhere
Tay-Sachs Disease	High in Ashkenazi Jews
Cystic Fibrosis	High in European and US Caucasians, Lower in Asian and African Populations

Examples of Polymorphic Loci with Different Allele Frequencies

Locus	Allelic Variation
ABO Blood Group	Wide Variation
HLA System	Numerous Alleles at Each Sub-Locus, Wide Variation
Other Blood Groups	Variation in Frequency of Common Alleles; Some Rare Alleles Show Restricted Distribution

Frequency of Cystic Fibrosis in Different Ethnic Populations

Group	Affected Individuals	Carrier
Ashkenazi Jews	1:2,500	1:25
Northern European	1:3,600	1:29
Hispanic	1:9,600	1:46
African American	1:15,000	1:65
Asian American	1:32,200	1:90

Factors Affecting Hardy-Weinberg Equilibrium

• Stratification: Subdivision of populations

• Assortative Mating: Non-random mating based on phenotype

• Consanguinity: Mating between relatives

• Mutations and Selection: Changes in fitness

• Gene Flow and Migration: Movement of alleles between populations

• Genetic Drift: Random changes in allele frequencies, including bottleneck and founder effects

Summary Table: Genotype Frequencies for WT & Mutant CCR5

Genotype	Number of People	Observed Relative Genotype Frequency
CCR5/CCR5	647	0.821
CCR5/ΔCCR5	134	0.169
ΔCCR5/ΔCCR5	7	0.009
Total	788	1.000

Allele Frequencies Calculation Example

• Total number of alleles in population = 788 x 2 = 1576

• Total CCR5 alleles = (2 x 647) + 134 = 1428

• Total ΔCCR5 alleles = (2 x 7) + 134 = 148

• % of CCR5 alleles = 1428/1576 = 90.6%

• % of ΔCCR5 alleles = 148/1576 = 9.4%

Allele Frequencies Table

Allele	Derived Allele Frequency
CCR5	0.906 = p
ΔCCR5	0.094 = q
Total	p + q = 1

Hardy-Weinberg Law and Equation

The Hardy-Weinberg Law states that the frequency of the three genotypes AA, Aa, and aa is given by the terms of the binomial expansion:

$(p + q)^2 = p^2 + 2pq + q^2 = 1$

Where:

• p = frequency of one allele

• q = frequency of the other allele

Use of Punnett Square to Understand Hardy-Weinberg Equation

Punnett squares can be used to visualize the distribution of genotypes in a population under random mating.

• For alleles A and a, the frequencies are p and q, respectively.

• The Punnett square shows the expected genotype frequencies: $p^2$ (AA), $2pq$ (Aa), $q^2$ (aa).

Summary of Hardy-Weinberg Conditions and Their Importance

• Allows estimation of gene and genotype frequencies when heterozygotes and homozygotes are indistinguishable.

• Explains persistence of mutant alleles in populations.

• Useful for genetic counseling and disease risk estimation.

Factors Affecting Hardy-Weinberg Equilibrium

• Stratification

• Assortative mating

• Consanguinity

• Mutations and selection

• Gene flow and migration

• Genetic drift (bottleneck, founder effect)

Additional info:

• Some context and definitions were expanded for clarity and completeness.

• Tables were reconstructed from slide data and summarized for academic use.