Genetics and Population Genetics: Mendelian Inheritance, Hardy-Weinberg Principle, and Evolutionary Mechanisms

Genetics and Mendelian Inheritance

Genetic Vocabulary

Understanding the basic terminology of genetics is essential for studying inheritance and population genetics.

• Homozygous: An organism with two identical alleles for a gene (e.g., PP or pp).

• Heterozygous: An organism with two different alleles for a gene (e.g., Pp).

• Genotype: The genetic information that constitutes a trait (e.g., PP, Pp, pp).

• Phenotype: The physical appearance resulting from the expression of the genotype (e.g., purple or white flowers).

• Allele: An alternative form of a gene.

Mendelian Inheritance: Monohybrid Cross

Mendel's experiments with pea plants demonstrated the principles of inheritance using crosses between plants with different traits.

• P Generation: Parental generation; homozygous purple-flowered (PP) crossed with homozygous white-flowered (pp).

• F1 Generation: All offspring are heterozygous (Pp) and display the dominant phenotype (purple flowers).

• F2 Generation: Crossing two Pp individuals yields a phenotypic ratio of 3:1 (purple:white) and a genotypic ratio of 1:2:1 (PP:Pp:pp).

Phenotype	Genotype	Count
Purple	PP (homozygous)	1
Purple	Pp (heterozygous)	2
White	pp (homozygous)	1

Ratio: Phenotype 3:1, Genotype 1:2:1

Population Genetics

Introduction to Population Genetics

Population genetics studies the distribution and change of allele frequencies under the influence of evolutionary processes.

• Population: The smallest biological unit that can evolve.

• Microevolution: Changes in allele frequency over time within a population.

• Genetic variation: The raw material for selection and evolution.

The Hardy-Weinberg Principle

The Hardy-Weinberg Principle provides a mathematical model to study genetic variation in a population under ideal conditions.

• For two alleles (A and a): let p = frequency of A, q = frequency of a.

• The sum of allele frequencies: $p + q = 1$

• Genotype frequencies: $p^2 + 2pq + q^2 = 1$

• Where $p^2$ = frequency of AA, $2pq$ = frequency of Aa, $q^2$ = frequency of aa.

Allele from Dad	Allele from Mom	Joint Probability	Genotype
A	A	p × p = p2	AA
A	a	p × q	Aa
a	A	q × p	Aa
a	a	q × q = q2	aa

Example: If $p = 0.6$, then $q = 0.4$. Genotype frequencies: $p^2 = 0.36$ (AA), $2pq = 0.48$ (Aa), $q^2 = 0.16$ (aa).

Assumptions of Hardy-Weinberg Equilibrium

Genotype proportions remain constant if the following conditions are met:

1. No mutation

2. Random mating

3. No selection

4. Very large population size

5. No gene flow (no immigration/emigration)

If any of these assumptions are violated, evolution can occur.

Mechanisms of Evolutionary Change

Agents of Evolutionary Change

• Mutation: Random changes in DNA that introduce new alleles.

• Non-random mating: Mating that is not random with respect to genotype or phenotype (e.g., assortative or disassortative mating).

• Gene flow: Movement of alleles between populations, which can homogenize genetic differences.

• Genetic drift: Random changes in allele frequencies, especially significant in small populations.

• Natural selection: Differential survival and reproduction of individuals with advantageous traits.

Types of Selection

• Directional selection: Favors individuals at one extreme of the phenotypic range.

• Disruptive selection: Favors individuals at both extremes of the phenotypic range.

• Stabilizing selection: Favors intermediate variants and acts against extreme phenotypes.

Example: Directional selection in pocket mice (dark vs. light fur), disruptive selection in seed-cracker birds (different beak sizes), and stabilizing selection in human birth weight.

Genetic Drift and the Bottleneck Effect

Genetic drift can lead to significant changes in small populations due to random chance. The bottleneck effect is a sharp reduction in population size, often due to environmental events, leading to loss of genetic diversity.

Summary Table: Hardy-Weinberg vs. Evolutionary Mechanisms

H-W Assumptions	Mechanisms of Evolution
Mating is random	Sexual selection
Population is very large	Genetic drift
No immigration/emigration	Gene flow
No mutation	Mutation occurs
Equal fitness	Natural selection

Key Takeaways

• Genetic variation is essential for evolution.

• The Hardy-Weinberg principle provides a null model for detecting evolutionary change.

• Five main processes can cause evolution: mutation, non-random mating, gene flow, genetic drift, and natural selection.

• Selection can be directional, disruptive, or stabilizing, each affecting population traits differently.