Mendelian Genetics and Population Genetics: Principles and Applications

Mendelian Genetics

Principles of Inheritance

Mendelian genetics explains how traits are passed from parents to offspring through discrete units called genes. Gregor Mendel's experiments with pea plants established the foundational laws of inheritance.

• Gene: A segment of DNA that codes for a specific trait.

• Allele: Alternative forms of a gene (e.g., P for purple flowers, p for white flowers).

• 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 and Phenotype

The genotype is the genetic makeup of an organism, while the phenotype is the observable physical or physiological trait resulting from the genotype.

• 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).

Mendelian Crosses: Pea Flower Color Example

Mendel's experiments with pea plants demonstrated the inheritance of flower color through generations.

• P Generation: Homozygous purple-flowered plant (PP) crossed with homozygous white-flowered plant (pp).

• Gametes: Purple plant produces P gametes; white plant produces p gametes.

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

• F2 Generation: Crossing two F1 plants (Pp x Pp) yields offspring with genotypes PP, Pp, and pp in a 1:2:1 ratio. The phenotypic ratio is 3 purple : 1 white.

Phenotype	Genotype	Ratio
Purple	PP (homozygous)	1
Purple	Pp (heterozygous)	2
White	pp (homozygous)	1
Phenotypic Ratio		3:1
Genotypic Ratio		1:2:1

Population Genetics

Introduction to Population Genetics

Population genetics studies the distribution and changes of allele frequencies in populations, which is the basis for understanding evolution. Evolution occurs at the population level, not the individual level.

• Population: The smallest biological unit that can evolve.

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

Hardy-Weinberg Principle

The Hardy-Weinberg principle provides a mathematical model to study genetic variation in populations. It predicts genotype frequencies under ideal conditions where evolution does not occur.

• Allele Frequencies: For two alleles (A and a), let p be the frequency of A and q be the frequency of a.

• Equation:

$p + q = 1$

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

• p2: Frequency of homozygous dominant genotype (AA).

• 2pq: Frequency of heterozygous genotype (Aa).

• q2: Frequency of homozygous recessive genotype (aa).

Example: If p = 0.6 and q = 0.4, then:

$p^2 = (0.6)^2 = 0.36$

$2pq = 2(0.6)(0.4) = 0.48$

$q^2 = (0.4)^2 = 0.16$

Genotype frequencies: AA = 0.36, Aa = 0.48, aa = 0.16

Assumptions of Hardy-Weinberg Equilibrium

For a population to remain in Hardy-Weinberg equilibrium, the following conditions must be met:

• No mutation

• Random mating

• No selection

• Large population size

• No gene flow (no immigration or emigration)

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

Agents of Evolutionary Change

Five main processes can cause changes in allele frequencies, leading to evolution:

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

• Non-random Mating: Mating patterns that are not random, such as assortative (similar individuals mate) or disassortative (different individuals mate).

• Gene Flow: Movement of alleles between populations through migration.

• 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

Selection can act on phenotypes in different ways:

• 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: In pocket mice, populations living on dark rocks favor dark coloration (directional selection), while those on sand favor light coloration. In African black-bellied seed-crackers, disruptive selection favors birds with either large or small beaks, depending on seed availability. Stabilizing selection is seen in human birth weight, where intermediate weights have the highest survival rates.

Summary Table: Mechanisms of Evolutionary Change

Mechanism	Description	Effect on Population
Mutation	Random changes in DNA	Introduces new alleles
Non-random Mating	Assortative or disassortative mating	Changes genotype frequencies
Gene Flow	Migration of individuals/alleles	Homogenizes populations
Genetic Drift	Random changes, especially in small populations	Loss or fixation of alleles
Natural Selection	Differential survival/reproduction	Increases frequency of advantageous alleles

Key Takeaways

• Genetic variation is essential for evolution.

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

• Five main mechanisms drive evolution: mutation, non-random mating, gene flow, genetic drift, and natural selection.

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