Sources of Genetic Variation and Patterns of Inheritance

Study Guide - Smart Notes

Tailored notes based on your materials, expanded with key definitions, examples, and context.

Sources of Variation During Inheritance

Overview

Genetic variation is fundamental to the process of inheritance and evolution. This variation arises from the behavior of chromosomes and genes during meiosis and fertilization. Understanding how genes are inherited, interact, and assort independently is crucial for explaining the diversity of traits observed in offspring.

Patterns of Single-Gene Inheritance

Monohybrid Crosses and Allelic Interactions

Single-gene inheritance involves one gene controlling one observable trait, such as flower color.
Each gene typically has two alleles (e.g., A or a), which may show complete dominance, incomplete dominance, or codominance.
Monohybrid crosses, pedigrees, and X-linked traits are classic examples of single-gene inheritance patterns.
Genotype combinations (e.g., AA, Aa, aa) result in different phenotypes depending on dominance relationships.

Heterozygous diploid cell with gene for flower color Genetic cross diagram for pea plant flower color inheritance

Dihybrid Crosses: Two Genes, Independent Traits

Law of Independent Assortment

In dihybrid crosses, two genes (e.g., G/g for color and W/w for shape) are considered, each controlling a different trait and located on different chromosomes.
The classic dihybrid cross (GgWw × GgWw) produces four phenotypes in a 9:3:3:1 ratio in the F2 generation.
This demonstrates Mendel’s Law of Independent Assortment: alleles of genes on different chromosomes assort independently during meiosis.

Cell with two pairs of homologous chromosomes for color and shape Dihybrid cross in peas showing 9:3:3:1 ratio

Two Genes, One Trait: No Interaction

Independent Gene Action

Sometimes, two genes both influence the same trait (e.g., body color), but each acts independently.
Each gene has two alleles (e.g., B/b and Y/y), and their effects are additive or independent.
The expression of one gene does not affect the expression of the other.

Two pairs of chromosomes for body color genes Wild type parakeet with BB YY genotype

Two Genes, One Trait: Gene Interaction (Epistasis)

Epistasis and Phenotypic Modification

In epistasis, two genes both influence a trait, but the expression of one gene depends on the presence of specific alleles of the other gene.
For example, two genes (C/c and W/w) may both affect eye color, but a recessive allele at one locus can mask the effect of the other gene.
This is known as recessive epistasis.

Two pairs of chromosomes for eye color genes with epistasis Diagram of pigment synthesis and transport in eye color epistasis

Sources of Genetic Variation During Inheritance

Independent Assortment and Recombination

Independent assortment refers to the random distribution of different chromosomes into gametes during meiosis, resulting in genetic variation among offspring.
Recombination (crossing over) is the physical exchange of DNA between homologous chromosomes during meiosis, creating new allele combinations.

Gametes in a Dihybrid Cross

Formation of Gametes and Allelic Combinations

In a dihybrid individual (e.g., GgWw), meiosis produces four types of gametes: GW, Gw, gW, and gw.
This is due to the independent segregation of each chromosome pair.

Meiosis in a dihybrid individual showing four gamete types

Chromosomal Basis for Independent Assortment (Mendel’s 2nd Law)

Random Alignment and Gamete Diversity

During Metaphase I of meiosis, homologous chromosomes align randomly at the metaphase plate.
This random alignment leads to different combinations of maternal and paternal chromosomes in gametes, supporting Mendel’s Law of Independent Assortment.
Each possible arrangement is equally likely, resulting in a 1/4 probability for each gamete type in a dihybrid cross.

Parental and Recombinant Gametes

Chromosome Shuffling and Genetic Diversity

Genes on different chromosomes can create both parental (original) and recombinant (new) gametes due to independent assortment and recombination.
For example, crossing pure breeding parents (gg ww × GG WW) produces F1 dihybrids (Gg Ww), which can generate both parental and recombinant gametes.

Mendel’s Law of Independent Assortment

Definition and Consequences

The alleles of genes on different chromosomes assort independently during meiosis.
Each gamete receives one chromosome from each pair, and the random alignment of chromosomes produces many possible allele combinations.

How Meiosis Creates Genetic Variation

Summary of Mechanisms

Movement of chromosomes into gametes during meiosis results in the shuffling of genetic material.
Recombination (crossing over) further increases genetic diversity by exchanging DNA between homologous chromosomes.