Extensions of Basic Principles

Introduction

This chapter explores how classical Mendelian inheritance is extended by more complex genetic phenomena, including incomplete dominance, codominance, gene interactions, epistasis, and non-Mendelian inheritance patterns. These concepts help explain the diversity of phenotypes observed in natural populations.

Degrees of Dominance

Complete Dominance

• Definition: The phenotype of the heterozygote is the same as one of the homozygotes.

• Example: Red and white flower color, where red is dominant.

Incomplete Dominance

• Definition: The heterozygous phenotype is intermediate between the two homozygotes.

• Example: Crossing red and white flowers produces pink flowers in the F1 generation.

Codominance

• Definition: The heterozygote expresses both alleles distinctly; both phenotypes are visible.

• Example: Human ABO blood group, where both A and B antigens are expressed in AB individuals.

Comparison Table

Penetrance and Expressivity

Penetrance

• Definition: The proportion of individuals with a specific genotype who display the expected phenotype.

• Incomplete Penetrance: Some individuals with the genotype do not show the phenotype.

• Formula:

Expressivity

• Definition: The degree to which a trait is expressed among individuals with the same genotype.

• Example: Severity of a genetic disorder may vary among individuals with the same mutation.

Lethal Alleles

Definition and Effects

• Lethal alleles: Cause death when present in a homozygous state, altering expected Mendelian ratios.

• Example: In mice, the yellow allele () is lethal when homozygous (), resulting in a 2:1 ratio of yellow to nonyellow offspring.

Multiple Alleles

Population Variation

• Definition: More than two alleles exist for a locus in a population, though individuals carry only two.

• Example: Duck coat color has three alleles (, , ) with a dominance hierarchy: .

Human ABO Blood Group Table

Gene Interaction

Definition

• Gene interaction: Multiple loci influence a single phenotype, often through metabolic pathways.

• Example: Fruit color in peppers is determined by two loci ( and ).

Epistasis

• Definition: One gene masks the effect of another gene at a different locus.

• Types:

  • Recessive epistasis: Example: Labrador coat color ( is epistatic to when homozygous).

  • Dominant epistasis: Example: Squash fruit color ( allele inhibits pigment production).

  • Duplicate recessive epistasis: Example: Snail shell pigmentation (both loci must have dominant alleles for pigment).

Modified Dihybrid Ratios Table

Complementation Test

Purpose and Method

• Definition: Determines if mutations producing similar phenotypes are at the same or different loci.

• Method: Cross homozygous mutant parents.

  • If F1 is wild-type, mutations are at different loci (complementation).

  • If F1 is mutant, mutations are at the same locus (no complementation).

Sex-Influenced and Sex-Limited Genes

Sex-Influenced Genes

• Definition: Autosomal genes expressed differently depending on sex.

• Example: Male pattern baldness, influenced by autosomal allele and X-linked testosterone receptor.

Sex-Limited Genes

• Definition: Genes expressed only in one sex.

Cytoplasmic Inheritance

Definition and Examples

• Definition: Traits determined by genes in mitochondria or chloroplasts, typically inherited maternally.

• Example: Leaf color in four-o'clock plants, determined by the phenotype of the maternal branch.

Genetic Maternal Effect

Definition

• Definition: Offspring phenotype is determined by the mother's genotype, often due to cytoplasmic factors (e.g., mRNA).

• Example: Shell coiling direction in snails.

Genomic Imprinting

Definition

• Definition: Epigenetic phenomenon where gene expression depends on the parent of origin due to DNA methylation or other modifications.

• Result: Only one functional copy of a gene is expressed.

Pleiotropy and Polygenic Traits

Pleiotropy

• Definition: A single gene influences multiple traits.

Polygenic Traits

• Definition: A single trait is influenced by multiple genes.

• Genotype Calculation:

  • 1 locus, 2 alleles: 3 genotypes

  • 2 loci, 2 alleles: genotypes

  • 8 loci, 2 alleles: genotypes

Practice Problems

Example 1: Corn Kernel Color

• Cross between homozygous yellow and purple corn; F1 intercrossed yields 119 purple and 89 yellow kernels.

• Question: What is the genotype of the yellow kernels?

Example 2: Squash Fruit Shape

• Disc-shaped crossed with long fruit; F2 yields 430 disc, 287 spherical, 46 long.

• Task: Assign genotypes and use chi-squared test for fit.

Example 3: Labrador Retriever Coat Color

• Yellow male × brown female yields all black puppies.

• Explanation: Use epistasis and genotype analysis to explain result.

Additional info:

• Tables and diagrams have been expanded for clarity and completeness.

• Practice problems are included to reinforce understanding of genetic principles.