Extensions of Mendelian Principles: Gene Interaction, Dominance, and Complex Inheritance

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

Type	Heterozygote Phenotype
Dominance	Same as one homozygote
Incomplete Dominance	Intermediate between homozygotes
Codominance	Includes both homozygote phenotypes

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: $ext{Penetrance} = rac{ ext{Number showing phenotype}}{ ext{Total with genotype}}$

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 ($Y$) is lethal when homozygous ($YY$), 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 ($M^R$, $M$, $m^d$) with a dominance hierarchy: $M^R > M > m^d$.

Human ABO Blood Group Table

Phenotype	Genotype	Antigen Type	Antibodies Made
A	IAIA or IAi	A	B
B	IBIB or IBi	B	A
AB	IAIB	A and B	None
O	ii	None	A and B

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 ($Y$ and $C$).

Epistasis

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

• Types:

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

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

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

Modified Dihybrid Ratios Table

Ratio	Genotype	Type of Interaction	Example
9:3:3:1	A_B_, A_bb, aaB_, aabb	None	Seed shape/color in peas
9:3:4	A_B_, A_bb, aaB_+aabb	Recessive epistasis	Coat color in Labradors
12:3:1	A_B_+A_bb, aaB_, aabb	Dominant epistasis	Color in squash
9:7	A_B_, A_bb+aaB_+aabb	Duplicate recessive epistasis	Albinism in snails

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: $3^2 = 9$ genotypes

  • 8 loci, 2 alleles: $3^8 = 6561$ 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.