Extensions of Mendelian Genetics: Chapter 4 Study Notes

Extensions of Mendelian Genetics

Chapter Overview

This chapter explores how genetic inheritance can deviate from the classic Mendelian patterns. It covers the effects of alleles at a single gene, interactions between multiple genes, molecular pathways, complementation analysis, and sex-linked inheritance, providing a comprehensive understanding of complex genetic traits.

Single Gene Inheritance and Mendelian Principles

Complete Dominance and Punnett Squares

In classic Mendelian genetics, traits are determined by alleles at a single gene locus, with one allele being dominant over the other. Punnett Squares are used to predict the possible combinations of alleles in offspring.

• Complete Dominance: The phenotype of the heterozygote is identical to that of the dominant homozygote.

• Punnett Square: A grid that shows all possible combinations of gametes and resulting offspring genotypes.

• Mendel's Law of Segregation: Each parent contributes one allele for each gene to their offspring.

Example: Eye color inheritance using Punnett Squares to determine the likelihood of a brown-eyed child.

Molecular Basis of Dominance

Enzyme Activity and Phenotype

The molecular basis of dominance often involves the production of a functional enzyme. In pea plants, a gene controls petal color by converting a precursor into pigment.

• Functional Enzyme: Presence of any functional enzyme leads to pigment production (dominant phenotype).

• Genotypes:

  • CC or Cc: Active enzyme, purple pigment.

  • cc: No active enzyme, white petals.

Additional info: Dominance is often due to haplosufficiency, where one functional allele produces enough enzyme for the dominant phenotype.

Incomplete Dominance

Intermediate Phenotypes

Incomplete dominance occurs when the heterozygote displays a phenotype intermediate between the two homozygotes.

• Definition: Neither allele is completely dominant; the heterozygote has a unique phenotype.

• Example: Flower color in four-o’clocks (Mirabilis jalapa):

  • Red (RR) x White (rr) → Pink (Rr) in F1

  • F1 (Rr) x F1 (Rr) → 1/4 Red (RR), 1/2 Pink (Rr), 1/4 White (rr)

Codominance and Multiple Alleles

ABO Blood Groups

Codominance occurs when both alleles in a heterozygote are fully expressed. The ABO blood group system is a classic example involving three alleles: IA, IB, and i.

• Codominance: Both IA and IB are expressed in AB individuals.

• Multiple Alleles: More than two alleles exist for a gene in the population.

Antigen	Phenotype
A	A
B	B
A, B	AB
Neither	O

Additional info: Six genotypes are possible: IAIA, IAi, IBIB, IBi, IAIB, ii.

Lethality and Pleiotropy

Lethal Alleles

Some alleles can cause death when present in certain genotypes, affecting observed ratios.

• Example: Manx cats have a tailless phenotype due to a lethal allele. Homozygotes for the lethal allele die as embryos.

Pleiotropy: A single gene affects multiple traits.

Polygenic Inheritance and Multiple Genes

Independent Assortment and Dihybrid Crosses

Many traits are influenced by multiple genes. Mendel’s dihybrid crosses revealed the principle of independent assortment.

• Independent Assortment: Genes on different chromosomes segregate independently during meiosis.

• Dihybrid Ratio: The classic F2 ratio for two independent genes is 9:3:3:1.

Example: Pea plant traits such as flower color and seed shape.

Gene Interactions and Modified Ratios

Epistasis and Pathway Interactions

Genes can interact in pathways, modifying expected Mendelian ratios.

• Epistasis: One gene masks or modifies the effect of another gene.

• Example: Skin coloration in corn snakes is determined by two genes, each affecting a different pigment pathway.

Modified Ratios: Gene interactions can produce ratios such as 9:3:4, 15:1, or 12:3:1 in the F2 generation.

Type of Gene Interaction	Modified Ratio
Complementary gene action	9:7
Recessive epistasis	9:3:4
Duplicate gene action	15:1
Dominant epistasis	12:3:1

Additional info: These ratios arise from specific gene interactions in metabolic pathways.

Complementation Analysis

Distinguishing Mutations in Different Genes

Complementation analysis helps determine whether mutations causing similar phenotypes are in the same or different genes.

• Definition: Production of a wild-type phenotype when two recessive mutations are combined in a diploid organism.

• Application: Used to identify whether mutants are defective in the same gene or in different genes.

Example: Crossing different white-flowered mutants to determine if they complement (indicating mutations in different genes).

Sex-Linked Inheritance

X-Linked Traits and Pedigree Analysis

Genes located on sex chromosomes exhibit unique inheritance patterns. X-linked traits show different transmission in males and females.

• Females: XX; Males: XY

• X-Linked Dominant: Affected males pass the trait to all daughters but not sons; affected females can pass the trait to both sons and daughters.

• X-Linked Recessive: More common in males; affected males cannot pass the trait to sons but can pass the allele to daughters.

Examples: Color blindness, hemophilia, hypophosphatemia.

Trait	Inheritance Pattern	Pedigree Features
X-linked dominant	Trait in every generation; affected fathers pass to all daughters	Both sexes affected, but more females
X-linked recessive	Skips generations; more males affected	Affected males from carrier mothers

Additional info: Reciprocal crosses produce different results for X-linked traits due to sex chromosome composition.

Meiosis and Chromosome Behavior

Basis of Genetic Inheritance

Understanding chromosome behavior during meiosis is fundamental to genetics. Meiosis ensures the segregation and independent assortment of alleles.

• Meiosis I: Homologous chromosomes separate.

• Meiosis II: Sister chromatids separate.

• Result: Four haploid cells, each with a unique combination of alleles.

Additional info: Review of mitosis and meiosis is recommended for understanding genetic principles.