week 3

Chapter Overview

• Purpose: Provide an overview of inheritance concepts, particularly those that relate to sex-linked inheritance, as exceptions to Mendelian laws.

Autosomes and Sex Chromosomes

• Animals: Possess two sets of chromosomes (diploid), with pairs of morphologically equal chromosomes.

• Chromosome Types:

  • Autosomes: Non-sex chromosomes that determine most phenotypic traits.

  • Sex Chromosomes: Determine an individual’s sex and may not have identical counterparts depending on the sex.

    • Example: Males (XY) are heterogametic; Females (XX) are homogametic.

    • ZW system in birds, where females are heterogametic (ZW) and males are homogametic (ZZ).

Mechanisms of Sexual Determination

• XY System (Humans, Mammals): Males (XY) are heterogametic, females (XX) are homogametic.

• ZW System (Birds, Reptiles, etc.): Females (ZW) are heterogametic, males (ZZ) are homogametic.

Recombination and Sex-Linked Genes

• Crossing Over in Meiosis: Minimal cross-linking occurs due to low morphological homology between sex chromosomes.

• Sex-linked genes are classified based on their location:

  • Holandric Genes: Located in the non-homologous region of the Y chromosome (XY system).

  • Hologynic Genes: Found on the X chromosome (in XY) or W chromosome (in ZW system).

  • Partial Sex-Linked Genes: Located in homologous regions, can be homozygous or heterozygous.

Sex-Linked Inheritance (SLI)

• Most sex-linked functional genes in mammals are found on the X chromosome; Y does not carry corresponding alleles.

• In males (XY), traits from X-linked genes are hemizygous, determined solely by the X chromosome.

Phenotypic Ratios in Sex-Linked Inheritance

• SLI vs. PSLI (Partially Sex-Linked Inheritance): Different inheritance patterns compared to autosomal genes.

• SLI leads to a 1:1 ratio for both males and females; PSLI results in a 1:1 for males and females with dominant traits.

Detection of Sex-Linked Inheritance

• Reciprocal Crosses: Assessing differences in offspring genotype/phenotype based on parental contributions helps detect sex-linked inheritance.

• Phenotypic ratios can identify the presence of sex-linked traits based on the differing contributions from male and female parents.

Gene Linkage Overview

• Gene linkage: Proximity of genes on the same chromosome prevents independent distribution during gamete formation.

• Example: Close genes segregate together; their recombination frequency indicates distances for constructing linkage maps.

Mendel’s Second Law

• Law of Independent Distribution: Each trait is expected to segregate independently unless genes are linked. Deviations indicate possible gene linkage.

Gene Linkage Mechanics

• Pachytene Stage during Prophase I: Where crossing over occurs, increasing genetic variability.

• Genes on the same chromosome show linkage, and their segregation patterns differ from Mendelian predictions due to proximity.

Linkage Detection Techniques

• Chi-Square Test: Determines if observed proportions deviate from expected Mendelian ratios, aiding in identifying linked genes.

• Detection of gene linkage occurs through the comparison of observed vs. expected phenotypic ratios.

Distance Between Genes

• Distance expressed in centimorgans (cM), reflecting the likelihood of crossing over occurring between linked genes.

• Backcrossing or self-pollination can help determine phenotypic ratios and linkages for the genes in study.