week 3

Chapter Overview

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

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).

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.

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.

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.

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.

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: 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.

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

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.

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 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.