Chapter 4 - Sex Determination_V2

Chapter 4: Sex Determination and Sex Chromosomes

Introduction

• Course: BIO208: Genetics - W25

• Instructor: Marina Rousseau, PhD

• Contact: mroussea@ubishops.ca

Overview of Sexual Reproduction and Genetics

• Sexual reproduction enhances genetic diversity.

• Sexual differentiation: Mechanism by which individuals develop as males (XY) or females (XX).

Factors in Sex Differentiation

1. Sex Chromosomes

2. Number of Sets of Chromosomes

3. Environment

1. Sex Chromosomes

• X-Y System:

  • Heterogametic sex (XY): produces two types of sperm, one with X and one with Y.

  • Homogametic sex (XX): produces eggs containing X chromosomes.

  • Human genome comprises 23 pairs, including one pair of sex chromosomes.

  • Male sex determination is linked to the Y chromosome, specifically the Sry gene.

• X-0 System (Insects):

  • Males have one X chromosome (X0) and females have two (XX).

  • Determination based on the ratio of sex chromosomes to autosomes.

• Z-W System (Some Birds and Fish):

  • Males are ZZ; females are ZW.

  • The homogametic sex here is male.

2. Number of Sets of Chromosomes

• Haplodiploid System (Bees, Wasps, Ants):

  • Males (haploid) develop from unfertilized eggs, while females (diploid) develop from fertilized eggs.

  • Only female honeybees result from sexual reproduction.

3. Environment

• Temperature Influence:

  • Example: Alligators and turtles' sex determination via incubation temperature (below 33°C = female; ≥33°C = male).

• Behavior Influence:

  • Example: Clownfish are protandrous hermaphrodites that can switch from male to female.

4. Dosage Compensation and X-Chromosome Inactivation in Mammals

• The X chromosome has important genes influencing protein interactions with autosomal genes.

• Dosage Compensation: Ensuring similar gene expression levels from X chromosomes in both sexes.

Dosage Compensation Mechanisms

1. Increased expression in heterogametic mammals (e.g., XY).

• Example: Drosophila males show two-fold X gene expression.

2. Decreased expression in homogametic mammals (e.g., XX).

• X-inactivation, where one X chromosome is inactivated in females to balance gene expression.

X-Chromosome Inactivation

• Mary Lyon's Hypothesis (1961): Inactivation occurs randomly in early development and is maintained.

• Cytology Study: Identified inactivated X chromosome as a Barr Body.

Example: Calico Cats

• Fur pattern resulting from random X-inactivation in early embryos with both alleles.

X-Chromosome Inactivation in Various Chromosome Compositions

X-inactivation Center (Xic)

• Mediates X-inactivation; includes the X-inactive specific transcript (Xist) gene.

• Phases of Inactivation:

  1. Initiation: Selection of X chromosome for inactivation.

  2. Spreading: Xist gene coats the chromosome, proteins condense the chromosome into a Barr body.

  3. Maintenance: Barr body persists through cell divisions.

5. Properties of X and Y Chromosomes in Mammals

• Sex determination relies on the presence of the Y chromosome (Sry gene) in the X-Y system.

• Genes unique to X or Y chromosomes:

  • X-linked genes: Specific to the X chromosome.

  • Y-linked genes (holandric): Specific to the Y chromosome.

  • Pseudoautosomal genes: Present on both X and Y.

Example: Mic2 Gene

• Found on both X and Y, exhibiting pseudoautosomal inheritance pattern.

6. Transmission Patterns of X-linked Genes

• Inheritance Patterns:

  • Y chromosome genes passed from fathers to sons.

  • X-linked genes passed from mothers to both daughters and sons.

Understanding X-linked Inheritance

• Males are hemizygous for X-linked genes, thus more likely affected by recessive X-linked disorders.

7. Morgan's Experiment

• Confirmed the specificity of genes to sex chromosomes using Drosophila melanogaster (fruit flies).

• Introduced a mutation (white eyes) via X-ray.

Results from Morgan's Experiment:

• Cross: White-eyed male with red-eyed female, resulting in red-eyed F1 generation.

• F2 Generation: 2459 red-eyed females, 1011 red-eyed males, 0 white-eyed females, 782 white-eyed males.

Conclusion and Applications

• Test Crosses and Reciprocal Crosses are vital techniques to determine X-linked gene inheritance.

• Example diseases include Duchenne Muscular Dystrophy and Hemophilia A, showcasing sex-linked inheritance patterns and implications in royal families (e.g., the British royal family).

• Understanding inbreeding and its effects on health within populations is pivotal, illustrated by historical figures.