Chapter 4 - Sex Determination

BIO208: Genetics - W25 Chapter 4 - Sex Determination and Sex Chromosomes

  • Instructor: Marina Rousseau, PhD

  • Contact: mroussea@ubishops.ca

Sexual Reproduction and Genetics

  • Overview of Sexual Reproduction:

    • Increases genetic diversity of a population.

    • Leads to sexual differentiation; the mechanisms that allow individuals to develop into biological males or females (XX or XY).

Factors in Sex Differentiation

  1. Sex Chromosomes

  2. Number of Sets of Chromosomes

  3. Environment

Detailed Factors in Sex Differentiation

1. Sex Chromosomes

  • X-Y System:

    • XY = heterogametic sex (produces two types of sperm: X and Y).

    • XX = homogametic sex (produces eggs with a single X chromosome).

    • Humans have 23 pairs of chromosomes (1 pair of sex chromosomes and 22 autosomes), with sex determined by sex chromosome from sperm (Y chromosome presence indicates male, influenced by Sry gene).

2. Number of Sets of Chromosomes

  • X-0 System (some insects):

    • Male has one X chromosome (X0); Female has two (XX).

    • Determined by the ratio of sex chromosomes to autosomes.

  • Z-W System (some birds and fish):

    • Male ZZ and Female ZW.

    • Similar to X-Y but with males as the homogametic sex.

3. Environment

  • Temperature Influence on Sex Determination:

    • Example: Alligator eggs; incubation below 33℃ yields females, at or above yields males.

  • Behavioral Influence:

    • Example: Clownfish can change sex from male to female; same chromosomal structure.

4.2 Dosage Compensation and X-Chromosome Inactivation in Mammals

Dosage Compensation

  • Importance:

    • Key genes on X chromosome need balance with autosomal gene levels.

  • Phenomenon:

    • Ensures expression levels are similar in both sexes.

  • Mechanisms:

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

    2. Decreased expression in homogametic mammals (e.g., human females) through X-inactivation.

X-Chromosome Inactivation

  • Mary Lyon’s 1961 Hypothesis:

    • One X chromosome in females is randomly inactivated (Barr Body).

  • Cytological studies enabled identification of inactivated chromosome.

Examples of X-Chromosome Inactivation

  • Calico Cats:

    • Fur pattern variation shows random X inactivation; resulting phenotype is patches of black and white fur due to different X chromosomes being active.

X-Chromosome Composition and Barr Bodies

Phenotype

Chromosome Composition

Number of X Chromosomes

Number of Barr Bodies

Normal Female

XX

2

1

Normal Male

XY

1

0

Turner Syndrome (female)

X0

1

0

Triple X Syndrome (female)

XXX

3

2

Klinefelter Syndrome (male)

XXY

2

1

Mechanism of X-Chromosome Inactivation

  1. Initiation:

    • Selection of X chromosome for inactivation during embryonic development.

  2. Spreading:

    • Xist gene expression on selected chromosome; coats chromosome, condenses it into a Barr Body.

  3. Maintenance:

    • Barr body remains through mitosis and cell division.

4.4 Properties of X and Y Chromosomes in Mammals

Chromosome Properties

  • Y chromosome, particularly Sry gene, is crucial for sex determination.

  • Genes Unique to Chromosomes:

    • X-linked genes (specific to X).

    • Y-linked genes (holandric, specific to Y).

    • Pseudoautosomal Genes:

    • Found on both X and Y; exhibit inheritance patterns similar to autosomal traits.

Transmission Patterns of X-Linked Genes

X-Linked Inheritance

  • Inheritance Patterns:

    • Fathers pass X to daughters only, while sons receive X only from mothers.

    • Males hemizygous for X-linked genes (single copy).

    • More likely to show recessive X-linked disorders (ex. red-green color blindness).

Morgan's Experiment

Overview

  • Conducted in early 1900s with Drosophila melanogaster.

  • Introduced mutations via X-ray, resulting in white-eyed offspring from red-eyed parental lines.

  • F1 Generation: All red-eyed flies; females heterozygous.

F2 Generation Results

  • Produced a mixture of red-eyed and white-eyed males and females in specific ratios (3:1 for red to white).

  • Confirmed it is an X-linked gene using Punnett squares.

Testing X-Linked Traits

Test Crosses

  • Cross between an individual with a dominant phenotype (red eyes) and one with a recessive phenotype (white eyes) confirms X-linked traits.

Example: Duchenne Muscular Dystrophy (DMD)

  • Inheritance Model:

    • X-linked recessive, more common in males.

    • Females act as carriers, with 50% affected sons from carrier mothers.

Inbreeding and Genetic Health

Risks and Implications

  • Increases recessive trait probabilities in populations.

  • Decreases heterozygotes and raises homozygous recessive trait occurrences.

  • Example: King Charles of Spain experienced health issues due to inbreeding, leading to lack of successors.