Sex Chromosomes and Sex Determination Systems

Sex Chromosomes and Sex Determination Systems

Diploid and Haploid States

• Diploid (2n): Organisms with two sets of chromosomes, one from each parent.

• Haploid (n): Organisms or cells with a single set of chromosomes.

• Examples:

  • Caenorhabditis elegans (Hermaphroditic species)

  • Humans (Homo sapiens)

  • Dioecious plants (e.g., Dioscorea gigantea)

Major Sex Determination Systems

• Chromosomal: Sex is determined by specific combinations of sex chromosomes (e.g., XX/XY, ZZ/ZW).

• Genic: Sex is determined by specific genes, not entire chromosomes.

• Environmental: Sex is determined by environmental factors (e.g., temperature in some reptiles).

Pseudoautosomal Regions

• Primary pseudoautosomal region: Region of sex chromosomes that pairs and recombines during meiosis.

• Secondary pseudoautosomal region: Additional region with similar recombination properties.

Sex Chromosome Systems

• Females: XX (Homogametic)

• Males: XY (Heterogametic)

• XX-XO System: Found in grasshoppers and some other insects. Females are XX, males are XO (only one X chromosome).

• ZZ-ZW System: Found in birds, snakes, butterflies, some amphibians, fish, and lepidoptera. Males are ZZ (homogametic), females are ZW (heterogametic).

• Haplodiploidy: Found in some social insects (e.g., bees, ants). Females are diploid, males are haploid.

Sequential Hermaphroditism

• Organisms change sex during their lifetime (e.g., some fish species).

Genic Balance System

• Sex is determined by the ratio of X chromosomes to sets of autosomes (A).

• Gene dosage: The number of copies of a gene present in a cell or nucleus.

Human Sex Chromosome Variations

• Turner Syndrome: 45, X (individuals have only one X chromosome).

• Klinefelter Syndrome: 47, XXY (males with an extra X chromosome).

• Poly-X females: 47, XXX or 48, XXXX (females with extra X chromosomes).

Sex Determining Region Y (SRY)

• The SRY gene on the Y chromosome triggers male development in humans.

Lyon's Hypothesis and Barr Bodies

• Lyon's Hypothesis: In female mammals, one of the X chromosomes is randomly inactivated in each cell, forming a Barr body.

• Barr bodies: Inactive X chromosomes visible in the nucleus of interphase cells.

• Number of Barr bodies: Equal to the number of X chromosomes minus one.

• Example: A female with genotype XXX will have two Barr bodies per cell.

Hemizygosity

• Hemizygous: Having only one allele for a gene in a diploid organism (e.g., genes on the X chromosome in human males).

Drosophila Autosomes

• Drosophila melanogaster (fruit fly) has three pairs of autosomes.