BOW MOD 4

Chapter 4: Sex Determination

• An understanding of sex determination could have averted significant historical events, such as the divorce of Catherine of Aragon and the execution of Anne Boleyn, who were blamed for not producing male heirs for King Henry VIII.

Chromosomal Sex Determination in Humans

• Humans utilize a chromosomal sex determination system.

  • 22 pairs of autosomes shared by both sexes.

  • Sex chromosomes are either XX (female) or XY (male).

  • Total of 23 pairs of chromosomes = 46 in normal diploid cells.

• During reproduction:

  • A woman contributes 22 autosomes + 1 X chromosome.

  • A man contributes 22 autosomes + either 1 X or 1 Y chromosome, which determines the offspring's sex.

Pseudoautosomal Regions (PAR)

• The pseudoautosomal regions are located on the X and Y chromosomes.

  • Regions include PAR1 and PAR2.

  • The SRY gene located on the Y chromosome plays a crucial role in sex determination.

Pathways Without SRY

• The presence of the SRY region leads to male development; its absence leads to female development:

  • Without SRY: XX, XO (Turner syndrome), and XXX (Trisomy X) result in females.

  • With SRY: XY result in males, as well as XXY and XYY (intersex conditions).

• Y0 is nonviable; the X chromosome is essential for individual survival.

Mammalian Sex Determination

• Sex differentiation is initiated by the Y-linked SRY gene affecting gonad development:

  • SRY absent: Ovaries and female reproductive organs develop (Müllerian duct).

  • SRY present: Testes and male reproductive organs develop (Wolffian duct).

Variations in Sex Determination

• Different organisms exhibit various mechanisms:

  • Birds: WZ system (ZZ for males, WZ for females).

  • Hymenoptera (Bees and Wasps): Haplodiploid system where males are haploid (from unfertilized eggs) and females are diploid.

Sex Determination in Drosophila (Fruit Flies)

• Uses an X/autosome ratio (X/A) for sex differentiation:

  • Males (X/A ratio of 0.5) and females (ratio of 1.0).

  • Females can be XX or XXY, while males can be XY, XYY, or XO.

Environmental Influences on Sex Determination

• Extreme sexual dimorphism seen in some invertebrates involves environmental factors:

  • Example: Bonellia viridis (marine annelid) where larval settlement determines sex based on the substrate: settling on sediment results in female, while attaching to a female results in male development.

Temperature-Dependent Sex Determination (TSD)

• Common in reptiles; temperatures during incubation influence sex:

  • Common in crocodilians, some turtles, and lizards.

  • Nutritional and social factors may also affect sex determination.

Overview of Mammals (Class Mammalia)

• Characteristics include:

  • Milk production for young, hair or fur, a neocortex in the brain, and three bones in the middle ear.

• Divided into three groups based on incubation methods:

  • Monotremes: Lay eggs (e.g., platypus).

  • Marsupials: Young born immature and nursed in a pouch (e.g., kangaroo).

  • Eutherians: Placental mammals.

Mammalian Sex Chromosomes

• Most therian mammals (marsupials + eutherians) have a single pair of sex chromosomes (XX for females, XY for males).

  • Monotremes: Have multiple pairs of sex chromosomes.

  • Genetic variations include sequences on sex chromosomes that may share homology with autosomes.

Genome and Sex Chromosomes

• X chromosome is larger and contains around 3,000 genes, while the Y chromosome has about 12 genes.

  • Males inherit only one X chromosome and are affected directly by X-linked mutations.

Examples of X-Linked Inheritance

• Colorblindness: Affects about 4% of males, caused by mutations in opsin genes.

• Hemophilia: Mutation in factor VIII gene results in nonfunctional clotting.

• Duchenne Muscular Dystrophy (DMD): Progressive muscle-wasting disease due to absence of the dystrophin protein.

Modified Sex Ratios

• Male and female survival rates differ:

  • Higher conception ratios for males.

  • Survival through different life stages is lower for males leading to skewed sex ratios as age increases.

Key Takeaways

• Various mechanisms of sex determination are observed across species, evolving in diverse ways and relatively recently.

• In eutherian mammals, the XY system is prevalent, and the presence of the SRY gene on the Y chromosome alters development.

• Males, having a single X chromosome, are vulnerable to deleterious mutations.