Sex Determination and Sex-Linked Characteristics

Introduction to Sex Determination

  • Many mechanisms involved in determining sex.

  • Inheritance of characters encoded by genes on sex chromosomes is different from autosomal inheritance.

Basics of Chromosomes

Sex Chromosomes

  • Definition: Chromosome whose presence/absence correlates with sex; plays a role in sex determination.

Autosomes

  • Definition: Any chromosome that is not a sex chromosome.

Mechanisms of Sex Determination

Chromosomal Sex Determination

  • Genes on specific chromosomes (sex chromosomes) determine sex.

Environmental Sex Determination

  • Environmental factors heavily influence sex determination through interaction with the genome.

Chromosomal Systems of Sex Determination

  • XX/XO

  • XX/XY

  • ZZ/ZW

Heterogametic and Homogametic Sexes

Definitions

  • Heterogametic sex: Produces two different types of gametes regarding sex chromosomes (e.g., XO, XY, ZW).

  • Homogametic sex: Produces one type of gamete regarding sex chromosomes (e.g., XX, ZZ).

Human Sex Determination

  • Males are the heterogametic sex (XY).

XX/XO System of Sex Determination

Historical Observations

  • Hermann Henking identified the X-body in male firebugs in 1891.

  • Nettie Stevens and E.B. Wilson showed in 1905 that the X-body was a chromosome determining sex.

Mechanism

  • Female: XX; Male: XO, where O indicates no X chromosome.

  • Female gametes carry one X chromosome; male gametes carry either an X or none.

  • Fusion of two X-chromosome bearing gametes produces females; males result from a female gamete fusion with a male gamete lacking an X chromosome.

XX/XY System of Sex Determination

  • Observed in mammals, including humans.

  • Males: XY; Females: XX. Males produce X and Y bearing gametes; females produce only X bearing gametes.

  • Males inherit X from mother, Y from father; females inherit X from both parents.

  • The Y chromosomes are always paternal. Must pair with X at meiosis.

Non-Disjunction Events

  • Can result in aneuploidy offspring:

    • XXX: Poly-X females, usually fertile but can have normal intelligence; higher chromosomal counts lead to more severe issues.

    • XXY: Klinefelter Syndrome, tends to have undeveloped testes and normal intelligence.

    • XO: Turner Syndrome, combination leads to normal intelligence, short stature, and sterility.

    • XYY: Above-average males, typically fertile, but some display personality disorders.

Summary of Human Sex Chromosomes

  • Essential X chromosome for development; female fertility requires two X chromosomes.

  • Y chromosome is male determining, producing TDF (Testis-Determining Factor).

The Human Y Chromosome

  • Contains SRY (sex-determining region Y) associated with TDF.

  • Comprises 25-30 protein-coding genes; hemizygous nature of Y means only one copy present.

Androgen Insensitivity Syndrome

  • XY chromosome structure resulting in phenotypic females due to receptor insensitivity to testosterone, discovered at puberty due to absence of menstruation.

X-Chromosome Inactivation Mechanism

  • One X chromosome in female mammals becomes inactivated.

  • Barr bodies arise from this inactivation - equal gene expression in both sexes is achieved.

Dosage Compensation in Mammals

  • Allows for better tolerance of extra X chromosomes compared to autosomal aneuploidies.

  • Most genes are expressed similarly in both sexes through inactivation. In Drosophila, no inactivation occurs; instead, male gene expression is doubled.

Drosophila Sex Determination

  • Sex determined by X chromosome count, regardless of Y presence.

Inheritance Patterns

  • Males inherit X from mother, Y from father; females inherit X from both.

  • X-linked traits show different inheritance patterns such as red-green color blindness, more common in males due to hemizygosity.

ZZ/ZW System of Sex Determination

  • In birds, snakes, etc., males are ZZ and females are ZW.

  • Z chromosome larger and with more genes than W; inheritance patterns differ from XX/XY.

Environmental Sex Determination

  • Interaction of environmental factors and genetics (e.g., temperature influences in sea turtles).

Summary of Mechanisms

  • Multiple independent strategies for sex ratio balance.

  • Some species express dosage compensation amid sex chromosomes.

Major Concepts of Sex Determination

  • Overview of chromosomal and environmental influences.

  • Understanding of X-linked vs. Y-linked inheritance, gene expression stabilization, etc.