lec 22

Key Terminology

  • Anisogamy

  • Hermaphroditic

  • Heteromorphic/Heterogametic

  • Aneuploidy

  • Nondisjunction

Learning Objectives

  • LO44: Use phylogeny to illustrate biodiversity in sex development; XY systems evolved independently.

  • LO45: Explain sex development patterns via sex-chromosome aneuploidy phenotypes in humans, comparing them with Drosophila and reptiles.

Genetic Mapping

  • Trihybrid crosses help order genes.

  • Interference reduces observed double crossover (DCO) gametes.

  • Coefficient of coincidence (C) = Observed DCO / Expected DCO.

  • Accurate mapping requires markers spaced < 10 cM apart.

Recombination & Sexual Reproduction

  • Recombination is key in sexual reproduction; it facilitates genetic diversity.

  • Asexual lineages often lead to extinction.

  • Facultative sexual reproduction occurs under stress (e.g. in Chlamydomonas).

Types of Sexual Reproduction

  • Anisogamy: Involves megagametes (eggs) and microgametes (sperms).

  • Hermaphrodites: Organisms producing both gamete types (e.g., Caenorhabditis elegans).

  • Sequential hermaphroditism: Size and social environment can influence sex change.

Diversity in Sex Development

  • Heteromorphic XY: Males with X and Y (e.g., mammals).

  • XO and ZW systems exist in other species (e.g., some insects, birds).

  • Temperature or environmental factors influence sex determination in some species.

Human Sex Development

  • Sex determined by Y chromosome and SRY gene (encodes testis-promoting factor).

  • Turner Syndrome: 45,X; female traits but usually sterile.

  • Klinefelter Syndrome: 47,XXY; male traits with sterility.

Y Chromosome Dynamics

  • Y chromosome structure is unique, degenerating over time.

  • No recombination leads to loss of genes and accumulation of unique Y-linked genes.

  • Differences between species (e.g. humans vs chimpanzees) reflected in Y chromosome evolution.