Phenotypic Expression: *Penetrance *Expressivity Gene expression often governed by genotype and environment Example: if 9/10 of individuals carrying an allele express the trait, the trait is said to be 90% penetrant *Onset of genetic expression *Temperature

Chapter 5: Sex Determination Life cycles and reproductive modes *Asexual reproduction *Primary sexual differentiation *Secondary sexual differentiation *Unisexual, dioecious and gonochoric *Bisexual, monoecious and hermaphroditic *Alternation of generations *Sexual reproduction Some additional terms:

Chlamydomonas

Maize (Zea mays)

Mutants of Maize tassel seed (ts) silkless (sk) barren stalk (ba)

Caenorhabditis elegans (C. elegans)

Review: Sexual Differentiation Life cycles and reproductive modes *Asexual reproduction –C.elegans hermaphrodite vegetative Chlamydomonas *Alternation in cycles -Chlamydomonas Maize *Sexual reproduction – Chlamydomonas C. elegans Maize Humans

X and Y Chromosomes: Early Studies 1906 Edmund B. Wilson and Protenor

X and Y Chromosomes: Early Studies Wilson and Lygaeus

X and Y Chromosomes: Early Studies Terms: Homogametic sex Heterogametic sex *The male is not always the heterogametic sex… Examples: Moths, butterflies, most birds, some fish, reptiles, amphibians, at least one species of plants

Chromosome Composition in Humans Karyotype XX XY

The Y Chromosome Pseudoautosomal regions (PAR) NRY Testis Determining Factor (TDF) *SRY – Sex determining region Is the Y chromosome a “genetic wasteland”? *Housekeeping genes *Testis development and function

SKALETSKY, et al. (2003) The male-specific region of the human Y chromosome is a mosaic of discrete sequence classes. Nature 423, The Y Chromosome