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 Maizetassel 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, 825 - 837

The Y Chromosome