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Phenotypic Expression:
Gene expression often governed by genotype and environment
*Penetrance
Example:
if 9/10 of individuals carrying an allele express the
trait, the trait is said to be 90% penetrant
*Expressivity
*Temperature
*Onset of genetic expression
Chapter 5: Sex Determination
Life cycles and reproductive modes
*Asexual reproduction
*Sexual reproduction
*Alternation of generations
Some additional terms:
*Primary sexual differentiation
*Secondary sexual differentiation
*Unisexual, dioecious and gonochoric
*Bisexual, monoecious and hermaphroditic
Chlamydomonas
Maize (Zea mays)
Mutants of Maize
silkless (sk)
tassel seed (ts)
barren stalk (ba)
Caenorhabditis elegans (C. elegans)
Review: Sexual Differentiation
Life cycles and reproductive modes
*Asexual reproduction – C.elegans hermaphrodite
vegetative Chlamydomonas
*Sexual reproduction – Chlamydomonas
C. elegans
Maize
Humans
*Alternation in cycles - Chlamydomonas
Maize
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
The Y Chromosome
SKALETSKY, et al. (2003) The male-specific region of the human
Y chromosome is a mosaic of discrete sequence classes. Nature
423, 825 - 837