Genes on the X Chromosome: *Sex chromosomes *Autosomes Example: In Drosophila and all mammals sex chromosomes designated as X and Y XX=female XY=male *1909 Thomas Hunt Morgan II III IV XX XY or

X-linkage - transmission and expression of genes on the X chromosome X-linkage in Drosophila: white mutation (eyes) Modifications of Mendelian Ratios sex-linkage - transmission and expression of genes on sex chromosomes

Sex-limited Inheritance: *Sex-limited trait – trait that is expressed in only one sex even though the trait may not be X-linked. *holandric genes: genes on the Y chromosome *autosomal genes Example: milk production in mammals; L=lots, l=little Example: hypertrichosis (excess ear hair)

*Sex-influenced trait – phenotypic expression that is conditioned by the sex of the individual Sex-influenced Inheritance: Examples: *cleft palate in humans *horns in sheep *pattern baldness in humans Summary: Sex-linked: on X or Y sex-chromosome Sex-limited: all or none expression by sex Sex-influenced: genotype + sex determines phenotype

Phenotypic Expression: *Penetrance – frequency with which individuals with a given genotype exhibit some degree of a phenotype associated with that trait. *Expressivity – the degree or range in which a phenotype for a given trait is expressed Gene expression often governed by genotype and environment 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 7: Sex Determination Life cycles and reproductive modes *Asexual reproduction – formation of new individuals from the cell(s) of a single parent. *Primary sexual differentiation *Secondary sexual differentiation *Unisexual, dioecious and gonochoric *Bisexual, monoecious and hermaphroditic *Alternation of generations – alternation of forms or of mode of reproduction in the life cycle of an organism. *Sexual reproduction – reproduction through the fusion of gametes. Some additional terms:

Chlamydomonas

Caenorhabditis elegans (C. elegans)

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 Homogametic sex – sex which produces uniform gametes with respect to sex chromosomes. Heterogametic sex – sex which produces gametes with unlike sex chromosomes. *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 XXXY