XY System • Most mammals and some insects use the “X” and “Y” chromosomes to determine sex • In humans, males are XY (heterogametic) while females are XX (homogametic) • In humans, a single gene (SRY) on the Y chromosome acts as a signal to set the developmental pathway towards maleness
ZW System • Birds, most fishe and some insects use the “Z” and “W” chromosomes to determine sex • The ovum determines the sex of the offspring • In birds, males are ZZ (homogametic) while females are ZW(heterogametic) • The W chromosome is thought to be essentail for female sex determination or at least contain female-determining genes.
X0 System • Crickets, grasshoppers and some insects use the number of X chromosomes to determine sex • The X0 system only has one sex chromosome – X • Males have only one X chromosome, (X0), females have two (XX) • The maternal gamete always contains an X chromosome. The sperm determines sex – containing either one X chromosome or no sex chromosomes at all.
Sex-Limited Traits• Autosomal, not found in the sex chromosomes• Genes are carried by both males and females, but only one sex would ever express them• Example: genes that influence how much milk a lactating mother produces when she’s nursing a baby• Another example: cryptorchidism (undescended testicles)
Sex-Influenced Traits• Autosomal, not found in the sex chromosomes• Genes are carried by both males and females, but there is a difference in the way the two sexes express them• Example: pattern baldness in humans. The baldness allele behaves like a dominant allele in males, while in females it behaves like a recessive allele
Sex-Linked Traits• Genes are found on the sex chromosomes• Sex-linked traits are behave differently from autosomal traits when sex chromosomes are non-homologous• May be dominant or recessive in nature• Phenotype is dependent also on the presence of a complete gene pair or just one allele• Example: Hemophilia (X-linked)