SEXRELATEDINHERITANCE

HOW ISSEX (GENDER) DETERMINED?

How is Sex Determined?

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.

SEXLIMITEDTRAITS

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)

SEXINFLUENCEDTRAITS

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

SEXLINKEDTRAITS

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)

Reference

Kent KawashimaBiology 11 Addison Wesley