4. 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
5. 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.
6. 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.
8. 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)
10. 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
12. 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)