NOTES 21 - Sex-Linked Inheritance In humans, sex is determined by the 23rd chromosome pair

How is sex determined in humans? Father’s 23rd pair is an X and a Y Mother’s 23rd pair is an X and an X Humans egg cells always have an X chromosome Human sperm cells have either an X or a Y chromosome If a sperm with an X fertilizes the egg, the 23rd pair will be XX and it will be a female If a sperm with a Y fertilizes the egg, the 23rd pair will be XY and it will be a male

Sex-linked traits – Traits carried on the X chromosome (23rd pair in humans) The X chromosome has many genes on it The Y chromosome has very few genes on it

Sex Chromosomes

Sex-linked traits are inherited differently in men and women Females have two X chromosomes, so they have 2 genes for all traits carried on the 23rd pair Males have one X chromosome, so they only have 1 gene for all traits carried on the 23rd pair

Inheritance of a Sex-Linked Trait

Whether or not recessive sex-linked traits get passed on depends on the sex of the parent and the sex of the offspring Males pass their X chromosome (and its genes) to all of their daughters and none of their sons Females pass one of their X chromosomes (and its genes) to all of their children Fathers cannot pass sex-linked traits to their sons, and always pass them to their daughters Males only inherit sex-linked traits from their mothers, so if a son receives an X chromosome with a recessive allele from his mother, he automatically will have the recessive trait

Carrier – a female with a recessive allele on one of her X chromosomes Ex. Color blindness is caused by a recessive allele on the X chromosome

XCXc = normal female carrier XcXc = color blind female Ex. Color blindness is caused by a recessive allele on the X chromosome XC = normal vision Xc = color blind XCXC = normal female XCXc = normal female carrier XcXc = color blind female XCY = normal male XcY = color blind male