NOTES 21 - Sex-Linked NOTES 21 - Sex-Linked InheritanceInheritance

In humans, sex is determined by the 23In humans, sex is determined by the 23rdrd chromosome pair chromosome pair

How is sex determined in How is sex determined in humans?humans?

Father’s 23Father’s 23rdrd pair is an X and a Y pair is an X and a Y Mother’s 23Mother’s 23rdrd pair is an X and an X pair is an X and an X Humans egg cells always have an X Humans egg cells always have an X

chromosomechromosome Human sperm cells have either an X or a Y Human sperm cells have either an X or a Y

chromosomechromosome If a sperm with an X fertilizes the egg, the If a sperm with an X fertilizes the egg, the

2323rdrd pair will be XX and it will be a female pair will be XX and it will be a female If a sperm with a Y fertilizes the egg, the If a sperm with a Y fertilizes the egg, the

2323rdrd pair will be XY and it will be a male pair will be XY and it will be a male

Sex-linked traitsSex-linked traits – Traits carried on the X – Traits carried on the X chromosome (23chromosome (23rdrd pair in humans) pair in humans)

The X chromosome has many genes on itThe X chromosome has many genes on it The Y chromosome has very few genes on itThe Y chromosome has very few genes on it

Sex ChromosomesSex Chromosomes

Sex-linked traits are inherited differently in men and Sex-linked traits are inherited differently in men and womenwomen

Females have two X chromosomes, so they Females have two X chromosomes, so they have have 2 genes2 genes for all traits carried on the 23 for all traits carried on the 23rdrd pairpair

Males have one X chromosome, so they only Males have one X chromosome, so they only have have 1 gene1 gene for all traits carried on the 23 for all traits carried on the 23rdrd pair pair

Inheritance of a Sex-Linked Inheritance of a Sex-Linked TraitTrait

Whether or not recessive sex-linked traits Whether or not recessive sex-linked traits get passed on depends on the sex of the get passed on depends on the sex of the parent and the sex of the offspringparent and the sex of the offspring

Males pass their X chromosome (and its genes) Males pass their X chromosome (and its genes) to all of their daughters and none of their sonsto all of their daughters and none of their sons

Females pass one of their X chromosomes (and Females pass one of their X chromosomes (and its genes) to all of their children its genes) to all of their children

Fathers cannot pass sex-linked traits to their Fathers cannot pass sex-linked traits to their sons, and always pass them to their daughterssons, and always pass them to their daughters

Males only inherit sex-linked traits from their Males only inherit sex-linked traits from their mothers, so if a son receives an X chromosome mothers, so if a son receives an X chromosome with a recessive allele from his mother, he with a recessive allele from his mother, he automatically will have the recessive traitautomatically will have the recessive trait

CarrierCarrier – a female with a recessive allele on – a female with a recessive allele on one of her X chromosomesone of her X chromosomes

Ex. Color blindness is Ex. Color blindness is caused by a recessive caused by a recessive allele on the X allele on the X chromosomechromosome

Ex. Color blindness is caused by a recessive Ex. Color blindness is caused by a recessive allele on the X chromosomeallele on the X chromosome

XXCC = normal vision = normal vision XXcc = color blind = color blind

XXCCXXC C = = normal femalenormal female XXCCXXcc == normal female carriernormal female carrier XXccXXcc = = color blind femalecolor blind female XXCCYY == normal malenormal male XXccYY = = color blind malecolor blind male