NOTES 21 SexLinked Inheritance In humans sex is

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

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

How is sex determined in humans? n n n Father’s 23 rd pair is

How is sex determined in humans? n n n Father’s 23 rd pair is an X and a Y Mother’s 23 rd 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 23 rd pair will be XX and it will be a female If a sperm with a Y fertilizes the egg, the 23 rd pair will be XY and it will be a male

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

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

Sex Chromosomes

Sex Chromosomes

Sex-linked traits are inherited differently in men and women n n Females have two

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

Inheritance of a Sex-Linked Trait

Inheritance of a Sex-Linked Trait

Whether or not recessive sex-linked traits get passed on depends on the sex of

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

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

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

Ex. Color blindness is caused by a recessive allele on the X chromosome XC = normal vision n n X CX C X CX c X c X CY X c. Y = = = Xc = color blind normal female carrier color blind female normal male color blind male