NOTES 21 - Sex-Linked Inheritance

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

2. 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

3. 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

4. Sex Chromosomes

5. 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

6. Inheritance of a Sex-Linked Trait

7. 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

8. 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

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