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. 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