NOTES 21 - Sex-Linked Inheritance
In humans, sex is determined by the 23rd chromosome pair
How is sex determined in humans?
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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
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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
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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
Ex. Color blindness is caused by a recessive allele
on the X chromosome
XC = normal vision
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XCXC
XCXc
XcXc
XCY
XcY
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=
=
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Xc = color blind
normal female
normal female carrier
color blind female
normal male
color blind male