Name:
Sex-Linked Inheritance
The study of inheritance of genes located on sex chromosomes was pioneered by T. H. Morgan and his
students at the beginning of the 20th century. Although Morgan studied fruit flies (Drosophila), the same
genetic principles apply to humans.
A. Some genes on sex chromosomes play a role in sex determination, but these chromosomes also
contain genes for other traits.
B. In humans, the term sex-linked usually refers to X-linked traits.
a. The human X-chromosome is much larger than the Y. Thus, there are more X-linked than Ylinked traits.
b. Most X-linked genes have no homologous loci on the Y chromosome.
c. Most genes on the Y chromosome no only have no X counterparts, but they encode traits
found only in males (i.e. testis-determining factor).
d. Examples of sex-linked traits in humans are color blindness and hemophilia
C. Fathers pass X-linked alleles to ONLY and ALL of their daughters.
a. Males received their X chromosome only from their mothers.
b. Fathers cannot, therefore, pass sex-linked (X) traits to their sons.
D. Mothers can pass sex-linked alleles to BOTH sons and daughters.
a. Females receive two X-chromosomes, one from each parent.
b. Mother pass on one X-chromosome (either the maternal or paternal homologue) to every
daughter and son.
E. If a sex-linked trait is due to a recessive allele, a female will express the trait only if she is
homozygous.
a. Females have two X chromosomes; therefore they can be either homozygous or
heterozygous for sex-linked alleles.
b. There are fewer females with sex-linked disorders than males, because even if they have
one recessive allele, the other dominant allele is the one that is expressed. A female that is
heterozygous for the trait can be a CARRIER, but show the recessive trait herself.
c. A carrier that mates with a normal male will pass the mutation to half her sons and half her
daughters.
d. If a carrier mates with a male who has the trait, there is a 50% chance that each child born
to them will have the trait, regardless of sex.
F. Because males have only one X-linked locus, any male receiving a mutant allele from his mother
will express the trait.
a. Far more males than females have sex-linked disorders.
Examples of Sex-Linked Inheritance:
Eye Color in Drosophila
Red eye color is normal in wild flies and dominant.
White eye color is mutant and recessive.
If we cross a homozygous red-eyed female (XRXR) with a white-eyed male (XrY), what color will the
offspring be?
Xr
Y
XR
R
XR
What color will the daughters be?______________________________
What color will the sons be?____________________________________
If we cross a homozygous white-eyed female (XrXr) with a red-eyed male (XRY)…
R
XR
Y
Xr
Xr
The daughters will be __________________________________________
The sons will be ________________________________________________
Hemophilia in Humans
Xh
Y
X
X
Xh
X
X
How many daughters
are carriers?
How many sons are
normal?
Y
How many daughters
are carriers?
How many daughters
are normal?
How many sons are
normal?
How many sons are
hemophilic?
Linked Inheritance Problem Set: (It may be VERY HELPFUL to underline or circle key words.)
1. A human female carrier who is heterozygous for the recessive, sex-linked trait causing red-green
color blindness marries a normal male. What proportion of their male offspring will have redgreen color blindness? Complete a Punnett square to help you show the evidence.
a.
b.
c.
d.
e.
100%
75%
50%
25%
0%
2. A female Drosophila of unknown genotype was crossed with a white-eyed male fly, of genotype
XrY (r = white eye allele is recessive, R = red-eye allele is dominant.) half of the male and half of
the female offspring were red-eyed, and half of the male and half of the female offspring were
white-eyed. What was the genotype of the female mother fly?
a.
b.
c.
d.
e.
XRY
XRXR
XrXr
XrY
XRXr
3. Women have sex chromosomes of XX, and men have sex chromosomes of XY. Which of a woman’s
grandparents could not be the source of any of the genes on either of her X chromosomes?
a.
b.
c.
d.
e.
Mother’s father
Father’s mother
Mother’s mother
Father’s father
Mother’s mother and mother’s father