Sex-linked Genes (Key)
Genes located on the X or Y
chromosomes
Many sex-linked genes are found
on the X chromosome.
The human Y chromosome is much
smaller and appears to contain only
few genes.
 Colorblindness
Colorblindness is the inability to
Distinguish certain colors.
Three human genes associated with
colorvision are located on the
X chromosomes.
Red-green colorblindness is the most
common form.
In the US, 1 in 10 males and
1 in 100 females have colorblindness.
Why the difference?
Males have just one X chromosome.
Therefore, their genotypes will be XCY or XcY.
Females have two X chromosomes. Therefore, their genotypes
would be XCXC (normal), XCXc (carrier), and XcXc is colorblind.
 Hemophilia
A sex-linked disorder in which a
protein for normal blood clotting
is
missing.
About 1 in 10,000 males has a
form of hemophilia.
Hemophiliacs can bleed to death
from minor cuts; may suffer
internal bleeding from bumps or bruises.
 Duchenne Muscular Dystrophy
A sex-linked disorder that results in the progressive weakening and
loss of skeletal muscle.
1 in 3000 males (US) has the condition.
Testcross – A Way to Determine Certain Genotypes
A testcross is a genetic cross of an organism showing the recessive
phenotype with an organism showing the dominant phenotype.
This is done to determine if the organism with the dominant
phenotype is homozygous dominant or heterozygous dominant.
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