Download Sex-Linked Trait Notes

Polygenic
• A trait that is controlled
by the interaction
between 2 or more
genes.
– Examples: skin color,
height, hair color
– Results in a continuum
of expressed
phenotypes.
Sex-Linked Traits
• Genes that are carried on the X chromosome.
• A female has XX chromosomes.
– XNXN – homozygous dominant
– XNXn – heterozygous – called carrier
– XnXn – homozygous recessive
• A male has XY chromosomes.
– XNY – dominant
– XnY – recessive
Sex-Linked Traits
• Sex-linked traits are more common among
males than females.
• Because:
– Usually sex-linked diseases are recessive.
– Females would need 2 copies of gene to have
disease.
– Males would only need 1 copy of gene to have
disease.
Sex-Linked Diseases
• Colorblindness
– The inability to perceive differences between some
colors that most can distinguish.
• Hemophilia
– Bleeding disorder in which people lack clotting factors
and therefore cannot stop bleeding if cut or injured.
• Huntington’s Disease
– Breakdown of the brain – leads to paralysis and death.
– Only dominant sex-linked disorder in humans.
– Hasn’t been eliminated because its onset is between
the ages of 30-50.
– Can be tested for, but is controversial.
Sex-Linked Punnett Squares
• Please remember that you need a MALE and
FEMALE to create a child.
• Do NOT try to cross two females – this is a
common mistake!! 
Colorblindness is a sex-linked recessive trait in humans. Janice is
a carrier for colorblindess and is married to Jonah, who is
colorblind. What is the probability of the two having a colorblind
daughter?
XN
Xn
Xn
X N Xn
Xn X n
Y
XN Y
X nY
Janice: XNXn
Jonah: XnY
• The couple has a 50% chance of having a daughter.
• Of the 2 “daughters”, 1 will be a carrier and 1 will be colorblind.
• Therefore, there is a 50% chance that they will have a daughter
who is colorblind (1 out of 2!!).