I.
X-Linkage/Sex-Linkage
A. Humans have 46 chromosomes or 23 pairs
in their genome.
i. The 23rd pair = sex-determining
chromosomes
ii. Mother passes down “X” chromosome
iii. Father passes down an “X” or “Y”
chromosome
a) XX  Female
b) XY  Male
B. There always will be an equal chance of
being born male or female…
C. Uniqueness of the X Chromosome
i. X chromosomes carry traits, such as…
a)
Color Blindness = Xc
b)
Baldness = Xb
c)
Hemophilia = Xh
ii. When certain traits are carried only on
the “X” chromosome, we can call this XLINKAGE or SEX-LINKAGE
D. “Y” chromosomes do not carry traits and
will not carry a superscript
E. The Importance of “Y”
i. Because “Y” does not carry an allele,
males which are “XY” are more
susceptible to many X-linked traits
ii. Instead of needing 2 recessive alleles,
the male only needs a single recessive on
the “X” to gain the trait
II. Examples…
C = Normal Vision
(Both X-Linked)
c = Colorblind
A woman has normal vision but is a carrier for
colorblindness. She has children with a male
who has normal vision. What are the parent’s
genotypes?
Mom:
Dad:
% Male
% Female
% Normal vision female (both dominant)
% Carrier female
% Colorblind female
% Normal vision male
% Colorblind male
Can there be a carrier male? Why or why not?
Full Hair = B
Both X-Linked
Baldness = b
A woman has a full head of hair, but does carry
the baldness allele that was passed down from her
family. The woman has children with a bald
man. What are the parent’s genotypes?
% Hair male
% Hair female
% Carrier male
% Carrier female
% Bald male
% Bald female