Sex Linked Genes
The Xs and Ys of Genetics
Sex Linked Genes
•  There are 23 pairs of
chromosomes and
one of those pairs are
the sex
chromosomes.
•  There are either X
chromosomes or Y
chromosomes.
Sex Linked Genes
•  These
chromosomes not
only carry the
genes that
determine male
and female traits,
but also those for
some other
characteristics as
well.
•  Genes that are
carried by either
sex chromosome
(X or Y) are said
to be SEX
LINKED
Males
•  Men normally have an
X and a Y
combination of sex
chromosomes.
•  Since only men inherit
the Y chromosome,
they are the only ones
that inherit Y-linked
traits.
Females
•  Women have
2X
chromosomes
X Linked Traits
•  Men and Women
can get the Xlinked traits since
both inherit X
chromosomes.
Sex inheritance patterns for male
and female children
X Chromosome Traits
•  Men only have one X chromosome
•  Therefore genes on that chromosome
are expressed in the male phenotype,
even if it is a recessive gene.
X-linked traits
•  In women, a
recessive allele
on one x
chromosome is
often masked on
their phenotype
•  May have a
dominant normal
allele on the other
carriers of X-linked
traits but more
rarely have them
expressed in their
own phenotypes
X-linked traits
•  In humans, at least
120 genes are known
to be X-linked.
•  Genes for hemophilia,
red-green color
blindness, congenital
night blindness, and
Duchene muscular
dystrophy
Hemophilia
•  An X-linked
genetically inherited
recessive disease
•  Can not clot their
blood
•  Severe bleeding from
minor cuts
•  Mostly in males
Red-Green Color Blindness
•  X-linked
•  Cannot perceive red and green
in the same
•  Most often afflicts males
Muscular Dystrophy (MD)
•  Characterized by a gradual irreversible
wasting away of skeletal muscle.
•  An X-linked trait most often passed on to
sons from their mothers.
•  Begins to weaken the legs of boys by age
3 and gets worse every year.
•  No cure and inflicted usually die by age
30.
Y-linked Traits
•  The Y chromosome is small and therefore
does not contain many genes
•  Y linked diseases are very rare
•  Only passed from father to son.
Hypertrichosis Pinnae.
•  Hairy ears
•  Can happen later in
life.
•  Y-linked