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.
• Example: Male infertility
Hypertrichosis Pinnae.
• Hairy ears
• Can happen later in
life.
• Y-linked