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
• The X and Y sex chromosomes carry the genes
that determine your gender
• Men have a combination of X and Y sex
chromosomes.
• Women have two X
XY
chromosomes. XX
This is what they look like
Before we move on,
• Draw a Punnett square
to show the probability
of having a boy in any
given pregnancy.
• Remember boys are XY
and girls are XX
Sex Linked Genes
• These • Genes that are
chromosomes not carried by either
only carry the sex chromosome
genes that (X or Y) are said
determine male to be SEX
and female traits, LINKED
but also those for
some other
characteristics as
well.
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 X-
linked traits,
Why?
Because they
both inherit X
chromosomes.
Sex inheritance patterns for male
and female children
X Chromosome Traits
• Men only have one
X chromosome
• Therefore genes XAY dominant, shown
XaY recessive, shown
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. Why?
• Because dominant normal allele may be
present on the other X chromosome.
• If the woman is heterozygous for the X-
linked disease, she is said to be a carrier.
This means she does not have the disease,
but can pass it to her children.
XA XA dominant, shown
XA Xa dominant masks the recessive, shown
Xa Xa recessive, not shown
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 the difference
between red and green
• 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 the individual afflicted with
the disease usually dies 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 or retinitis
pigmentosa (causes progressive loss of
sight.
Hypertrichosis Pinnae.
• Hairy ears
• Can happen
later in life.
• Y-linked
Some thinking, …
• Which parent determines the sex of a
baby?
• What will be the sex of a child produced
when an egg is fertilized by a sperm that
has a Y chromosome?
• Based on the knowledge you have
acquired thus far, can the sex of a child be
predicted based on the number of males
or females in a family? Why?
