Sex Linked
Traits
Male or Female?
•The gender of a baby is determined by
genes on chromosomes.
• There are 23 pairs of chromosomes in each
of our cells.
•One of pair of chromosomes are called sex
chromosomes.
•The sex chromosomes
determine whether a person
is male or female.
•The sex chromosomes are
the only pair of
chromosomes that
do not always match.
Females
The two sex chromosomes
match - XX
Males
The two sex chromosomes do
not match - XY
Female
Homozygous
Male
Heterozygous
•Since both of a female’s sex
chromosomes are X, all eggs
carry one X chromosome
•Males have two different sex
chromosomes, so sperm cells
will either carry an X or a Y
chromosome
Sex-Linked Genes
Some human traits occur more
often in one gender than the other.
Sex-Linked Genes: Genes on the
X and Y chromosomes, whose
alleles are passed from parent to
offspring on sex chromosomes
What is a sex-linked trait?
•Sex-linked traits are due to genes located
on sex chromosomes.
As we know, male sex chromosomes are XY
while female sex chromosomes are XX.
•The X chromosome contains over 1000
genes while the Y chromosome contains
as few as 26. Therefore, many sex-linked
traits are discussed in terms of the Xchromosomes.
Carriers
Because females have two copies of the X
chromosome, it is possible to have certain traits
“hidden” by a dominant copy.
However, because males only have one X
chromosome, the observable phenotype is
obvious and identifies the genotype.
Carriers
 When
a female contains a recessive allele
that is hidden by the dominant allele, we
call them carriers.

A carrier maintains the ability to pass on
a trait even if they do not express/show
it.
In females, a dominant
allele on one X
chromosome will
mask a recessive allele
on the other X
chromosome.
In males, there is no
matching allele on the X
and Y chromosome. As a
result, any allele on the X
chromosome will produce
the trait in a male
who inherits it.
Because males have only
one X chromosome, males
are more likely than
females to have a sexlinked trait that is
controlled by a recessive
allele.
Example of sexlinked trait controlled
by a recessive allele:
red-green Color
Blindness affects
more males than
females.
Examples of X-linked genes
 Other
than determining sex, genes on the
X chromosome are responsible for traits.
Some examples are:





Hemophilia
Red-green color blindness
High blood pressure
Muscular dystrophy
Fragile-X syndrome
Recessive Lethals

In some rare instances, recessive traits are
lethal-- meaning the organism is born very weak
and sickly or dies not long after birth.


When looking at non-sex linked chromosomes, we
can apply standard Punnett Square probability.
However, when looking at sex linked chromosomes
we see that males have a much higher tendency for
recessive lethals. Why?