Sex Linked Traits

Punnett Squares:
Sex-Linked Traits

Sex-Linked Traits

• Sexually reproducing organisms have one special set of chromosomes called sex chromosomes. Traits that are coded on the sex chromosomes are called sex-linked traits.

• There are genes on the X chromosome that control non-gender related traits. 

• Females have two copies of these genes (because they have two X chromosomes) whereas males have only one copy of these genes (because they have one X chromosome).

Sex-Linked Traits

• Traits coded on the X chromosome are not inherited the same way with males and females. 

• Males are affected be X-linked genetic disease if they inherit one copy of the disease whereas females can inherit one copy disease and not be affected because they have a normal copy on their second X chromosome.

• Colorblindness and hemophilia (a blood clotting disorder) are sex-linked genetic diseases.

Sex-Linked Traits

colors.

If you cannot see the

number 74 in the dots to

the right, you may be

color blind and should be

tested.

Color

Blindness Test
What number do you

see in the dots?

What is a carrier?

A person who carries a
recessive gene, but does
not express the trait in
their phenotype. This
recessive gene can be
passed down to future
offspring.

There is an important vocabulary word that you will

need to understand as we talk about this trait.

This pedigree shows how color

blindness is passed down in a family.

What patterns do you notice?

Females seem

more likely to be

carriers while

males seem more
likely to get this

disorder!

Color Blindness Pedigree

Males with disorder =

Females with disorder =

Female carriers =

3

0

5

Color Blindness Pedigree

Whether or not you get a trait like color

blindness is linked to your x chromosome. Males are color blind far more often than females. Because of

this, we call it a ______________.

sex-linked trait

The reason that some traits are linked to the

sex of an organism is because they are

located on one of the __23__ chromosomes.

male

female

For a male, the 23rd pair of chromosomes is
unique because the X and Y chromosomes do

not contain the same information.

Males only get one copy of every
gene on the X chromosome. What
would happen to them if one of

these genes had a mutation?

The allele would HAVE to be

expressed in the male’s phenotype because
that is the only copy that he has to show!

Why would females be more likely
to be carriers of the gene instead of

showing the trait themselves?

females have two X chromosomes so if one has a mutation,

the normal allele on the other chromosome will take over.

So does this
mean that
females will
NEVER get a
sex-linked

trait?

Females CAN have a sex-linked recessive

trait, but it is rare. For a female to acquire a
sex-linked recessive trait like color blindness,

her father must have the trait AND her

mother must at least be a _______.

Female with
sex-linked

trait

They ALL have the sex-linked trait!

If a woman has

a sex-linked
trait like color
blindness, what

is true of her

sons?

Female with
sex-linked

trait

Why would her sons have to have this sex-linked trait?

Because males always get their X chromosome from

their mom, and both of her X chromosomes carries the

mutated allele.

It is important to point out that we are only covering

sex-linked recessive traits. While sex-linked

dominant traits exist, they are rare and hard to

identify from a pedigree because they can appear to
be inherited just like autosomal traits unless large

family histories are created.

The main identifying factor of
ANY sex linked trait carried on
the X chromosome, whether

dominant or recessive, is that it

is NEVER passed from

father to son!

Remember: In humans, the 23rd pair of
chromosomes is the sex chromosomes. The
karyotype XY means the person is a male. If
a person has the karyotype XX, the person is
a female.

male

female

1. What percentage of offspring will be
color blind males? _______________

2. What percentage of offspring will be
female carriers? _________________

3. What percentage of the offspring will
be completely normal? ___________

Hemophilia

Hemophilia is carried on the X
chromosome, and the blood

of people who have this

disease does not coagulate or

clot properly. This causes
people to bleed excessively

when they have been injured.

A mutation in the gene
for coagulation factor

VIII is the most common

cause of hemophilia.

1. What percentage of offspring will be
hemophiliac males? _______________

2. What percentage of offspring will be
female carriers? _________________

3. What percentage of offspring will be
hemophiliac females?___________

Multiple Choice

 The inheritance pattern for an autosomal dominant trait is shown in the pedigree. Shaded symbols represent individuals that express the dominant trait.

• I -1: aa

  I -2: Aa

• I -1: AA

  I -2: Aa

• I -1: Aa

  I -2: aa

• I -1: aa

  I -2: AA

Multiple Choice

Hemophilia is a sex-linked trait. Sylvia knows that she is a carrier for the disease, and she knows her husband is normal. What are the chances of Sylvia and her husband having a child with hemophilia?

• 50

• 100

• 0%

• 75%

Multiple Choice

Why is it not possible for a male human to carry the color blind trait without expressing it?

• The Y chromosome hides the expression of the trait

• The allele for colorblindness is X-linked recessive, therefore it will always be expressed if there is only one X chromosome.

• The colorblind allele never affects males

• The color blind allele is carried on the Y chromosome