Sex Linked Inheritance

Wait…. What?????
• What’s a “mosaic”?
• Remember the FLUID
MOSAIC MODEL????
– The Plasma Membrane
– P.S. What is a Plasma Membrane?
• An individual (organism, artwork, organelle)
made up of smaller, different ”pieces”…
• So, in a way, WE ARE MOSAICS, too!

Calico Cats
• Calico Cats can ONLY BE Female!

Why????
• Because a cat’s fur color is linked to two different
alleles on the “X” chromosome
• And…For a cat to have more than two colors…
• It must have TWO X CHROMOSOMES!

Sex-Linked Inheritance
• Sex-Linked Inheritance means that a
gene (or multiple genes) are carried on
one of the sex chromosomes

Sex-Linked Inheritance
• Remember, Females have XX and Males are XY.
– The Y has very few genes - mainly those that contribute to male
characteristics. (Only about 87 genes total.)
– The X much more genetic information – for gender and other
characteristics. (About 2050 genes!)

Sex-Linked Inheritance
• Some genes for a disorder/disease are located on
one of the sex chromosomes
• If the gene is linked to the “Y” chromosome,
women will not have the disorder.
– Why? _________________________________
• If the gene is linked to the “X” chromosome, and it
is recessive, women will not express the disorder.
– Why?
______________________________________
• If the gene on “X” is dominant… both will express
it!

Who is usually affected by
Sex-Linked Disorders?
• MEN! Genes for certain traits are on
the X chromosome only…
– Since Men only have one X chromosome
then they are more likely to have the
disorder
– Women are somewhat protected since
they have two X chromosomes and are
less likely to inherit receive disorders.
• If women receive a recessive gene on one X
chromosome, they are called “carriers”
because they “carry” the gene but don’t
express the disorder

Sex-Linked Disorders
• Affected males never pass the disease to their
sons
– Men give their “Y” to their sons!
• Affected males pass the defective X
chromosome to all of their daughters, who are
described as carriers
– This means they carry the disease-causing allele
but generally show no symptoms
• Female carriers pass the defective X
chromosome to…
– half their sons (affected by the disease)
– half their daughters (who are carriers)
– The other children inherit the normal copy of
the chromosome

How do you solve Sex-linked
Problems?
• You determine which trait
(or disorder) is dominant or
recessive
• Set up a punnett square
using XX for females and
XY for males.
– Assign alleles for X only!
• Solve as usual, keeping in
mind that the Y
chromosome has no allele!
Genotypes: XRXr , XRY
Phenotypes: All offspring
have red eyes.
If Red eyes are dominant and
sex-linked, show the cross
between a homozygous red eyed
female and a white eyed male.

Your Turn!
• Hemophilia is a sex-linked trait where XH gives
normal blood clotting and is dominant to the
hemophilia allele Xh
• Identify the genotypes of…
1) a woman with normal blood clotting whose father
had hemophilia
2) a normal man whose father had hemophilia.
• What is the probability that a mating between these
two individuals will produce a child, regardless of sex,
that has hemophilia?

Set up the Punnett Square!
- a woman with normal blood clotting whose
father had hemophilia  XHXh
- a normal man whose father had hemophilia 
XHY
Genotypes:
Phenotypes:

What are some X-linked disorders?
• ALD
• Hypertrichosis
• Duchenne muscular dystrophy
• Hunter Syndrome
• Menkes disease(kinky hair syndrome)
• Hemophilia
• Color blindness
• Inherited diabetes (Type 1)

ALD (Adrenoleukodystrophy)
• A deadly genetic disease that is a result
of fatty acid buildup caused by the enzymes
not functioning properly
• Causes damage to the nerves, resulting in
neurological issues and, later, death

Hypertrichosis
• An abnormal amount of hair growth over the
body
– Informally called werewolf syndrome, because the
appearance is similar to the mythical werewolf

Hypertrichosis

Are you colorblind?

Color Blindness
• Having a decreased ability to see some
colors or differences in colors
• Red-Green is most common

Think how difficult some things
would be if you have colorblindness

What about this….
Imagine life without
being able to see
true colors!

Menke’s Disease
(Kinky Hair Syndrome)
• Rapid deterioration of the nervous system.
• Weak muscle tone, sagging facial features,
seizures, developmental delay, and
intellectual disability caused by enzymes not
functioning
• Children with Menkes syndrome typically
begin to develop symptoms during infancy
and often do not live past age 6

Menke’s Disease

Hemophilia
• The “Royal Blood Disease” impairs the body's
ability to make blood clots
– Clots are needed to stop bleeding
• This results in people bleeding longer after an
injury and an increased risk of bleeding inside
joints or the brain

Hemophilia
• Remember the Royal Family?

Tay-Sachs
• Tay-Sachs disease is a rare inherited disorder
that progressively destroys nerve cells
(neurons) in the brain and spine
• Enzymes are the culprit AGAIN!
• Paralysis of the nerves cause eventual death

Tay-Sachs
Tay Sachs

Duchene Muscular Dystrophy
• Duchenne muscular dystrophy (DMD) is a
genetic disorder characterized by progressive
muscle degeneration and weakness
• Until relatively recently, boys with DMD
usually did not survive much beyond their
teen years
– Advances in cardiac and respiratory care means
that survival into the early 30s is becoming more
common

Duchenne Muscular Dystrophy

PKU (PhenylKetonUrea)
• Decreased metabolism of the amino
acid phenylalanine
– Enzymes can’t break it down!
• Untreated PKU can lead to intellectual
disability, seizures and death

PKU

Cystic Fibrosis (CF)
• A genetic disorder that affects mostly the
lungs, but also the pancreas, liver, kidneys,
and intestine Cystic Fibrosis
• The enzymes required to break down mucous
are not functioning, so the mucous collects
and clogs…

SCID (Severe combined
immunodeficiency)
• Do you know “David, the Boy in the Bubble”,
who was born without a working immune
system?

SCID
• What happens when your immune system
doesn’t work???
– https://www.nytimes.com/video/u
s/100000004077071/the-boy-in-
the-bubble.html

Albinism
• Albinism is an inherited genetic condition that
reduces the amount of melanin pigment
formed in the skin, hair and/or eyes. Albinism
occurs in all racial and ethnic groups
throughout the world.
– As well as other species!

Albino Animals

Different degrees of Albinism…

Y linked
• Hairy ears