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SexLinkage2.ppt
1. SEX DETERMINATION
• The sex of an individual is determined by
the sex chromosomes contributed to the
zygote by the sperm and the egg
2. SEX DETERMINATION
• An egg can donate an X
• A sperm can donate an X or Y
• Therefore the sperm determines the sex
of a child
3. SEX-LINKED INHERITANCE
• Using fruit flies as
test subjects,
Thomas Morgan
studied eye colour
using simple
monohybrid
crosses.
• Red eyes (R) are
dominant over
white eyes (r).
4. SEX-LINKED INHERITANCE
• When he crossed
purebred white-
eyed males with
red-eyed females,
he was unable to
produce a female
with white eyes.
• He concluded that
the gene must be
located on the X
chromosome.
5. SEX-LINKED INHERITANCE
• Some traits are
located on the
sex
chromosomes,
so the
inheritance of
these traits
depends on the
sex of the parent
carrying the trait.
6. SEX-LINKED INHERITANCE
• Most known sex-
linked traits are
X-linked (carried
on the X
chromosome).
This is probably
because the X
chromosome is
much larger than
the Y chromosome.
7. SEX-LINKED DISORDERS
• Some sex-linked traits are associated
with disorders.
• Most are found on the X chromosome,
Y-linked disorders are rare.
• Males are at a much greater risk for
inheriting sex-disorders because they
only inherit one X, so if the X has the
allele for the disorder, they will suffer
from the disorder.
• Recessive lethal X-linked traits result
in death.
8. EXAMPLES OF SEX-LINKED
TRAITS and DISORDERS
• Male pattern baldness, red-green colour
blindness, myopia, night blindness,
hemophilia
9. SEX-LINKED INHERITANCE
Punnett squares are used to
predict the outcome of sex-
linked inheritance.
Assume the trait is X-linked
unless told otherwise!
Most disorders are recessive,
some are dominant, the
question will tell you.
A “carrier” is a female who is
heterozygous for the trait.
10. EXAMPLE
• Hemophilia is a recessive X-linked
trait. What is the probability of a
couple having a hemophiliac child if
the man does not have hemophilia
and the woman is a carrier?
13. X – linked Recessive Inheritance
• Refers to those situations where a recessive
allele on the X chromosome can lead to a
trait/condition or disorder.
14. X – linked Recessive Inheritance
• Males are affected more often than females. Ratio of
8:1.
• Affected males will transmit the allele to all
daughters, but not to sons.
• Homozygous recessive females can arise only from
matings in which the father is affected and the
mother is affected or a carrier.
15. X – linked Recessive Disorders
• Hemophilia which is the inability of the blood
to clot properly.
• Duchenne Muscular Dystrophy which
causes progressive and degenerative
muscle weakness.
16. X – Linked Dominant Inheritance
• Refers to situations where a single dominant
allele on the X chromosome can lead to a
trait/condition.
• Very uncommon.
17. X – Linked Dominant Inheritance
• 1. Twice as many females are affected as males.
• 2. Usually half the children of an affected female will be
affected, regardless of sex.
• 3. All the daughters of an affected male will be affected but
none of the sons.
18. X – Linked Dominant Example
• Vitamin D resistant rickets which can
lead to bone deformities, particularly in
the lower limbs (bowed legs).
19. 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)
20. 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
21. Hypertrichosis
• An abnormal amount of hair growth
over the body
– Informally called werewolf syndrome,
because the appearance is similar to the
mythical werewolf
27. 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
29. 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
31. 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
33. 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
35. 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
37. 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…
39. SCID
• What happens when your immune system
doesn’t work???
–
https://www.nytimes.com/video/u
s/100000004077071/the-boy-in-
the-bubble.html
40. 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!
