Sex-Linked
Traits
Genetic
Counseling
Sometimes it’s a good
idea to know the odds
. . . . Especially when
dealing with sex-linked
traits.
It’s all about the SEX!
• Sex-linked genes are located on the 23rd
chromosome (sex chromosome).
• Sex chromosomes are X and Y.
• XY = Boy
• XX = Girl
• Sex-linked genes are shown as SUPERSCRIPTS
on the X chromosome.
Sex-Linked Traits
• Sex-linked traits can be Dominant or Recessive.
•
A = dominant a = recessive
•
What would be the genotypes of a male and female that have a Sex-linked
Dominant trait and do not express (have) the trait?
•
•
Expresses Trait:
No Expression:
•
What would be the genotypes of a male and female that have a Sex-linked
Recessive trait and do not express the trait?
•
•
Expresses Trait:
No Expression:
Male - XA Y
Male - Xa Y
Male - Xa Y
Male - XA Y
Female - XA XA or XA Xa
Female - Xa Xa
Female - Xa Xa
Female - XA XA or XA Xa
(Carrier)
• Most Sex-linked traits are Recessive!
Sex-Linked Recessive Traits
• More males are affected than females.
• An affected son can have parents who have the
normal phenotype. (XAY, dad) x (XAXa, mom)
• For a daughter to have the trait, her father must also
have it. Her mother must have it or be a carrier.
(XaY, dad) (XaXa or XAXa, mom)
• The trait often skips a generation from the
grandfather to the grandson.
• If a woman has the trait (XaXa), ALL of her sons will
be affected.
• Pedigrees show only female carriers--no male
carriers.
Examples of Sex-Linked Recessive Disorders
• Red/Green Colorblindness – Difficulty perceiving differences between
colors (red or green, blue or yellow).
• Hemophilia – Absence of one or more proteins necessary for normal
blood clotting.
• Deafness
• Cataracts – opacity in the lens that can lead to blindness
• Night blindness – (Nyctalopia) rods do not work so that can not see in the
dark
• Glaucoma – pressure in the eye that can lead to optic nerve damage and
blindness
• Duchenne Muscular Dystrophy – progressive weakness and degeneration
of skeletal muscles that control movement due to absence of dystrophin
(protein that maintains muscle integrity). Mainly in boys, onset 3-5 yrs, by
12 years can’t walk, and later needs respirator.
To See or Not To See
Color-Blind
-- cannot see the
numbers or designs in
the circles.
-- What do you see?
Color Blind: Recessive SL Trait
Color-Blind Pedigrees
• Draw a pedigree showing a cross between a
man with normal vision and a woman who
carriers the color-blind trait.
• Color-blind is a recessive trait.
• Genotypes of Parents:
• Male = XR Y Female = XR Xr
Does yours look like this?
• Pedigree of color-blind trait.
• Genotypes of Parents:
• Male = XR Y Female = XR Xr
XRY
XRY
XRXr
X rY
XRXR
XRXr
Can a female be color-blind?
• Think about it.
• If so, how? Why?
YES—she can!
• If dad (XrY) is
color-blind and
mom (XRXr) is a
carrier =
50% chance XrXr
• She’s color-blind.
Hemophilia
•It is a rare blood disorder.
•The blood does not clot.
• A hemophiliac will bleed
“freely”, while a normal
person will eventually stop
bleeding because a scab
forms.
•Hemophiliacs require
medical intervention to stop
the flow of blood.
•Usually they are given
clotting factors, which help
them form scabs.
•Hemophilia is inherited.
•About 1 out of 5,000 males
are born with hemophilia
each year.
Pedigree Royale: Hemophilia