Download CHAPTER 12 SEX-LINKED TRAITS

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:
Male - XA Y
Male - Xa Y
Female - XA XA or XA Xa
Female - Xa Xa
• 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
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
• Duchene 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.
Karyotype: a picture of
chromosomes.
Body Cells vs. Sex Cells
• Autosomes: the first 22
homologous pairs of chromosomes.
• Autosomes are the same for both
males and females.
• Sex cells: Determine sex of
offspring
Is it Male or Female? Explain
• Sex chromosomes: determines the
sex of the individual.
The
sex chromosomes are the 23rd pair
of chromosomes.
XX =female
XY=male
Which parent determines the sex
of an offspring?
• DAD
• Why?
All moms have the genotype
XX. When egg cells are made,
they will all carry a single X
chromosome.
• All dads have the genotype XY.
When sperm cells are made, 50%
will have an X chromosome and 50%
will have a Y chromosome.
• Therefore, males and females are
born in roughly a 50:50 ratio.
SEX-LINKED TRAITS
• Those traits that are controlled by
genes on the X or Y chromosomes.
• NOTE: The Y chromosome is much
smaller than the X chromosome and
only contains a few genes. Most
sex-linked traits are on the X
chromosome.
Genetic Sex-Linked
Disorders
2- Hemophilia A and B
Recessive disorders that affect 1 of
5,000 males. These interfere with
normal blood clotting and occur on the
X chromosome.
In humans, hemophilia is a sex-linked
trait. Having hemophilia is recessive (Xh) to
being normal (XH). The heterozygous female is
called a carrier. Cross a carrier female with a
normal male.
__ XHXh __ X __ XHY ___
male
XH
Y
XH
Xh
XH XH
XH Xh
XH
Y
Xh Y
Results:
Genotypic ratio:
1 XHXH :1 XHXh : 1 XHY :1Xh Y
Phenotypic ratio:
2 normal females:
1normal male:
XH
1 male with
hemophilia
Y
XH
Xh
XH XH
XH Xh
Normal Female
Normal Female
XH
Y
Normal Male
Xh Y
Male w/
hemophilia
Now You Try
• Cross a carrier
female with a
male with
hemophilia.
XHXh X XhY
Results:
Genotypic ratio:
1 XHXh :1 XhXh : 1 XHY :1 Xh Y
XH
Phenotypic ratio:
Xh
1 normal female:
1 female w/ Hemopholia:
XH
1normal male:
1 male with hemophilia
Y
XH
XH
Normal Female
XH
Y
Normal Male
Xh Xh
Female w/
hemophilia
Xh Y
Male w/
hemophilia