Download day 11 sex linked traits

Sex Link Traits:
Genes on the X and Y chromosomes
Copy what
is in RED
Lets recall how we determine sex…
 Sex Chromosomes: X and Y
Female: XX
Male: XY
 The sperm determines the sex
because it gives an X or Y to the
gametes. The egg only gives an X
to the gamete
 The X chromosome is Bigger
What are sex-linked genes?
 genes found on a sex
chromosome
• X-linked genes are genes found
on the X chromosome,
symbolized by Xr, XR.
• Y-linked genes are found on the Y
chromosome, (we won’t look at
any of these)
• Thomas Morgan experimented
with the eye colour of fruit flies
(Drosophilia) to determine Xlinkage
Y-linked Genes
• Y-linked genes are uncommon because the Y
chromosome is so small and does not contain
many genes, Y-linked diseases are rare.
• In humans, only males have a Y chromosome
so traits / diseases are passed only from father
to son
X-linked Genes
• X-linked traits can be dominant or recessive XD,
Xd
• If the trait is DOMINANT only one copy is
needed for the trait to be expressed in either
females or males: XDX, XDY
• If the trait is RECESSIVE two copies are
needed for the trait to be expressed in females
XdXd. But only one copy is needed in males:
XdY
X-linked Genes
• X-linked recessive traits will show up more
often in males because they have only one
X chromosome
• Example 1: female carrier of an x-linked gene
and a normal male
• What will the genotypes be from the following
cross?
XXr and XY
Set up the Punnett Square
X
X
Y
r
X
Here’s the results
• For girls: 0% have the
trait
• For boys 50% have it.
• Probability is higher
for boys because
whatever X they get
determines the trait,
for girls they have to
get 2 recessive X’s.
X
Y
X
r
X
XX
X Xr
XY
r
XY
Examples of X-linked traits in
humans:
1. Red/Green Colour Blindness
2. Hemophilia blood disease
3. Muscular Dystrophy
4. Icthyosis simplex (scaly skin)
Colorblindness
• A person with normal colour vision sees a number seven in
the circle above.
• Those who are colour blind usually do not see any number
at all.
Colorblindness
• RED-GREEN
COLORBLINDNESS:
• People with red-green
colour blindness see
either a three or nothing
at all.
• Those with normal colour
vision see an 8.
Hemophilia
Hemophilia- Lacking in the
ability to clot blood
– There is a gene on the “X”
chromosome that control
blood clotting
– People who have
hemophilia are missing
the protein to clot blood
– They can bleed to death by
minor cut.
Muscular Dystrophy
- Results in
weakening/loss of muscles
- Caused by defective
version of gene that codes
for muscle
X-linked Genes
• Example 2: an unaffected male marries an
affected female
• What will the genotypes be from the following
cross?
XrXr and XY
Set up the Punnett Square
r
X
X
Y
r
X
Here’s the results
• For girls: 0% have the
trait but 100% are
carriers
• For boys 100% have
it.
r
X
r
X
X
X Xr
X Xr
Y
r
XY
r
XY
Phenotypes of Children
100% girls normal fingers
100% boys with webbed fingers
Practice
• http://www.ksu.edu/biology/pob/genetics/xl
inked.htm
• http://www.zerobio.com/drag_gr11/sexlink.
htm
• http://www.biology.arizona.edu/mendelian
_genetics/problem_sets/sex_linked_inherit
ance/sex_linked_inheritance.html
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