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Journal 10/26/16
What determines whether someone becomes male or
female? (genetically speaking).
How do you think this works? Do you think you could
come up with a better system?
Objective
To learn about genes that
pass on with gender only.
Tonight’s Homework
p 126: 1-3, 8-10
Sex-Linked Traits
Most genes are identical in men and women.
However, there’s one that’s different between
the genders.
Women have two, larger “X” chromosomes,
while men only have 1 larger “X” chromosome,
and the shorter “Y” chromosome.
There are some genes that appear on these “X”
chromosomes that do not appear on the “Y”
one.
We call these special genes “sex-linked traits”.
Sex-Linked Traits
For example:
Hemophilia is a disease that stops your blood
from clotting. This means a simple bruise (which
is where you bleed a little internally) never
stops bleeding, and you can die within a few
days.
This disease is treatable nowadays (you simply
get shots that add that clotting chemical to your
blood) but used to be very dangerous.
This is also a sex-linked recessive genetic trait.
Let’s see how it passes on.
Sex-Linked Traits
Example:
Mom has one dominant gene and one recessive
one. We write this as:
XG
Xg
Dad only has one gene that can carry the trait.
His “X” gene. His “Y” gene doesn’t have the part
that includes this trait.
XG
Y
Let’s make a Punnett square for their possible
children.
Sex-Linked Traits
XG
Xg
XG
XG
XG
XG
Xg
Y
XG
Y
Xg
Y
Take a look at the daughters. One has 2
dominant genes, the other has 1 dominant and
1 recessive. Neither daughter would have
hemophilia.
For the sons, only the mom’s genes are passed
on. Since the Dad’s “Y” gene doesn’t have this
trait, he can’t pass it on.
Note that 1 son will have hemophilia.
Sex-Linked Traits
Because sex-linked traits are attached to the “X”
gene, a man can never pass on the trait to his
sons. The mom can pass it on to sons and
daughters, but the dad can only pass it to
daughters.
Practicing with sex linked traits
Do the following combinations and tell me all the
genotype and phenotype probabilities:
1)
2)
3)
4)
5)
6)
Homozygous dominant mother with recessive father
Heterozygous mother with recessive father
Homozygous recessive mother with recessive father
Homozygous dominant mother with dominant father
Heterozygous mother with dominant father
Homozygous recessive mother with dominant father.
Exit Question
Which of the following is impossible?
a) A male passing on a sex-linked gene to his daughter
b) A male passing on a sex-linked gene to his son
c) A female passing on a sex-linked gene to her daughter
d) A female passing on a sex-linked gene to her son
e) All of the above are impossible
f) None of the above are impossible