Biology 207 Workshop 8
1. . Describe the mode of inheritance of the genetic disease in the following
pedigrees. Is the mutation dominant/recessive and autosomal/ Sex-linked? Which
are possible? Which is most likely?
a.
X-linked recessive
b.
Autosomal recessive
c.
Since the mutant phenotype skips a generation it has to be recessive. There appears
to be no bias between males and females and therefore is autosomal. The best
description describes this mutation as autosomal recessive.
d.
X linked because all females appear to inherit the mutation from their affected
fathers. If the trait is autosomal it should affect both females and males at an equal
frequency. The trait appears to be dominant because there is not an instance where
it skips a generation.
2. Suppose that an allele, b, of a sex-linked gene is recessive and lethal. A man marries a
woman who is heterozygous for this gene. If this couple had a large number of normal
children, what would be the predicted sex ratio of these children? (ie. Ratio of male
children to female children)
1:2
3. a. If a child has an autosomal dominant trait, what can you say about the parents?
One parent will be affected, the other can be unaffected.
b. If two parents have an autosomal dominant trait, what can you say about their
children?
3/4 of their children should be affected.
c. If two parents have an autosomal recessive trait, what can you say about their
children?
All of their children will be affected.
d. If a child has an autosomal recessive trait but the parents do not, what can you say
about the parents?
Both the parents must be carriers.
e. Why do autosomal recessive traits skip generations?
B/c both parents need to be carriers in order to have affected children. And since
it’s recessive, the children only have a 1/4 chance of being affected.
f. What can you conclude about male children of mothers with an X-linked recessive
trait? What can you say about female children?
All male children will have the trait. The female children will not, unless their dad
is also affected.
g. What can be said about father-to-son transmission of X-linked traits?
It doesn’t happen, b/c fathers pass their Y onto their male children.
h. How do the number of males with X-linked recessive traits compare with the
number of females with the trait? What about for X-linked dominant?
For x-linked recessive, more males will be affected. For x-linked dominant,
males and females will be equally affected.
4. A young woman is worried about having a child because her mother’s only sister had
a son with Duchenne Muscular Dystrophy (DMD), a rare X-linked recessive disorder.
The young woman has no brothers or sisters.
a. Draw the relevant parts of the pedigree of the family described above. (Be sure to
include the grandmother, and the husbands of all the women mentioned above)
b. State the genotype of everyone in the pedigree.
c. Calculate the probability that the young woman’s first child will have DMD.
a and b)
XRXr
XRY
XRY
XR?
XRY
XRXr
XRY
XR?
Xr Y
?
c) probability = 1/2 (that her mother inherited the allele) x 1/2 (that she inherited the
allele) x 1/2 (that her child will inherit the allele) x 1/2 (that the child is a boy)
= 1/16
5. Tay-Sachs disease is a rare human disease in which toxic substances accumulate in
nerve cells. The recessive allele responsible for this disease is inherited in a simple
Mendelian manner. A woman is planning to marry her first cousin, but the couple
discovers that their shared grandfather’s sister died in infancy of Tay-Sachs disease.
a. Draw the relevant parts of the pedigree and show the genotypes.
b. What is the probability that the cousins’ first child will have Tay-Sachs?
Tt
Tt
TT
T2
/3
/2 T-
1
T-
tt
/2
1
TT
TT
1
T-
/2
/2
1
1
/2
/2
1
T-
?
c) probability = 2/3 x 1/2 x1/2 x1/2 x 1/2 x 1/2 x 1/2
= 1/96
6. An inversion is present in chromosome 2 of mice. The normal form and the inverted
form of chromosome 2 are illustrated below:
Normal form:
Inverted form:
A
B
A
D
C
C
D
E
B
E
a) What type of inversion is this?
Pericentric inversion.
b) Diagram the chromosomes of a female mouse heterozygote for this inversion at
metaphase of meiosis I.
C
B
D
C
B
D
A
E
A
E
c) Assume that a 4-strand double cross-over (one between B and the centromere, the
other between C and D) occurred during meiosis, diagram the chromosomes at
anaphase of meiosis I.
A
B
C
D
A
E
B
C
D
E
A
B
C
D
A
E
B
C
D
E
d)How many of the gametes produced (in part c) are expected to produce viable
progeny?
None are viable.