Genetics 320 (EEB/MCB) TEST ONE
(1) [10 points] Two parents who are both carriers (heterozygotes) for a
recessive disease have a one in four chance that their child has the disease.
Unfortunately, genetic counselors often hear such parents say this is not a
problem as they only plan to have three children.
What is the probability that at least one of their three children show the
disease?
Suppose their first two child were disease-free. What is the chance that the
third has the disease?
(2) [10 points] In class it was mentioned that Mendel likely "cheated" as
he only considered seven traits, exactly the number of pea chromosomes. If
more traits were considered, they would not have shown independent
assortment. What is the probability in peas of picking seven traits at
random and having them all fall on different chromosomes (assume that all
pea chromosomes have an equal number of genes)?
#3) [25 Points] 2000 Gametes from an AbC / aBc individual are scored, with
the following numbers seen:
ABC 162 aBC 22
ABc 165 aBc 661
AbC 657 abC 155
Abc 20 abc 158
Compute (1) the gene order, (2) the recombination frequency between all sets
of loci, (3) the coefficient of coincidence, and (4) the interference.
#4) [15 Points] The A and B loci are linked, with recombination frequency
1/4. A cross is made with Ab/aB as the father, ab/aB as the mother.
(a) What are the gamete frequencies for the father?
(b) What are the gamete frequences for the mother?
(c) What is the chance of getting an Aabb offspring?
(d) What is the chance of getting a AaBb offspring?
#5) [5 points] Define both Penetrance and expressivity. How are these
different?
#6) [10 points] A child has blood type O. There are two possible fathers, Joe
Nosering (a mega-million dollar rock star) and Joe Sixpack (an unemployed
truck driver). The mother (blood type B) claims Nosering (blood type A) is
the father.
a. Based on this data, can Nosering be excluded? Why?
b. Joe Sixpack had an AB child from a previous marrage. Can Sixpack be
excluded? Why?
#7) [10 points] Fungal mapping
(a) Consider the following 1000 linear octads and their frequencies
1 AAAAaaaa 350
2 AAaaAAaa 150
3 aaAAaaAA 150
4 aaaaAAAA 350
Which octads are first division segregrants?
Which are second division segregrants?
What is the recombination frequency between the marker and centromere?
(b) Suppose the observed frequency of second division segregrants is 2/3.
What can we say about the recombination frequency?
#8) [5 points] Suppose a parent is a carrier for a dominant autosmal
disease gene, where D = disease allele, d= normal. D is closely linked
(recombination frequency c) to a marker locus with alleles M and m. If the
parent has phase DM/md, what is the probability that an M-bearing gamete
from this parent also carries the D allele? (Be sure and show how you got
your result!)
#9) [10 points] Consider chromosomes 1 and 2 below and the resulting
translocation between them.
Draw the chromosome pairing at metaphase in a translocation heterozygote.
Use arrows to show how an example of how the centromeres could segregate
to give rise to a set of normal euploid gametes.