Practice Questions – Nondisjunction, Complementation, DNA
1) A mother is a carrier for an X-linked recessive disease. The father is normal. The mother gives birth
to an XXY Klinefelter son. Her son happens to be afflicted with the disease. Which of the following
errors most likely occurred?
(a) Write down the genotypes of the mother and father:
Mother: ________________
Father: _________________
(b) Write down the genotype of the son:
Son: ______________
(c) Write down the gametes produced by the mother and father that led to the Klinefelter son:
Mother: ______________
Father: _______________
(d) This nondisjunction event occurred during (select TWO of the following):
A. Mitosis
B. Meiosis I
C. Meiosis II
D. Spermatogenesis (sperm formation)
E. Oogenesis (egg formation)
(e) When a sperm that is aneuploid from a nondisjunction event in meiosis I fertilizes a euploid
(normal) egg, which of the following could result: (Circle all that apply)
A. Turner (X0) female
B. XYY male
C. Trisomy X (XXX) female
D. Klinefelter (XXY) male
(f) When a euploid (normal) sperm fertilizes an egg that is aneuploid from a nondisjunction event in
meiosis I, the zygote that results could be which of the following: (Circle all that apply)
A. Turner (X0) female
B. XYY male
C. Trisomy X (XXX) female
D. Klinefelter (XXY) male
2) In a female mammalian somatic cell, we know that only one X chromosome is active at any given
time. The other X chromosome is inactivated.
(a) In placental mammals such as humans and mice, the choice of which X chromosome is inactivated for
an individual cell is random. In marsupials (a classification of mammals that includes kangaroos and
opossums), the X chromosome that is inactivated is not random. Examine the following pedigree for an
X-linked recessive trait in kangaroos.
Determine whether it is the paternal or the maternal X chromosome that is inactivated in marsupials.
Explain and justify your answer.
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(b) The inactivated X chromosome(s) in females are called ______________.
(c) X chromosome inactivation is an essential process. This is evidenced by the fact that mice that
possess two active X chromosomes in their cells usually die during embryonic development. Propose an
explanation for why two active X chromosomes can be lethal.
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3) You have discovered a new Drosophila gene, which you name DWW. In Drosophila with a wild-type
version of this gene, the flies are hairy. You have created knockout flies in which both alleles of the
DWW gene are deleted. Further, you have identified a mutant allele of this gene (dww), and you seek to
characterize this mutation by performing the following crosses:
P:
F1:
F2:
P:
F1:
F2:
Pure-breeding wild-type flies
x
Knockout flies
↓
100% hairy flies
↓
305 hairy flies and 107 flies with little or no hair
Pure-breeding wild-type flies
x
Pure-breeding mutant flies
↓
100% hairy flies
↓
274 hairy flies and 70 semi-hairy flies
F1 flies (from second cross) x F1 flies (from first cross)
↓
296 hairy flies and 99 flies with little or no hair
(a) True / False: The mutant allele (dww) is a dominant allele. _______________
(b) Which of the following best describes the mutant allele:
A. Hypermorphic
B. Hypomorphic
C. Amorphic
D. Neomorphic
You have identified five other mutations that affect hairiness. You name the mutations mut1, mut2,
mut3, mut4, and mut5. You want to discover whether any of these mutations and the dww mutation are
allelic. So you perform crosses as follows and examine the F1 which are either all semi-hairy or all wildtype (hairy) or all super-hairy.
dww/dww
mut1/mut1
mut2/mut2
mut3/mut3
mut4/mut4
mut5/mut5
wild-type
dww/dww
semi-hairy
mut1/mut1
semi-hairy
semi-hairy
mut2/mut2
hairy
hairy
semi-hairy
mut3/mut3
semi-hairy
semi-hairy
hairy
semi-hairy
mut4/mut4
hairy
hairy
semi-hairy
hairy
semi-hairy
mut5/mut5
super-hairy
super-hairy
super-hairy
super-hairy
super-hairy
super-hairy
wild-type
hairy
hairy
hairy
hairy
hairy
super-hairy
hairy
(c) Disregarding mut5/mut5, how many complementation groups are there? _______
(d) You cannot determine whether mut5 is allelic to mut4 from the test above. Describe the cross you
would need to perform to determine whether mut5 and mut4 are allelic and state the expected
outcome of the cross if mut5 and mut4 are indeed allelic.
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(e) True / False: dww and mut2 are allelic. _______
4)
(a) The nitrogenous base on the right (with the arrow pointing towards it) is ____________________
(b) If a DNA molecule is 30% cytosine, what is the percentage of adenine in the DNA molecule? ______
(c) Which of the sugar molecules above is deoxyribose (the sugar found in DNA)? _________
(d) DNA polymerase can NOT add new nucleotides to a nucleotide that contains which sugar(s)?
(write down all that apply): _________
Examine the following diagram, representing DNA replication.
(e) i. Which letter(s) represent the current site of action of DNA polymerase III?
(choose all that apply): _____________
ii. Which letter(s) represent the current site of action of DNA ligase?
(choose all that apply): _____________
iii. Enzyme B is _____________ and enzyme C is _____________
iv. In the diagram above, circle the location of the most recently added primer.
(f) A mutant E. coli strain exhibits the following abnormal phenotype: DNA is unwound to form a
replication fork but no nucleotides are added to synthesize a new DNA strand. Give the name of the
enzyme that this mutation affects: ________________________
(g) A mutant E. coli strain exhibits the following abnormal phenotype: RNA is never removed from the
newly synthesized DNA strands during DNA replication. Give the name of the enzyme that this mutation
affects: ________________________
(h) A mutation has inactivated the 3' → 5' exonuclease activity of DNA polymerase III. What will result?
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5) Duchenne muscular dystrophy (DMD) is an X-linked recessive disorder, which results in muscle
degeneration due to muscle tissue lacking an important structural protein. Consider a healthy male and
a female carrier for DMD, who both have blood types AB. What is the probability of their having a
healthy child with type B blood?
_________
Hint: Remember blood types:
Phenotype:
Genotype(s):
IAIA or IAi
Type A blood
IBIB or IBi
Type B blood
IAIB
Type AB blood
ii
Type O blood
6) Alien dogs can have black fur or white fur, have long ears or short ears, and have red eyes or yellow
eyes. The genes for fur color, ear length, and eye color are all located on the X chromosome. Black fur,
long ears, and red eyes are dominant.
We cross a pure-breeding black-fur, long-eared, red-eyed female alien dog with a white-fur, shorteared, yellow-eyed male alien dog. We then take a female F1 offspring and cross it with its father. The
offspring of this cross are listed below:
black fur, long ears, red eyes
white fur, short ears, yellow eyes
black fur, short ears, yellow eyes
white fur, long ears, red eyes
black fur, short ears, red eyes
white fur, long ears, yellow eyes
black fur, long ears, yellow eyes
white fur, short ears, red eyes
19
21
18
22
7
8
8
7
(a) Which genes (if any) are linked and what is the distance (in cM) between the linked genes?
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(b) Which genes (if any) assort independently? _______________________________________________