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7.012 Problem Set 2
Question 1
In unicorns, coat color (brown or white) is controlled by a single gene with two alleles,
A and a. The brown phenotype is dominant over the white phenotype. Height (tall or short)
is controlled by a single gene with two alleles, H and h. The tall phenotype is dominant over
the short phenotype. These two loci are on different chromosomes.
a) What is the phenotype of an HhAa unicorn?
b) What is the phenotype of an hhAa unicorn?
c) If an HhAa unicorn mates with an hhAa unicorn, what fraction of the progeny will be
short and brown?
i) Show how you would answer this question using a Punnett square.
ii) Show how you would use the laws of probability to answer the same question.
Question 2
For each of the following pedigrees, give the likely mode of inheritance (autosomal dominant,
autosomal recessive, sex-linked dominant, or sex-linked recessive). Assume that individuals
marrying into a family do not carry the mutant allele.
a) Mode of inheritance:
b) Mode of inheritance:
c) Mode of inheritance:
Question 3
A rare and delicious fish lives in the shallows surrounding a remote desert island soon to be
famous as the site for a SURVIVOR sequel. This fish has two phenotypes of interest: shininess
and speed. The shiny phenotype is dominant over the dull phenotype, and the fast phenotype
is dominant over the slow phenotype. The producers of SURVIVOR hired you to breed large
numbers of shiny slow fish to increase the chance that the new contestants can catch a meal.
Phenotype
shininess
speed
alleles
D or d
E or e
You cross a true breeding shiny slow fish to a true-breeding dull fast fish to get an F1 class of
progeny that is entirely shiny and fast.
a) What is the genotype of the true-breeding shiny slow fish?
b) What is the genotype of the true-breeding dull fast fish?
c) What is the genotype of the F1 shiny fast fish?
d) You then perform a test cross with an F1 fish. What are the genotypes of the individuals in
this test cross?
e) If the genes controlling these two traits are unlinked, what phenotypes and in what ratios
would you expect as a result of this test cross?
f) If the traits were completely linked, what phenotypes and in what ratios would you expect
as a result of this test cross?
Question 3, continued
f) What you actually see when you complete the test cross is:
48 shiny fast fish
199 shiny slow fish
208 dull fast fish
45 dull slow fish
i) Circle the recombinant phenotypes.
ii) What is the recombination frequency between the shininess and speed loci?
g) Interestingly, you find that these same fish can be either sharp-eye or blind, where the
sharp-eyed phenotype is dominant over the blind phenotype. Use B or b to designate the
alleles of the eyesight locus.
You determine that the loci for shininess and for eyesight are linked with a recombination
frequency of 4.5%. Draw the two potential maps for the D, B, and E loci. On each map,
express the distance between the loci as the recombination frequency.
h) What would you have to do to determine which of these maps is the correct one?
Question 4
On the same island is a seaweed (a diploid organism) that exists in three different colors (this
seaweed will soon be woven into bikini tops and breech cloths and will become THE fashion
statement in Paris). This seaweed can be gold, green or purple. The color is controlled by a
pathway of two distinct enzymes encoded by the F and G genes. For questions a) and b)
below, use F and G to designate the wild-type alleles. Use f and g to designate the loss-offunction alleles. See the partial pathway below.
You cross two true-breeding plants, one that is purple and one that is green. The F1 plants are
all gold.
a) Given only the information above, what are two possible genotypes for the purple parent?
b) Given only the information above, what are two possible genotypes for the green parent?
c) What is the genotype of the F1 progeny?
You cross two F1 progeny and expect to see a 9:3:3:1 ratio of phenotypes in the F2 generation.
Surprisingly, you notice that the F2 progeny have the following ratio of phenotypes:
9 gold : 4 purple : 3 green
d) Given this data, circle the correct pathway.