Answers ■ A13Q3: Draw a Punnett square for a genetic cross of two heterozygous,
black-coated dogs. What are the phenotype and genotype ratios of
their offspring?
A3: Phenotype ratio is 3:1 black to brown. Genotype ratio is 1:2:1
BB to Bb to bb.
Figure 7.6
Q1: List all the possible offspring genotypes and phenotypes.
A1: Round, yellow (dominant dominant): RRY Y, RrY Y, R RYy,
RrYy.
Round, green (dominant recessive): RRyy, Rryy.
Wrinkled, yellow (recessive dominant): r rY Y, r rYy.
Wrinkled, green (recessive recessive): rryy.
Q2: What is the offspring phenotype ratio?
A2: 9:3:3:1.
Q3: Complete a Punnett square for a genetic cross of two true-
breeding Portuguese water dogs—one with a black, wavy coat
(homozygous dominant, BBW W) and one with a brown, curly
coat (homozygous recessive, bbww). What is the phenotype ratio
of their offspring (F 1 )? Now fill out another Punnett square,
crossing two of the offspring. What is the phenotype ratio of the
F 2 generation?
A3: F 1 -generation phenotype ratio is 3:1 black wavy to brown curly.
F 2 -generation phenotype ratio is 9:3:3:1 black wavy to black curly
to brown wavy to brown curly.
Figure 7.7
Q1: Boxers are far more inbred than poodles. Why does that
inbreeding make the former a better target for genetic studies of
disease than the latter?
A1: Since they are more inbred, boxers are more likely than
poodles to be homozygous for traits of interest.
Q2: Explain why a geneticist interested in finding a gene linked to
cancer would want to look at the DNA of senior golden retrievers
with and without cancer?
A2: To know whether there was a genetic difference between
goldens with and without cancer, the DNA of cancer-free dogs
would have to be known.
Q3: Obsessive-compulsive disorder (OCD) in humans is
characterized by obsessive thoughts and compulsive behavior,
such as pacing. Canine compulsive disorder (CCD) is characterized
by compulsive behavior such as “flank sucking,” sometimes seen
in Doberman pinschers. Would you predict that the medications
given to humans with OCD would decrease compulsive behaviors
in CCD dogs? Why or why not?
A3: One would predict that they might, since dogs and humans
share many genes and therefore OCD and CCD may share a
common genetic basis and be treatable by the same means.
Figure 7.8
Q1: What are the genotypes of a large and a small dog?
A1: LL (large) and ll (sma ll).
Q2: Is it possible to have a heterozygous large dog? Explain why or
why not.A2: No, because Ll is a medium-sized dog.Q3: Crossing a Great Dane and a Chihuahua is likely to be
unsuccessful, even though they are members of the same species
(and thus have compatible sperm and egg). Why is that? What are
some potential risks of such a cross?A3: It would be difficult for a Great Dane and a Chihuahua to
mate, because of the size difference. One risk would be that if a
female Chihuahua were to become pregnant this way, the pups
would be too large for her to carry safely to term.Figure 7.9
Q1: What are the possible genotypes (at both genes) of the black
dog? The yellow dog? The brown dog?A1: Black dog: BBEE, BbEE, BBEe, BbEe.
Yellow dog: BBee, Bbee, bbee.
Brown dog: bbEe, bbEE.Q2: Draw a Punnett square showing possible matings between
the black dog (assuming it is heterozygous at both genes) and the
yellow dog (assuming it is heterozygous at the B gene). List all
the possible phenotypes of their offspring. (See Figure 7.6 for an
example of a Punnett square made with two traits.)A2: There are three possible offspring phenotypes: black, yellow,
and brown.Q3: If you wanted the most variable litter possible, what colors of
Labrador retrievers would you cross?A3: If they were true-breeding (homozygous), you would cross
yellow and brown.Figure 7.10
Q1: The gene that brings about the pale Siamese body fur is also
responsible in part for the typical blue eyes of the species. What is
the term for this type of inheritance?A1: Pleiotropy.Q2: Siamese kittens that weigh more tend to have darker fur on
their bodies. Why might this be?A2: It could be that larger kittens have lower core temperatures,
thus allowing more melanin to be produced on their bodies.Q3: The Siamese cat pictured is called a “seal point” because it
has seal-colored (dark brown) extremities. Some Siamese cats
show the same color pattern, but the dark areas are of a lighter
color or even a different shade—for example, lilac point, red point,
blue point. What results would you predict if the experiments
described in the text (shaving the cat and then increasing or
decreasing temperature) were conducted on cats with these
color patterns?A3: The results should be the same, since even though the color of
the melanin has changed, presumably the mechanism for laying
down the melanin (based on temperature) has not.