TLW 7e Chapter 10 Answers
Foundations of Genetics
Apply Your Understanding: Additional Genetics Problems
1. Silky feathers in chickens is a single-gene recessive trait whose effect is to produce
shiny plumage. If you had a normal-feathered bird, what would be the easiest cross to
perform to determine if a bird is homozygous or heterozygous for the silky allele?
A test cross would be the best mating to determine if a normal bird was homozygous or
heterozygous for the silky trait. If a cross of the normal bird with a bird that was silky
(homozygous recessive) produced no silky birds, the normal feathered bird was
homozygous dominate. If the cross produced some offspring that were silky, the normal
feathered bird was heterozygous for the silky trait. Approximately half of the offspring in
this cross would be expected to be silky.
2. Among Hereford cattle there is a dominant allele called polled; the individuals that
have this allele lack horns. Suppose you acquire a herd consisting entirely of polled
cattle, and you carefully determine that no cow in the herd has horns. Some of the calves
born that year, however, grow horns. You remove them from the herd and make certain
that no horned adult has gotten into your pasture. Despite your efforts, more horned
calves are born the next year. What is the reason for the appearance of the horned calves?
If your goal is to maintain a herd consisting entirely of polled cattle, what should you do?
Somewhere in your herd you have cows and bulls that are not homozygous for the
dominant gene "polled." Since you have many cows and probably only one or some small
number of bulls, it would make sense to concentrate on the bulls. If you have only
homozygous "polled" bulls, you could never produce a horned offspring regardless of the
genotype of the mother. The most expedient thing to do would be to keep track of the
matings and the phenotype of the offspring resulting from these matings and render
ineffective any bull found to produce horned offspring.
3. Brachydactyly is a rare human trait that causes a shortening of the length of the fingers
by a third. A review of medical records reveals that the progeny of marriages between a
brachydactylous person and a normal person are approximately half brachydactylous.
What proportion of offspring in matings between two brachydactylous individuals would
be expected to be brachydactylous?
Based on this information, one can conclude that brachydactyly is dominant trait. If it
were a recessive trait, only present in people who are homozygous recessive, the
inheritance pattern of offspring of a brachydactyl person and a normal person would be
less than half. It would only be half if all of the normal people were heterozygous for the
trait, and because the trait is rare, it is not likely that the entire population of normal
people carry the recessive gene. The proportion of offspring being brachydactylous
between a mating of two brachydactyl people would depend on the genotypes of the
parents. A brachydactyl person could be homozygous dominant or heterozygous and still
exhibit the trait. If at least one of the parents was homozygous dominant, all of the
offspring would be brachydactylous. If both parents were heterozygous, 3/4 of the
offspring would be brachydactylous and 1/4 would be normal.
4. Your instructor presents you with a Drosophila (fruit fly) with red eyes, as well as a
stock of white-eyed flies and another stock of flies homozygous for the red-eye allele.
You know that the presence of white eyes in Drosophila is caused by homozygosity for a
recessive allele. How would you determine whether the single red-eyed fl y was
heterozygous for the white-eye allele?
Breed the fly to one from the white-eyed stock. If half of the offspring are white-eyed, then
your fly is a heterozygote.
5. Hemophilia is a recessive sex-linked human blood disease that leads to failure of blood
to clot normally. One form of hemophilia has been traced to the royal family of England,
from which it spread throughout the royal families of Europe. For the purposes of this
problem, assume that it originated as a mutation either in Prince Albert or in his wife,
Queen Victoria.
a. Prince Albert did not have hemophilia. If the disease is a sex-linked recessive
abnormality, how could it have originated in Prince Albert, a male, who would have been
expected to exhibit sex-linked recessive traits?
b. Alexis, the son of Czar Nicholas II of Russia and Empress Alexandra (a
granddaughter of Victoria), had hemophilia, but their daughter Anastasia did not.
Anastasia died, a victim of the Russian revolution, before she had any children. Can we
assume that Anastasia would have been a carrier of the disease? Would your answer be
different if the disease had been present in Nicholas II or in Alexandra?
a. It could have originated as a mutation in his germ cell line.
b. Since their son Alexis was a hemophiliac, the disease almost certainly originated with
Alexandra; Nicholas II would have contributed only a silent Y-chromosome to Alexis’s
genome. There is a 50% chance that Anastasia was a carrier.
6. A normally pigmented man marries an albino woman. They have three children, one
of whom is an albino. What is the genotype of the father?
Albinism, a, is a recessive gene. The mother’s genotype would be aa therefore the
father’s genotype had to be Aa for this couple to produce an albino child.