Download Biology 162 Discussion section Week 8 Problems in Mendelian

Biology 162
Discussion section
Week 8
Problems in Mendelian Genetics
1. In roses, the allele for red flowers (represented by the letter A) is completely dominant to
the allele for white flowers (represented by the letter a). The three possible parental
genotypes are AA, Aa, and aa. There are six possible crossings that can be made between
these genotypes (for example, AA × AA, AA × Aa, etc.). Show the expected proportions
of the genotypes and phenotypes in the F1 offspring from all six possible crossings.
2. Suppose that the allele for red flowers was incompletely dominant to that of white
flowers, such that the heterozygotes were pink. With this assumption, show the expected
proportions of the F1 phenotypes from the crossings in problem 1.
3. Assume that white color is completely dominant to yellow color in squash. If pollen from
the anthers of a heterozygote white-fruited plant is placed on the pistil of a yellow-fruited
plant, what would be the expected proportion of all genotypes and phenotypes from this
cross?
4. In humans, brown eyes are usually completely dominant over blue eyes. Suppose a blueeyed man marries a brown-eyed woman whose father was blue-eyed. What proportion of
their children would you predict will have blue eyes?
5. If a brown-eyed man marries a blue-eyed woman and they have four children, all browneyed, can you be certain that the man is homozygous? If they had ten children, all browneyed, would that prove what the father's genotype is?
6. A brown-eyed man whose father was brown-eyed and whose mother was blue-eyed
married a blue-eyed woman whose father and mother were both brown-eyed. The couple
has a blue-eyed son. For which of the individuals mentioned can you be sure of the
genotypes? What are their genotypes? What genotypes are possible for the others?
7. In the fruit fly, Drosophila melanogaster, vestigial wings and hairy body are produced by
two recessive genes located on different chromosomes (not-sex-linked). The normal
alleles, long wings and hairless body are completely dominant. Suppose a vestigialwinged hairy male is crossed with a homozygous normal female. What types of progeny
would be expected? If the F 1 offspring from this cross are permitted to mate randomly
among themselves, what progeny would be expected in the F2 ? Show the proportions of
genotypes and phenotypes for each generation.
8. Given the genes described in above, suppose that a hairy female which is heterozygous at
the wing locus is crossed with a vestigial-winged male which is heterozygous at the body
hair locus. What will be the proportions of all genotypes in the F1 generation?
9. In Poinsettias, the allele for tall plants (T) is completely dominant over the allele for short
plants (t). The allele for lobed leaves (L) is completely dominant over the allele for
lobeless leaves (l). In the following cross: tall, lobed × short, lobed the following progeny
were found:
Number
tall, lobed
604
tall, lobeless
201
short, lobed
599
short, lobeless
198
What are the most probable genotypes for the parents?
10. Suppose there are 4 genes, all known to be located on different chromosomes. In the
following cross:
AaBBCcDd × AaBbCcdd
What will be the expected frequencies of offspring with the following genotypes?
AABBCCdd
aaBbccdd
AaBBccDd
AaBbCcDd
AABBCCDD
11. If a man with blood type B, one of whose parents had blood type O, marries a woman
with blood type AB, what would be the expected proportion of their children having each
blood type? Show the genotypes of the parents and their possible children.
12. Both Mrs. Smith and Mrs. Jones had babies the same day in the same hospital. Mrs.
Smith took home a baby girl, whom she named Sharon. Mrs. Jones took home a baby
girl, whom she named Jane. Mrs. Jones began to suspect, however, that the child had
been accidentally switched with the Smith baby in the nursery. Blood tests were made;
Mr. Smith was type A, Mrs. Smith was type B, Mr. Jones was type A, Mrs. Jones was
type A. Sharon was type 0, and Jane was type B. Had a mix-up occurred?
13. The white-eyed gene in Drosophila is sex-linked. The white-eyed allele is recessive to
the wild type allele (red eyes). A white-eyed female and a red-eyed male are crossed.
What would be the phenotypes and sexes of the F1 offspring? (Hint: in Drosophila, as in
humans, the male is the heterogametic sex).
14. In humans, a sex-linked recessive allele p is responsible for red-green color blindness. Its
dominant allele p+ is required for normal vision. A woman with normal vision whose
father had red-green color blindness marries a color-blind man. What is the probability
that their daughter will be color blind?
15. A man and his wife both have normal color vision, but a daughter has red-green color
blindness. The man sues his wife for divorce on grounds of infidelity. Can genetics
provide evidence supporting his case?
16. Suppose that a pigeon breeder finds that about one-fourth of the eggs produced by one of
his prize pairs do not hatch. Of the young birds produced by this pair, two-thirds are
males. The pigeon breeder suspects that a lethal allele may be involved. Describe a
possible explanation for these results. (Hint: in birds, the male is the homogametic sex,
and the female is heterogametic).
17. It is exceedingly difficult to determine the sex of very young chickens, but it is easy to
tell by visual observation whether or not they are barred. The barred pattern is inherited
as a sex-linked dominant. Set up a cross so that the sex of all chicks can be determined
when they hatch. (Remember the hint in problem 16).
18. Gray feather color in African Gray Parrots is determined by the dominant allele G. Birds
which are homozygous for the recessive allele g are orange in color. A second gene also
determines feather color in African Grays such that the birds which are homozygous for
the recessive allele r have red feathers, irrespective of the genotype at the G/g locus.
These two genes assort independently. Two double heterozygotes are mated together.
What would be the expected proportions of phenotypes in the next generation?
19. If the dominant gene K is necessary for hearing, and the dominant gene M results in
deafness no matter what other genes are present, what proportion of the offspring
produced by the cross kkMm × Kkmm will be deaf? (Assume that there is no linkage).
20. The cross-over frequency between linked genes A and B is 40%; between B and C, 20%;
between C and D, 10%; between C and A, 20%; and between D and B, 10%. What is the
sequence of the genes on the chromosome?
Problems in Population Genetics
21. What genetic factors must not be occurring for a Hardy-Weinberg equilibrium to exist?
22. (a) The frequency of an allele A is 0.90, and the frequency of the alternative allele a is
0.10. Calculate the expected genotypic frequencies at Hardy-Weinberg equilibrium?
(b) If two alleles have equal frequencies at a certain locus, what are the expected
genotypic frequencies at Hardy-Weinberg equilibrium?
23. In a population of 1000 individuals examined at a particular gene locus, there were 300
individuals with genotype AA, 500 individuals with genotype Aa, and 200 individuals
with genotype aa.
(a) Calculate the three genotypic frequencies for this locus.
(b) Calculate the allelic frequencies for the two alleles.
24. The M-N blood locus was examined in a sample population of 730 Australian
Aborigines. There were 22 individuals with blood type MM, 216 with type MN, and 492
with type NN.
(a) Calculate the genotypic frequencies for this locus.
(b) Calculate the frequencies of the M and N alleles.
25. Cystic fibrosis is a genetic disorder in homozygous recessives that causes death during
the teenage years. If 4 in 10,000 newborn babies have the disease, what are the expected
frequencies of the three genotypes in newborns, assuming the population is at HardyWeinberg equilibrium? Why is this assumption not strictly correct?