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A general strategy in solving such problems is:
a. Decide on symbols to represent the alleles, and which is dominant (if given).
b. Write out the genotypes as completely as possible all individuals.
c. See what's asked in the problem.
d. Take steps to find the answer (maybe Punnett square or backtracking).
e. Check to make sure the result is consistent with the conditions of the problem.
1. In the fruit fly, vestigial wings and hairy body are produced by recessive alleles on two different chromosomes. If a
vestigial winged, hairy male is crossed with a homozygous normal female, what ratios of phenotype and genotype do you
expect in the F1 and F2 generations?
2. When two short-tailed cats mate, about half of the kittens have short tails, a quarter have long tails, and a quarter have
What is the simplest genetic explanation for the inheritance of this trait?
3. In peas, the allele for tall plants (T) is dominant over the allele for short plants (t). On a separate chromosome, the allele
for smooth peas (S) is dominant over the allele for wrinkled peas (s). Calculate phenotypic ratios for the following
TtSs x TtSs
TtSs x ttss
ttSs x Ttss
TTss x ttSS
4. In roses, flower color is determined by red and white alleles, which are codominant. If a breeder wants to produce off-
spring that are all pink, what types of plants should he cross? Why aren't these pink plants true breeding?
5. What is the probability of having four children of the same sex?
6. Allele "b" is sex-linked, recessive, and lethal. Suppose a man marries a woman who is heterozygous for this gene. What
is the chance that the next son or the next daughter will carry the allele? What will be the ratio of surviving boys to
7. In the "ABO" blood system, alleles A and B are each dominant to O, but codominant with each other (remember that a
single person has two alleles, so the possible phenotypes are A, B, AB, and O). In a paternity suit, a woman with the A
blood type claims her son (type O blood) was fathered by a man with type AB blood. As a geneticist called to testify,
what do you tell the judge? What are the possible genotypes of the mother?
8. A woman, whose maternal grandfather had hemophilia (X-linked recessive), has parents who seem to be normal. She,
too, seems normal, as does her husband. What are the genotypes of the woman, her parents, her maternal grandparents,
and her husband? What are the chances that her first son will be normal?
9. The diagrams show 2 pairs of homologous chromosomes, with 5 genes marked out. (Capitals show dominant types and
lower case letters show recessive types).
Which gene(s) are homozygous? _________________.
Which are heterozygous? _______________________.