1. In the cross TtSs X TtSs, what is the probability that a given offspring will have the
genotype TTSS? How about TTss? How about ttss?
In humans, cystic fibrosis is inherited as an autosomal recessive trait. Persons with the
genotype CC are normal. A person who is cc has the disease.
2. What is the phenotype of a person who is Cc? The parents of a child with cystic fibrosis
express disbelief because the disease has not occurred in the family of either parent. How
would you explain this to them? What are the genotypes of these parents? What are the
chances their next child will also have the disease?
3. If a woman with cystic fibrosis marries a normal man who does not have the recessive
allele, what are the possible genotypes of their children?
In humans, the ability to digest milk sugar (lactose, which is a double sugar) is based on an
autosomal dominant allele (L). Persons who are homozygous recessive (ll) cannot digest
lactose when they reach adulthood. Heterozygotes are phenotypically normal.
4. If two parents are both Ll, what are the odds that any child will be unable to digest
lactose when grown? If these parents have 8 children, how many will probably be able to
digest lactose as adults? Is it possible that all 8 would be unable to digest lactose? What
are the odds?
5. In the cross CcLl X CCLL, can a child be produced who has cystic fibrosis AND cannot
digest lactose? How about in the cross CcLl X ccll? How about the cross CcLl X CcLl?
Huntington's disease is a rare disease of the central nervous system resulting from a dominant
autosomal allele (H). The disease does not begin until about age 40. Persons who are hh are
normal. Persons who are Hh have the disease, while persons who are HH do not occur
6. A couple have 4 children. At age 43, the father develops Huntington's disease (the mother
is normal). What is the father's genotype? The mother's? What is a given child's chances
of having the disease? How many of the 4 children probably have the disease? Could all
4 have it? Explain.
7. Incomplete dominance describes a condition wherein more than one allele exists, but one
is not dominant over the other and the resulting phenotype is a mix. In snapdragons, RR
codes for red flowers, WW for white, and RW for pink. What would be the result of a
cross of two red-flowered plants? Two pink-flowered plants?
8. Some genes possess more than two alleles (multiple alleles). The gene for the ABO blood
group has 3 alleles. The allele designated as IA makes A antigen on red blood cell
membranes, IB makes B antigen, and i0 codes for no antigen at all. The IA and IB alleles
are co-dominant to each other, and dominant over i0. Write the 6 genotypes possible with
this system. What is the phenotype (blood type) of each genotype? What are the chances
that the cross IAIB X IAi0 will produce a type A baby? A type B baby? A type AB baby?
A type A mother with a type A husband brings home a type O baby from the hospital.
Could the hospital have made a mistake? Did the hospital make a mistake? What if the
baby were type B? What if the mother were type O and the baby was AB?
Mary's mother is type O and Mary is type B. George's blood type is A. Can George and
Mary have a baby with type O blood? How about type A? Type B? Type AB?
9. In humans a condition known as hypercholesterolemia is inherited on the basis of
incomplete dominance. The allele b calls for abnormally high levels of blood
cholesterol; the allele B calls for normal levels. Persons who are bb usually die of heart
attack by age three, Bb individuals have moderately high levels of cholesterol, and BB
individuals are phenotypically normal. In the case where two Bb individuals marry, what
is the probability that a given offspring will have the more serious form of the disease?
The milder form of the disease? Be completely normal? Could the cross Bb X BB
produce a severely diseased baby? How about a mildly diseased baby?
10. Sickle cell anemia is another incompletely dominant trait. The allele h results in the
production of abnormal hemoglobin; the allele H calls for normal hemoglobin.
Individuals who are Hh are mildly diseased. Those who are hh are severely diseased and
usually die in childhood. Two mildly diseased person are planning to marry. What
might a genetic counselor advise this couple as to the various probabilities concerning
sickle cell anemia in their children? What assumptions would you make concerning the
genotypes of the four parents of this couple?
11. If two animals, heterozygous for a single gene, are mated and have 200 offspring, about
how many will have the dominant phenotype?
12. Two long-winged flies were mated and the offspring included 77 with long wings and 24
with short wings. Is the short- winged condition dominant or recessive? What are the
genotypes of the parents?
13. If we cross a heterozygous tall pea plant with a dwarf pea plant, what proportion of the
F1 will be expected to be tall?
14. Suppose we cross one dwarf pea plant with another dwarf one. What proportion of the F1
will be expected to be dwarf?
15. In peas green pod color is dominant over yellow. A plant with green pods is crossed with
one having yellow pods. Some of the offspring have yellow pods. What was the genotype
of the green podded parent?
16. In rabbits, spotted coat (S) is dominant to solid color (s), and black (B) is dominant to
brown (b). A brown spotted rabbit is mated to a solid black one and all the offspring are
black spotted. What are the genotypes of the parents? What would be the appearance of
the F2 if two of these F1 black spotted rabbits were mated?
17. The long hair of Persian cats is recessive to the short hair of Siamese cats, but the black
coat color of Persians is dominant to the black-and-tan coat of Siamese. If a pure black,
long- haired Persian is mated to a pure black-and-tan, short-haired Siamese, what will be
the appearance of the F1? If two of these F1 cats are mated, what is the chance of
obtaining in the F2 a long-haired, black-and-tan cat?
18. In horses, black is due to a dominant gene B, chestnut to its recessive allele b. The
trotting gait is due to a dominant genet, pacing to its recessive allele. A homozygous
black trotter is crossed with a chestnut pacer. What sort of foals will result in several
crosses of this sort so far as coat and gait are concerned?
19. If two of the F1 animals from the cross in Problem 18 are crossed, what kinds of colts can
be produced? In what proportion will each phenotype be expected to appear?
20. A spotted rabbit and a solid-colored were crossed. They produced all spotted off-spring.
When these F1 rabbits were crossed, the F2 consisted of 32 spotted and 10 solid-colored
rabbits. Which of these characteristics is determined by a dominant gene?
21. What proportion of the F2 spotted rabbits on the above problem would be homozygous?
How many of the F1 solid- colored rabbits would be expected to be homozygous?
22. In summer squash, colorless fruit is due to a dominant gene W; colored fruit is due to its
recessive allele w. Dish-shaped fruit is determined by a dominant gene S, sphere-shaped
fruit by its recessive allele s. How many genotypes may squash plants have in regard to
color and shape of fruit? How many categories of phenotypes could be expected from
their genotypes? How many different homozygous genotypes are possible? What
phenotypic ratio would you expect from a cross between two doubly heterozygous
plants?