Part B Genetic problem sheet-Monohybrid Crosses (complete and incomplete dominance)
In #1-7 of the following problems, consider the cross given to the P generation. Then state the kinds of
gametes and F1 generation (including phenotype) which they produce. Also note the kinds of gametes
derived from the F1. Construct the F2 checkerboard. Name the F2 phenotypes and show, by a ratio or
table, the relative frequency of the outward appearance. Dominance is complete except where
otherwise indicated. In problems 1-6, the F1 generation is crossed with its own type.
1. Maïze:
Tall variety (DD) x dwarf variety (dd)
2. Human: Dark brown (BB) x blue eyes (WW). Dominance is incomplete in this case.
3. Four o’clocks (Marabilis jalopa)flowers may be either RED, white or pink in colour. Make the
following cross: White (WW) x red (RR). Dominance is incomplete in this case also.
4. Human: short limbs (ll) x normal limbs (LL)
5. Human nasal ridge is high, medium or normal. Make the following cross: High (QQ) x normal (NN) .
Dominance is incomplete in this case.
6. Dog: Stub tail (SS) x long tail (ss)
7. Poultry :
P generation: Broody (BB) x non-broody (bb). The F1 generation thus produced is then back-crossed
with non-broody (bb). What is the expected F2 phenotype ratio?
8. Swine: A mule-footed male (Mm) is crossed with a mule-footed female (Mm). Consider the result of
this cross the F1 generation. Calculate the expected F2 phenotype ratio. Swine having mm are twofooted.
9. Rabbit: A rex-coated male (rr) is crossed with a normal-coated female (Rr). Consider this the P
generation. Calculate the expected F1 phenotype ratio.
10. Cat: A short-haired (normal) cat (Ll) is crossed with a long-haired angora (ll). Consider the result
of this union the F2 generation. Calculate the expected F3 phenotype ratio.
Part A-Practice problems: Complete dominance Monohybrid Cross
FOR ALL PROBLEMS, CREATE A
PUNNET SQUARE AND GIVE THE PHENOTYPE AND GENOTYPE RATIO
1. In pea plants, the allele for tallness is dominant (T) and shortness (t) is recessive. a) A homozygous
dominant (TT) is cross with a homozygous recessive (tt).
b) A heterozygous tall plant is crossed with another heterozygous tall plant.
c) A heterozygous tall plant and a short plant are crossed with each other.
2. Yellow pea pods (Y) are dominant over green pods (y). A heterozygote is corssed with a homozygous
dominant plant.
3 a)A homozygous cream colored mouse (dd) is crossed with a heterozygous (Dd) dark mouse. B) What are
the odds that this couple will have a cream colored baby? C) What are the odds of a dark mouse?
I4. n sheep, white is due to a dominant gene (W), black is due to its recessive allele (w). A white ewe mated
to a white ram produces a black lamb. How does this happen? What are the genotype and phenotype
ratios?
5. The polled (hornless) condition (P) is dominant over horned (p). A certain polled bull is crossed to 3 cows.
With cow A, which is horned, a polled cow is produced; With cow B, also horned, a horned cow is
produced; with cow C, with is polled, horned cow is produced. What are the genotypes of the three calves
and the four parents?
6. An albino is a person with no pigmentation in his skin, eyes or hair. Normal pigmentation is (A) is dominant
over albino (a). A normal pigmented man (whose mother was albino) marries a homozygous normal woman.
What would the expected appearances of the children be?
7. If the man in problem #8 married a normal woman (whose father was albino), what percent of the
children would be expected to be albino?
8. In chickens, yellow legs (Y) are dominant over white legs (y). A yellow legged rooster was crossed with a
white legged hen. Both kinds of offspring were produced. What are the genotypes of the parents and the
offspring?
9. A yellow legged rooster was crossed with a white legged hen. All fifteen of the offspring were yellow
legged. What is the genotype of the rooster?
10. In a cross between two white mice, in several litters, 12 out of 50 offspring were black. a. What color is
dominant? b. What are the genotypes of the parents?
c. What are the genotypes of the black offspring?
11. What would be the genotypes of both parents in problem #12 if all fifty offspring were white?
12. If one fruit fly is heterozygous (red) and the other is homozygous recessive (white), what percent of the
offspring would be expected to have white eyes?