Monohybrid Crosses
1. In Clarkia elegans plants, alleles for white flowers is recessive to the allele for orange
flowers. Pollen from a heterozygous orange flower is placed on the stigma of a white
flower. What is the expected ratio of phenotypes in the progeny?
2. Albinism in humans is inherited as a simple recessive trait. For the following
families, determine the genotypes of the parents and offspring. When two alternative
genotypes are possible, list both.
a) Two non-albino parents have four children, three normal and one albino.
b) A normal male and an albino female have six children, all normal.
3. The black and white pattern is dominant over the red and white pattern in Holstein
cattle. True breeding black and white Holstein cattle have the highest market vale. A
black and white bull was mated to many pure breeding black and white cows. The
calves sired by this bull with different cows (F1) were all black and white in
appearance. When pairs of his offspring were interbred (selfing of the F1), some pairs
produced all black and white offspring (F2), and some pairs produced black and white
offspring at a frequency of about 75% and red and white progeny at a frequency of
about 25%. This bull was soon removed from the stud lists of the Holstein cattle
breeders. Explain precisely why, using symbols in a Punnett Square.
4. In pea plants, tall (T) is dominant to dwarf (t). Show the cross between a
heterozygous tail and a dwarf plant. What are the expected phenotypes and in what
proportion?
5. In pea plants, inflated pods are dominant to constricted pods. Show the cross between
a true-breeding inflated pod plant with a true-breeding constricted pod plant. What
are the expected phenotypes and in what proportion?
6. If two constricted pod plants are crossed, what are the expected phenotypes of the
offspring and in what proportion?
7. In humans, tongue rolling is a dominant trait, those with the recessive condition
cannot roll their tongues. Bob can roll his tongue, but his mother could not. He is
married to Sally, who cannot roll her tongue. What is the probability that their first
born child will not be able to roll his tongue?
8. In goats, a recessive gene causes the goat to “faint” when they are startled. A farmer
breeds two goats (that have never fainted) and their first offspring faints two days
after its birth. What must the parent’s genotypes have been? Show the cross to prove
it.
9. In certain trees, smooth bark is dominant over wrinkled bark. Cross two trees that are
heterozygous for smooth bark. If there are 100 offspring produced, how many will
have wrinkled bark?
10. Imagine that in seals, the length of the whiskers has two alleles. The dominant allele
(L) codes for long whiskers and the recessive allele (l) codes for short whiskers.
a) What percentage of offspring would be expected to have short whiskers from the
cross of two long-whiskered seals, one that is homozygous dominant, and one that
is heterozygous?
b) If one parent seal is pure long-whiskered and the other is short-whiskered, what
percent of offspring would have short whiskers?
11. In Purple People Eaters, one horn is dominant and no horns is recessive. Draw a
Punnett Square showing the cross of a purple people eater that is hybrid for horns
with a purple people eater that does not have horns. Summarize the genotypes and
phenotypes of the possible offspring.
12. Consider the trait Speckled (S) in lizards, which is completely dominant to the other
allele at this locus, Plain (s). What is the genotype of a homozygous speckled lizard?
How many gamete genotypes can this lizard produce?
13. Two black dogs are mated many times over the years. Over their life span, they have
produced 31 black and 9 white offspring. Give the genotypes of parents and progeny,
explaining how you got your answer.
14. Achondroplasia is a form of dwarfism inherited as a completely dominant trait. Two
achondroplastic dwarves have a dwarf child; later, they have a second child who is
normal. What are the genotypes of the two parents in this mating? What is the
probability that their next child will be dwarf?
15. In humans, free earlobes (E) are dominant over attached earlobes (e). A heterozygous
free earlobed male marries a female with attached earlobes. What will be the possible
phenotypes and genotypes of the offspring?
16. A recessive mutant allele, black, causes a very dark body in Drosophila (fruit flies)
when homozygous. The dominant colour is described as grey. What F1 phenotypic
ratio is predicted when a black female is crossed to a grey male whose father was
black?
17. In a cross between a black and a white guinea pig, all members of the F1 generation
are black. The F2 generation is made up of approximately 3/4 black and 1/4 white
guinea pigs. Which allele is dominant? Determine the genotypes and phenotypes of
each generation.
18. If all of the offspring of a particular set of parents have Tt for their genotype, what is
the genotype of the parents?
19. A green seed of a pea plant is planted in a greenhouse. It grows, flowers, the flowers
are fertilized and when the fruits (next generation of peas) appear, all of the seeds in
its pods are yellow. What must have happened? Remember that green seed