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MONOHYBRID CROSSES
Dominant- Recessive
Steps in solving problems:
1. Determine symbols for each allele.
2. Write the genotype for each parent.
3. Determine the gametes that each parent can produce.
4. Make a punette square and combine gametes.
5. Answer the question!
1. A homozygous plant for tall is crossed with one homozygous for dwarf. Tall is dominant over
dwarf.
a. What will be the genotypic and phenotypic proportions of the F1
b. Cross to F1 to find the F2
c. A cross between the F1 and its tall parent
d. A cross between the F1 and its dwarf parent?
2. A tall plant crossed with a dwarf one produces offspring of which about 1/2 are tall and 1/2 are
dwarf. What are the genotypes of the parents?
3.
4. In poultry, rose comb is dominant over single comb. A farmer believes that some of his rosecombed chickens may carry a factor for single comb. Can you suggest a method for finding out
which chickens are heterozygous?
5. Lack of pigment in the human body is an abnormal recessive trait called "albinism". Using "N"
and "n" to represent the dominant (normal) allele and the recessive (albino) allele respectively,
three genotypes and two phenotypes are possible, list them.
6. A man and a woman plan to marry and wish to know the probability of their having any albino
children. What would you tell them if:
a. both the man and the woman are normally pigmented, but each has one albino parent.
b. the man is an albino; the woman is normal but her father is an albino
c. the man is an albino, and there has never been any albinism in the woman's family.
7. An extra finger in humans (polydactylism) is rare but is due to a dominant gene. When one
parent is normal and the other parent has an extra finger but is heterozygous for the trait, what is
the probability that their first child will be normal? Their second child? Their third child?
8. A dominant gene "G" is responsible for the gray body color of fruit flies: its recessive allele
"g" produces black body color. A cross of a gray female with a black male produces 52 black and
58 gray in the F1. If the gray F1 females are crossed to their black F1 brothers, what genotypic
and phenotypic ratios would be expected in the F2?
Blood Typing
9. Write all the possible genotypes for these blood types:
A
B
AB
O
10. Explain why it is impossible for a man with AB blood type to have a child with O blood
type.
11. A woman with A blood has 2 children. One with O and the other with type B blood. What
type blood must the father of these 2 children have? Also, give the genotypes that the mother
and the children have.
12. Suppose 2 newborn babies were mixed up in the hospital and there was a question as to what
parents the babies belonged to. From the following blood types, determine which baby belongs
to which parents. Also determine the genotype of each of the 6 people.
Baby 1
Baby 2
Ms. Brown
Mr. Brown
Ms. Smith
Mr. Smith
type O
type A
type B
type AB
type B
type B
Sex Linked (X-linked) Problems
13. In humans the gene for hemophilia is sex linked and recessive to the gene for normal
clotting. Diagram the genotypes of the parents of the following crosses and then determine the
expected phenotypic ratios for the crosses.
a.
b.
c.
d.
hemophilic woman X hemophilic man
normal heterozygous woman X hemophilic man
normal homozygous woman X hemophilic man
normal heterozygous woman X normal man
14. Colorblindness is a sex-linked recessive trait. A family has two colorblind and on normal
vision sons. What are the genotypes of the boys and their parents?