PRACTICE GENETICS PROBLEMS TO BE COMPLETED
1. In tomatoes R = red fruit dominant to r = yellow fruit. If a tomato plant
homozygous for red fruit is crossed with one with yellow fruit, what will be the
genotypes and phenotypes in the F1 generation? Cross two members of the F1 and list
the genotypes, phenotypes and ration expected in the F2.
Which of the F2are
homozygous? Heterozygous?
2. In a man, the gene for non-blue eyes (brown, hazel, gray, etc.) is dominant over the
gene for blue eyes. If two non-blue eyed parents have four children, two non-blue eyed
and two blue eyed, what are the genotypes of the parents? How could you explain this
ratio?
3. Mr. and Mrs. Jones had just had a child and went to visit Mr. and Mrs. Smith who also
just had a child. Each couple wanted to show their child to the other couple. Mr. Jones
had blue eyes as did his wife. Mr. Smith had brown eyes and his wife had blue eyes.
Baby A had blue eyes and baby B had brown eyes. Which baby belonged to which
couple?
4. Brachydactyly is a rate human trait that causes a shortening of the fingers. Various
investigations have shown that approximately half the progeny of brachydactyly x
normal marriages are brachydactylous. What proportion of brachydactylous offspring
would be expected in matings between two brachydactylous individuals?
5. In Cocker Spaniels, solid coat color is dominant over spotted coat. Suppose a truebreeding, solid-colored dog is crossed with a spotted dog, and F1 dogs are interbred.
What is the probability that the first puppy will have a spotted coat? What is the
probability that, if four puppies are born, all of them will have a solid coat?
6. Sickle cell disease is caused by a genetic defect in the coding for the amino acid
sequence of hemoglobin, causing abnormal red blood cells. Heterozygous persons
appear normal, but when the blood is held under low oxygen concentration, their red
blood cells also become sickle-shaped. A young woman has a brother who died of sickle
cell disease, and she is concerned about the chance of the condition appearing in her
children. When blood samples are taken and placed under low-oxygen concentration, her
red blood cells become sickled, but those of her prospective husband remain normal.
What is the probability of their children having this disease?
7. In flowers with incomplete dominance use R = red flowers and r = white flowers. A
heterozygous pink flower is crossed with a white flower. In such a cross what % would
be red? What % pink? What % white?
8. We have spoken of alleles as being dominant or recessive as if there were no other
possibilities. But somehow one allele of a pair is not completely dominant over the
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other. In shorthorn cattle, for instance, when a red bull (RR) is crossed with a white cow
(rr), their heterozygous offspring (Rr) are neither red nor white, but roan (having
intermingled red and white hairs). If a roan bull (Rr) is crossed with a roan cow (Rr), the
calf has one chance in four of being white (rr), two chances in four of being roan (Rr),
and one chance in four of being red (RR).
a) Using symbols work out a cross of roan with red.
b) A farmer wants to establish a pure strain of roan cattle that breeds true. Why
is this impossible?
9. In poppies, R = red flowers, r = white flowers, but R is not dominant to r. Instead
R and r work together when they exist in the same cell and produce a “blend” in that Rr
produces pink flowers. If two pink flowers are crossed, what are the genotypes,
phenotypes and ratios expected? This is an example of non-dominance or incomplete
dominance..
10. A man’s maternal grandfather has type AB blood while all other grandparents are
type O. What are the chances of the man being: A? B? AB? O?
11. John has type O blood, his father type B, and his mother type A. What are the
genotypes of John’s parents?
12. Suppose two newborn babies are accidentally mixed up in the hospital and there was
a question of which baby belonged to which parents. From the following blood types,
determine which baby belongs to which parents:
Baby 1
Baby 2
Mrs. Brown
Mr. Brown
Mrs. Smith
Mr. Smith
Type O
Type A
Type B
Type AB
Type B
Type B
Now determine the genotype of each of the six people in the problem.
13. In guinea pigs, black coat is dominant to white coat and short hair is dominant to long
hair. A man was given a black short-haired guinea pig. In an effort to find out the
animal’s genotype, the man performed a test cross with a white long-haired animal
several times. The results of the crosses were as follows: 18 black short-haired and 21
black long-haired. Using B for black, b for white, S for short and s for long hair, what is
the genotype of the man’s black guinea pig? Of the test cross animal? Of the offspring?
14. Congenital deafness in man is due to the homozygous condition of either or both of
the recessive factors d and e. Both dominant factors D and E are necessary for normal
hearing. A deaf man married a deaf woman and all seven children have normal hearing.
What are the genotypes of the parents and children?
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15. In moose, H = antlered dominant to h = not antlered. If B = bearded dominant to b =
non-bearded in the same animal, what parents would a non-antlered, non-bearded moose
have (genotypically) is both parents were antlered and bearded?
16. A man’s maternal grandmother has normal vision; his maternal grandfather was
colorblind; his mother is colorblind; his father has normal vision. What are the genotypes
as to vision of the parents and grandparents mentioned? What type of vision has the man
himself? What type do his sisters have? If he should marry a woman genotypically like
one of his sisters, what type of vision would be expected in the offspring?
17. The bleeders disease, hemophilia, is sex linked. A man and his wife have 2 boys and
4 girls. One of the boys has hemophilia. The other boy and the girls do not. What are
the genotypes of the parents?
18. Why is it not possible for a male to heterozygous for colorblindness?
19. Color black and yellow in cats is sex linked. Possession of allele b produces black, y
produces yellow.The combination (by) produces tortoise shell or tricolor (black, yellow,
and white markings)
a) Cross a black male with a tortoise shell female and describe the phenotypes and ratios
expected.
b) Cross a black male with a yellow female and give genotypes, phenotypes and ratios
expected.
20. As Mendel discovered, gray seed color in peas is dominant to white. In the following
experiments, parents with known phenotypes but unknown genotypes produced the listed
progeny:
Progeny
Parents
(a)
(b)
(c)
(d)
(e)
gray x white
gray x gray
white x white
gray x white
gray x gray
Gray
White
82
118
0
74
90
78
39
50
0
0
Using the letter G for the gray gene and g for white, give the most probable
genotype of each parent.
21. In crosses (b), (d), and (e) of Problem 20, indicate how many of the gray progeny
produced by each cross would be expected to produce white progeny when self-fertilized.
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22. The following is a pedigree of a fairly common human hereditary trait (shaded
symbols):
I.
II.
III.
a) Indicate whether you think the gene differences causing this effect are
dominant or recessive, e.g., A (dominant) or a (recessive).
b) Designate the genotype of each individual in the pedigree, and indicate the
choice of genotypes if you think more than one genotype if possible for an
individual.
23. In four-o’clock plants, the allele for red flower color has an effect that is incompletely
dominant over the effect of the white color allele. If a cross between two plants produced
18 red, 32 pink, and 15 white plants, what are the phenotypes of the parents?
24. In cattle, the effect of the allele producing red coat color (R) in incompletely
dominant over the effect of the allele producing white coat color (r), the heterozygote
being roan-colored (Rr). On the other hand, the effects of alleles for the absence of horns
show complete dominance; HH and Hh are hornless or “polled”, and hh ir horned. On
the assumption that these two gene pairs assort independently:
a. What would be the phenotype of the F1 derived from a mating RRHH x
rrhh?
b. What would be the phenotypes and their proportions in an F2 derived from
crossing F1 x F1?
c. What would be the phenotypic proportions among the progeny derived
from crossing F1 individuals to the original white horned stock?
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25.Petrakis, Molohon, and Tepper investigated the inheritance of different types of
earwax (cerumen) among American Indians living in the United States and observed
that an individual could have one of the two types of earwax, dry or sticky, which were
inherited in the fashion shown by the following data:
Offspring
Parental
Combination
sticky x sticky
sticky x dry
dry x dry
Number of
Matings
Sticky
Dry
10
8
12
32
20
0
6
9
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On the basis of these data explain whether the inheritance of dry earwax is caused
by dominance, recessiveness, partial dominance, co-dominance, or some other
mode.
26.Some investigators found that matings between horses with pale cream coat color to
horses with chestnut coat color produced exclusively “palomino” individuals with
intermediate coat color. A number of matings between palominos themselves produced 19
pale cream, 21 chestnut, and 44 palominos.
a) Define gene symbols and suggest genotypes for the three coat colors.
b) If a palomino horse breeder wanted to eliminate pale cream individuals both as parents
and as offspring, but nevertheless wanted to obtain as many palominos as possible, what
breeding method would you suggest that he follow?
27.What phenotypes and ratios would you expect among the offspring of the following
crosses:
a. IA IA x ii ?
b. IA IA x IAIB ?
c. IA IA x IBi ?
d. IA IA x IAi ?
e. IAi x IAi ?
f. IAi x IB IA ?
g. IAi x ii ?
28. A common kind of red-green color blindness in humans is caused by the presence of
a sex-linked recessive gene c, whose normal allele is XC. Using these genes, what
are all the possible colorblind genotypes and their corresponding phenotypes in males
and in females?
29. (a) Can two colorblind parents produce a normal son? (b) A normal daughter?
(c) Can two normal parents produce a colorblind son? (d) A colorblind daughter?
(d) Can a normal daughter have a colorblind father? (e) A colorblind mother?
(f) Can a colorblind daughter have a normal father? (g) A normal mother?
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30. A colorblind woman marries a normal-visioned man. They have two children, a boy
and a girl:
a. what will be the genotype and phenotype of the boy?
b. what will be the genotype and phenotype of the girl?
31. A woman of A blood type and normal color vision produced five children as follows:
a.
b.
c.
d.
e.
male, A blood type, colorblind;
male, O blood type, colorblind;
female, A blood type, colorblind;
female, B blood type, normal color vision;
female, A blood type, normal color vision.
Of the two husbands that may have mated with this woman at different times, no. 1 had
AB blood type and was colorblind, and no. 2 had A blood type with normal color vision.
Which of these men is the most probable father in each case?
32. Mary has blue eyes (a recessive phenotype), but both her mother and father have
brown eyes. What are the chances that a child in this family will have blue eyes?
brown eyes?
33.
Red flowers are dominant to white flowers in peas. Inflated pods are dominant to
constricted pods. If a pea, which is heterozygous for red flowers and for inflated
pods, is crossed with a pea with white flowers and constricted pods, what will the
ratio of red-inflated to red-constricted to white-inflated to white-constricted be?
34.
The only member of Judy’s family who is color blind is her brother Jim. What is
Jim’s genotype? The father’s genotype? The mother’s genotype? What is
Judy’s genotype is she later has a color blind son?
35.
A child wonders if she is adopted and hence compares her blood type to those of
her “parents.” Her father is type AB and her mother is type O. What blood type
of the child would indicate that she might have been adopted?
36. A couple unable to conceive in any other way chooses to have a fertilization
accomplished in a test tube, followed by uterine implantation. The procedure is
successful, but after birth, the woman claims the child is not hers and the doctor
mixed up the ova. In the resulting maternity suit, the following data came to light.
The woman’s father was color blind, her blood type is B, and she suffers from
sickle cell disease. The man has blood type A, is not color blind, and is a sickle
cell carrier. The child, a male, has blood type AB, is color blind, and does not
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carry the sickle cell trait, either heterozygously or homozygously. Which of the
following is (are) true?
a.
b.
c.
d.
e.
The data from the blood typing prove the woman is the mother.
The color blindness data prove the woman is the mother.
The sickle cell trait data prove the woman is the mother.
The data indicates that the woman is not the mother.
The data are insufficient to determine the maternity of the mother.
37. About 70% of Americans get a bitter taste from the drug phenyl thiocarbamide
(PTC); the others do not. The ability to taste this drug results from a dominant
gene, while taste blindness is recessive. A normally pigmented woman who is
taste blind and has an albino father who is a taster. She marries an albino man
who is a taster, but who has a mother who is taste blind. What type of children
might this couple have?
38.
In garden peas, tall stem is dominant over short stem, green pods are dominant
over yellow pods, and smooth seeds are dominant over wrinkled seeds. Suppose
a homozygous short, green, wrinkled pea plant is crossed with a homozygous tall,
yellow, smooth one. What will be the appearance of the F1 generation? What
proportion of the F2 generation will have the tall, yellow, and wrinkled traits?
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