Genetics Problems
1. Suppose a father of blood type A and a mother of blood type B have a child of type O. What are the possible blood
types of the mother and father?
Father: IAi
Mother: IBi
2. Suppose a father of blood type B and a mother of blood type O have a child of type O. What are the chances that
their next child will be blood type O? Type B? Type A? Type AB? K/U T/
type O: 50% or ½
Type B: 50% or ½
Type A: O%
Type AB: O%
I
3. Why is blood type inheritance an example of both codominance and complete dominance?
Alleles A and B are codominant – they are expressed equally (at the same time) when present in a cell (genotype
IAIB) , but both alleles A and B are dominant over allele O, when present in the heterozygous genotype (IAi or IBi)
4. Sickle-cell anemia is a condition in which the red blood cells of an individual can become shaped like the letter
“C.” This shape prevents the red blood cells from moving easily through blood vessels. It can result in the cells
clumping, blocking blood flow and causing pain, infection, and organ damage. The allele that causes sickle-cell
anemia is autosomal recessive (s), and the dominant allele can be represented by S.
(a) For the following families, determine the genotypes of the parents and offspring. When it is not possible to
decide which genotype an individual is, list both. C
(i)
Two normal parents have four normal children and one with sickle-cell anemia.
Parents both Ss
normal children: either Ss or SS
child with sickle cell anemia: ss
(ii) A normal male and a female with sickle-cell anemia have six children, all normal.
normal male: most likely SS, but could be Ss (sample size is too small to tell for sure);
female: ss
6 normal offspring: Ss
(iii) A normal male and a female with sickle-cell anemia have six children; three are normal, and three have sickle-cell
anemia.
Normal male: Ss
female with sickle cell anemia: ss
3 normal children: Ss
3 children with sickle cell anemia: ss
(b) Construct a pedigree chart for the families in (ii) and (iii).
ii)
ii)
5. Examine the pedigree charts in Figures 13 and 14. T/I
(a) Determine whether the mode of inheritance for the traits is
autosomal dominant or autosomal recessive. Explain.
Figure 13: autosomal recessive
Figure 14: autosomal dominant
(b) Label the genotype of each individual in the pedigree chart.
Assume that the dominant allele is A and the recessive allele is a.
Figure 13: I-1: Aa; I-2: Aa; II-1: Aa or AA; II-2: aa; II-3: Aa or AA
Figure 14:
I-1: Aa; I-2: Aa; II-1: aa; II-2: aa; II-3: aa; II-4: Aa
III-1: aa; III-2: Aa; III-3: Aa
6. Hairy ears is a rare condition that is sex linked. Let H be the dominant allele (non-hairy ears) and h be the recessive
allele (hairy ears). T/I
(a) Examine the pedigree chart in Figure 15. Determine if the condition
is X-linked or Y-linked.
X-linked, because Dad (I-2) would have to have the disorder as well as
the other 2 male offspring in generation II, if it was Y-linked
(b) Label all possible genotypes.
I-1: XHXh; I-2: XHY; II-1: XHXh; II-2: XHY; II-3: XHY; II-4: XhY; II-5:
XHXH or XHXh; III-1: XHXh; III-2: XHXh; III-3: XHY, III-4: XHY
7. The alleles for human blood types A and B are codominant, but both are dominant over the type O allele. The Rh
factor is separate from the ABO blood group and is located on a separate chromosome. The Rh+ allele is dominant
over the Rh-. Indicate the possible phenotypes from mating a woman with type O, Rh-, with a man with type A, Rh+.
The man is homozygous for type A and heterozygous for Rh factor.
man: IAIA, Rr
woman: ii rr
type A+: 1/2
type A-: 1/2
8. Why is sample size important in a genetic investigation? Larger sample size allows the actual outcome to more
closely match the predicted outcome (calculated probability)
9. Is it possible to have a trihybrid cross? Explain your thinking. T
Yes. You would simply be looking at the inheritance of 3 separate genes located on three separate chromosomes.
Each of the genes would have to assort randomly.
10. A test cross was carried out to determine if a bull displaying a dominant trait was heterozygous or homozygous.
The alleles for the trait are designated by the letters T and t. The offspring display both dominant and recessive
phenotypes. Draw a Punnett square that shows the alleles produced by both parents and the genotypes of the
offspring.
Bull: Tt
Cow: tt (always the homozygous recessive phenotype in test crosses)
Offspring: dominant phenotypes: Tt ; receesive phenotypes: tt
11. A child is born with a trait that is not present in either of the child’s biological parents. C A
(a) Is the child’s genotype for the trait homozygous recessive, homozygous dominant, or heterozygous?
How do you know?
homozygous recessive
(b) Is the mother’s genotype for the trait homozygous recessive, homozygous dominant, or heterozygous?
How do you know?
heterozygous
(c) Is the father’s genotype for the trait homozygous recessive, homozygous dominant, or heterozygous?
How do you know?
heterozygous
(d) The alleles for this trait can be represented with the letters B and b. Draw a Punnett square that shows this cross.
In the completed Punnett square, circle the genotype that represents the child described in the question.
Parents: Bb x Bb
child in the question: bb
other offspring: ¼ BB, ½ Bb
(e) What is the probability of these parents having another child with the same trait as the child described in the
question?
¼
12. In foxes, two alleles, Pp and P s, influence survival and hair colour. P pP p zygotes do not survive. P pP s
individuals have normal health and platinum coat colour. P sP s individuals also have normal health and
have a silver coat. What are the expected genotype and phenotype ratios of offspring from the cross of a
platinum and a silver fox?
platinum x silver
P pP s x
P sP s
F1 genotypes: ½ P p P s F1 phenotypes: ½ platinum coat
½ P sP s
½ silver coat
Multiple Choice
13. In humans, which blood type is the result of codominance?
(a) type A
(c) type AB
(b) type B
(d) type O
14. What term describes an individual who carries only one
type of allele for a gene?
(a) heterozygous
(c) homozygous
(b) monohybrid
(d) dihybrid
15. What pattern of heredity is demonstrated when, in
shorthorn cattle, a cross between a red bull and a white cow
results in roan calves?
(a) complete dominance
(c) incomplete dominance
(b) codominance
(d) X-linked dominance
16. In a cross, the P generation consists of two truebreeding
pea plants: one with purple flowers and one with white
flowers. How many different genotypes will be present in the
F1 generation?
(a) 1
(b) 2
(c) 3
(d) 4
17. What phenotype ratio is expected in a dihybrid
cross?
(a) 3:1
(b) 1:2:1
(c) 9:3:3:1
(d) 3:1:9
18. On a pedigree chart, what symbol indicates an affected
male?
(a) shaded square
(c) unshaded square
(b) shaded circle
(d) unshaded circle
19. How is the probability of two independent events both
occurring calculated? /U
(a) Multiply the probabilities of the events.
(b) Add the probabilities of the events.
(c) Subtract the probabilities of the events.
(d) Divide the probabilities of the events.
20. A pedigree chart shows two unaffected parents who have
two affected offspring and two unaffected offspring. What are
the genotypes of the parents for the trait?
(a) heterozygous
(c) homozygous dominant
(b) homozygous recessive
(d) impossible to determine