GENETICS BOOK PROBLEMS #2 (pg. 272-273; #1, 5, 6, 11, 13, 14, 17)
1. In some plants, a true-breeding, red-flowered gives all pink flowers when crossed with a white-flowered
strain: CRCR (red) X CrCr (White)  CRCr (Pink). If flower position (axial or terminal) is inherited as it is in
peas (see Table 14.1), what will be the ratios of genotypes and phenotypes of the F 1 generation resulting
from the following cross: axial-red (true-breeding) X terminal-white? What will be the ratios in the F2
generation?
A= axial (dominant) a= terminal (recessive)
R= red, r=white, Rr= pink (pink can also be notated as RW if you call the white allele “W”)
P= AARR x aarr  F1= 100% AaRr (genotype); 100% axial-pink (phenotype) to get F2, cross AaRr x AaRr
AR
AARR
AARr
AaRR
AaRr
AR
Ar
aR
ar
Ar
AARr
Aarr
AaRr
Aarr
Genotypic Ratio:
1/16 AARR
2/16 AARr
2/16 AaRr
4/16 AaRr
2/16 Aarr
aR
AaRR
AaRr
aaRR
aaRr
ar
AaRr
Aarr
aaRr
aarr
1/16 aarr
2/16 aaRr
1/16 aaRR
1/16 Aarr
Phenotypic Ratio:
3/16 Axial-red
6/16 (3/8) Axial-pink
3/16 Axial-white
1/16 Terminal-red
2/16 (1/8) Terminal-pink
1/16 Terminal-white
5. A man with type-A blood marries a woman with type-B blood. Their child has type-O blood. What are the
genotypes of these individuals? What other genotypes, and in what frequencies, would you expect in
offspring from this marriage?
Man= AO (IA IO)
Woman= BO (IB IO)
Child= OO (i i)
B
I
iO
Phenotypic ratios:
¼ AB
¼A
¼B
¼O
IA
AB
AO
IO
BO
OO
6. Phenylketonuria (PKU) is an inherited disease caused by a recessive allele. If a woman and her husband,
who are both carriers, have three children, what is the probability of each of the following?
a. All three children are of normal phenotype.
(¾ x ¾ x ¾) = 27/64
b. One or more of the three children have the disease.
1 – 27/64 = 37/64
Note: Remember that the probabilities of all possible outcomes always add up to 1.
c. All three children have the disease.
(¼ x ¼ x ¼) = 1/64
d. At least one child is phenotypically normal.
1 – 1/64 = 63/64
11. Imagine that a newly discovered, recessively inherited disease is expressed only in individuals with typeO blood, although the disease and blood group are independently inherited. A normal man with type-A blood
and a normal woman with type-B blood already have one child with the disease. The woman is now pregnant
for a second time. What is the probability that the second child will also have the disease? Assume that both
parents are heterozygous for the gene that causes the disease.
d= inherit disease D= no disease
Man: AODd
Woman: BODd
Child #1: OOdd
Child #2: 1/16 chance will have the disease
¼ chance OO x ¼ chance dd
13. In corn plants, a dominant allele I inhibits kernel color, while the recessive allele i permits color when
homozygous. At a different locus, the dominant allele P causes purple kernel color, while the homozygous
recessive genotype pp causes red kernels. If plants heterozygous at both loci are crossed, what will be the
phenotypic ratio of the offspring?
P: IiPp x IiPp
IP
Ip
iP
ip
IP
IIPP
IIPp
IiPP
IiPp
Ip
IIPp
IIpp
IiPp
Iipp
iP
IiPP
IiPp
iiPP
iiPp
Phenotypic ratio: 12 no color: 3 purple: 1 red (3/4 no color: 3/16 purple: 1/16 red)
ip
IiPp
Iipp
iiPp
iipp
14. The pedigree below traces the inheritance of alkaptonuria, a biochemical disorder. Affected individuals,
indicated here by the colored circles and squares, are unable to break down a substance called alkapton,
which colors the urine and stains body tissues. Does alkaptonuria appear to be caused by a dominant allele or
by a recessive allele? Fill in the genotypes of the individuals whose genotypes can be deduced. What
genotypes are possible for each of the other individuals?
Recessive allele
George: Aa
Arlene: aa
Sandra: A- (AA or Aa)
Tom: aa
Sam: Aa
Wilma: aa
Ann: Aa
Michael: Aa
Daniel: Aa
Alan: Aa
Tina: A- (AA or Aa)
Carla: aa
Christopher: A- (AA or Aa)
17. In mice, Black (B) is dominant to white (b). At a different locus, a dominant allele (A) produces a band of
yellow just below the tip of each hair in mice with black fur. This gives a frosted appearance known as agouti.
Expression of the recessive allele (a) results in a solid coat color. If mice that are heterozygous at both loci
are crossed, what is the expected phenotypic ratio of their offspring?
Cross: AaBb x AaBb
AB
Ab
aB
ab
AB
AABB
AABb
AaBB
AaBb
Phenotypic ratio: 9 agouti: 3 solid black: 4 white
Ab
AABb
AAbb
AaBb
Aabb
aB
AaBB
AaBb
aaBB
aaBb
ab
AaBb
Aabb
aaBb
aabb