Chapter 8 Answers
Foundations of Genetics
Visual Understanding
Figure 8.10
Using the four gametes shown from the F1 generation, how many possible crosses are
there? Remember that a gamete type could cross with another of the same type (such as
RY X RY). Draw a Punnett square for each cross, and list the ratios of genotypes and
phenotypes for the F2 generation of that cross.
There are 10 possible crosses:
RY X RY
Ry X Ry
RY X Ry
Ry X rY
RY X rY
Ry X ry
RY X ry
rY X rY
rY X ry
ry X ry
This is because the cross RY X Ry is exactly the same as the cross Ry X RY, so the 16
possible pairings of gametes are really only 10 different types of crosses.
The results of the top row are easy to see:
RY X RY = RRYY, round, yellow seeds // homozygous dominant for both traits
Ry X Ry = RRyy, round, green seeds // homozygous dominant for shape, homozygous
recessive for color
rY X rY = rrYY, wrinkled, yellow seeds // homozygous recessive for shape, homozygous
dominant for color
ry X ry = rryy, wrinkled, green seeds // homozygous recessive for both traits
RY
Ry
RRYy
Round, yellow seeds
Homozygous dominant for
shape, heterozygous for color
RY
ry
RrYy
Round, yellow seeds
Heterozygous for both traits
RY
rY
RrYY
Round, yellow seeds
Heterozygous for shape,
homozygous dominant for
color
Ry
rY
RrYy
Round, yellow seeds
Heterozygous for both traits
Ry
ry
Rryy
Round, green seeds
Heterozygous for shape,
homozygous recessive for
color
rY
ry
rrYy
Wrinkled, yellow seeds
Homozygous recessive for
shape, heterozygous for color
Figure 8.19
Use Punnett squares to illustrate whether a type A female and a type B male can have a
child with type O blood.
A woman with type A blood could have the genotype of IA IA or the genotype IA i.
A man with type B blood could have the genotype of IB IB or the genotype IB i
If both of them were heterozygous for blood type, then the cross would be IA i. X IB i
IA
i
IB
IA IB
IBi
i
IAi
ii
They would have a 1 in 4 chance of having a child with blood type O.
Challenge Questions
Mendel
As Mendel struggled with understanding inheritance and formed his laws, how would the
outcome have been different if he had only chosen five characteristics to study—flower
position, pod shape, plant height, flower color, and seed color?
When Mendel proposed his two laws, he stated that the alleles for one trait sorted
independently of the alleles for a second trait. He did not know about chromosomes, and
the fact that there are many traits on each chromosome. Three of the five traits listed are
all found on one chromosome and the other two are found on a second chromosome. If
these were the only traits Mendel had studied he would have been very confused at his
results – the alleles for pod shape and plant height and flower position would generally
(except for crossover!) have all sorted exactly the same way through many generations,
and the alleles for seed color and flower color would have sorted together most of the
time. He probably would have been very confused about why some traits seemed to
associate and others to sort independently.
From Genotype to Phenotype
Kim and Su-Ling are doing fruit fly crosses in their biology class. Kim wants to test
whether the female they have is homozygous or heterozygous for red eyes by mating with
a red-eyed male. How should Su-Ling explain the difficulty to him?
Red eyes are dominant over white eyes. Since it is a sex-linked trait, the only allele the
male has to pass on is the allele for red eyes. Thus, in the first (F1) generation, all the
female offspring will have red eyes, whether the female is homozygous or heterozygous.
If she is heterozygous then they should see white-eyed males as long as many eggs were
laid. It would be much easier to use a white-eyed male for the cross. Then if female is
heterozygous about half of all of the F1 generation will have white eyes, both males and
females. If she is homozygous then they will all have red eyes.
Chromosomes and Heredity
Your biology class is collecting information on heredity. Michael realizes that he, along
with three of his four brothers, are color blind, but his four sisters are not, and neither are
his parents nor his grandparents. Can you help Michael understand what happened?
His mother carried the recessive allele for color blindness, as did her mother. She
passed the allele on to several of her children. In the girls, since they received a normal
allele from the father, they all have normal vision; only a genetic test would show which,
if any, of Michael’s sisters are carriers of the allele for color blindness. Four of the boys
only have the color blind allele, and so they express the trait; the fifth boy received the
mother’s normal allele.
Human Hereditary Disorders
Kuzungu is a child orphaned by civil war in her country and raised in a group home. She
has sickle-cell anemia, and type AB blood. Two couples who believe they are her
grandparents ask you, a genetic counselor, to help them determine the truth. What do you
suggest?
You do not have the parents, but you can determine the parents’ possible blood types by
working with the two sets of grandparents. Kuzungu is also homozygous for the sicklecell allele, which means that she received the allele from both of her parents. Therefore
at least one person in each set of grandparents would have to carry the sickle-cell allele.
These two traits, blood type and the presence of the sickle-cell allele, are not decisive, but
will be a first step at seeing if the couples could possibly be her grandparents.