Notes to Instructors
Chapter 15 The Chromosomal Basis of Inheritance
What is the focus of these activities?
Many students have difficulty solving genetics problems. This is especially true for
problems that include both autosomal and sex-linked genes.
What are the particular activities designed to do?
Activity 15.1 Solving problems when the genetics are known
This activity is designed to give students practice in solving autosomal genetics problems,
Activity 15.2 Solving problems when the genetics are unknown
The types of questions presented in Activity 15.1 provided students with practice solving
What misconceptions or difficulties can these activities reveal?
Activity 15.1
Question 2b: The answer is zero. Given that, many students automatically think this
question was designed to trick them. You may present the following scenario to point out
Questions 3 and 4: Many students have difficulty solving sex-linked genetics problems
because they try to solve them using the alleles alone. In other words, they do not indicate
Activities 15.2 and 15.3
Both of these activities are designed to give students practice in some of the actual types
of problems/situations that might be encountered by geneticists.
Answers
Activity 15.1 Solving Problems When the Genetics Are Known
Refer to Activity 14.3 and to Chapters 14 and 15 in Campbell Biology, 9th edition, to
complete this activity.
1. An organism that has the genotype AaBbCc is crossed with an organism that has the
genotype AABbCc. Assume all genes are on separate sets of chromosomes (that is,
they are not linked).
a. What is the probability that any of the offspring will have the genotype
AABBCC? (Hint: To get the answer, consider the six possible types of autosomal
crosses. Determine the individual probabilities of getting AA offspring from the
monohybrid cross. Then do the same to determine the probabilities of getting BB
offspring and CC offspring. Multiply these probabilities together.)
b. What is the probability that any of the offspring will have the genotype AaBbcc?
2. Consider the cross AaBbCcddEe ⫻AABBccDDEe.
a. What is the probability that any offspring will have the genotype AaBBCcDdEE?
b. What is the probability that any offspring will have the genotype AABBCCDDee?
Activity 15.1 93
3. In fruit flies (Drosophila melanogaster), the most common eye color is red. A
mutation (or allele) of the gene for eye color produces white eyes. The gene is
located on the X chromosome.
a. What is the probability that a heterozygous red-eyed female fruit fly mated with a
white-eyed male will produce any white-eyed offspring?
Xw+Xw⫻XwY ⫽heterozygous red female crossed with a white male
94 Activity 15.1
b. What is the probability that the mating in part a will produce any white-eyed
females?
The probability that the cross will produce any white-eyed females is 1⁄2. (Note:
c. What is the probability that this mating will produce any white-eyed males?
4. A heterozygous brown-eyed human female who is a carrier of color blindness
marries a blue-eyed male who is not color-blind. Color blindness is a sex-linked
trait. Assume that eye color is an autosomal trait and that brown is dominant over
blue. What is the probability that any of the offspring produced have the following
traits?
Bb X+Xcb (female) ⫻bb X+Y (male)
f. Blue-eyed, color-blind females 1⁄2 ⫻0 ⫽0
g. What is the probability that any of the males will be color-blind?
h. Why do males show sex-linked traits more often than females?
Males have only one X chromosome. The X chromosome carries many more
Activity 15.2 Solving Problems When the Genetics
Are Unknown
An understanding of Mendelian genetics allows us to determine the theoretical
probabilities associated with normal transmission of autosomal and sex-linked alleles
during reproduction. This understanding provides us with strategies for solving genetics
problems. In real-life situations, geneticists use these strategies to determine the genetics
behind specific phenotypic traits in organisms. They do this by conducting controlled
crosses of experimental organisms (e.g. Drosophila) or by analyzing family pedigrees (as
for humans).
Controlled Crosses
Two problems are presented below. In each, you are given:
Activity 15.2 95
For each of the problems, analyze the results in each cross and answer the questions that
follow.
96 Activity 15.2
Cross 1: Male Ambler ⴛFemale Wild Type
1. Problem One
Wild population Wild type Ambler Total
Male 33 17 50
Female 31 19 50
Total 64 36 100
Offspring Vial 1 Wild type Ambler Total
Male 29 24 53
Female 29 31 50
Total 58 55 113
a. What does cross 1 tell you about dominance versus recessiveness of the alleles?
Because you get equal numbers of both phenotypes, it is impossible to determine
b. What does cross 1 tell you about placement of the alleles on autosomes vs. sex
chromosomes?
Because the numbers of males and females in each phenotype is approximately
Cross 2: Female Ambler ⴛMale Wild Type
Offspring Vial 2 Wild type Ambler Total
Male 0 32 32
Female 32 0 32
Total 32 32 64
a. What does cross 2 tell you about dominance versus recessiveness of the alleles?
b. What does cross 2 tell you about placement of the alleles on autosomes vs. sex
chromosomes? (In your answer show the chromosomal genotypes for the parents
in this cross.)
a. What does cross 1 tell you about dominance versus recessiveness of the alleles?
These results indicate that Mt and Bt are codominant and the heterozygote is Tr.
b. What does cross 1 tell you about placement of the alleles on autosomes vs. sex
chromosomes?
Since all males and females are hybrid, the Mt and Bt alleles must be autosomal.
Activity 15.2 97
Cross 2: Monocle, Spinner Female ⴛTrifocal, Spinner Male
Mt ⫽Monocle; Bt ⫽Bifocal; Tr ⫽Trifocal; Sp ⫽Spinner; Sh ⫽Shing
Offspring
Vial 2
Mt, Sp Mt, Sh Bt, Sp Bt, Sh Tr, Sp Tr, Sh Total
Male 8 8 0 0 8 8 32
Female 23 0 0 0 15 0 38
Total 31 8 0 0 23 8 70
Cross 1: Bifocal, Spinner Female ⴛMonocle, Shiny Male
Mt ⫽Monocle; Bt ⫽Bifocal; Tr ⫽Trifocal; Sp ⫽Spinner; Sh ⫽Shing
Offspring
Vial 1
Mt, Sp Mt, Sh Bt, Sp Bt, Sh Tr, Sp Tr, Sh Total
Male 0 0 0 0 31 34 65
Female 0 0 0 0 34 38 72
Total 0 0 0 0 65 72 137
2. Problem Two
Mt ⫽Monocle; Bt ⫽Bifocal; Tr ⫽Trifocal; Sp ⫽Spinner; Sh ⫽Shing
Wild
Population
Mt, Sp Mt, Sh Bt, Sp Bt, Sh Tr, Sp Tr, Sh Total
Male 10 6 6 0 22 3 47
Female 19 1 9 1 20 4 54
Total 29 7 15 1 42 7 101
a. What does cross 2 tell you about dominance versus recessiveness of the alleles?
b. What does cross 2 tell you about placement of the alleles on autosomes vs. sex
chromosomes?
This indicates that the Sp and Sh alleles are on the X chromosome.
98 Activity 15.2
2 Sp females: 1 Sp males: 1 Sh males.
Analysis of Pedigrees
Analyze the pedigree and answer the questions that follow.
The diagram below shows a pedigree of three generations in a family. Black
circles/squares indicate persons with a genetic disorder. A square indicates a male and a
circle indicates a female. The two males in generation 1 are siblings.
Generation 1
Generation 2
Generation 3
AB
3. Looking only at the generation 2 offspring (of the two generation 1 brothers), what
can you say about the gene(s) controlling the genetic disorder? Is the disorder
caused by a gene that is dominant or recessive, autosomal or sex-linked?
The gene is most likely dominant. If it is dominant, the gene may be either
4. What additional information do you gain from examining the generation 3
offspring?
The mating between two affected individuals (lineage A – Generation 2) produces
Activity 15.3 How can the mode of inheritance be determined
experimentally?
Outline the experimental crosses you would need to make to solve each problem.
1. Three new traits have been discovered in a population of Drosophila:
The positions of the three genes on the chromosomes are unknown. Given two pure
breeding (homozygous) lines and using an initial cross of normal, normal, normal
females with tapping, single stripe, angular males, describe the appropriate genetic
experiments needed to establish whether any of these traits are caused by genes that are:
a. Autosomal or sex-linked
First, mate normal, normal, normal homozygous females with tapping, single
stripe, angular males. The phenotypes of the F1individuals will indicate which
Activity 15.3 99
100 Activity 15.3
b. Linked on the same chromosome or unlinked
If the genes are not linked, we expect the probability of offspring with a given set
of phenotypes—for example, normal, one stripe, angular—to be equal to the
Activity 15.3 101
2. A genetics student chose a special project involving a three-gene cross to check the
relative positions and map distances separating three genes in Drosophila that she
thought were all on the third chromosome. To do this, she mated Drosophila females
that were homozygous for the recessive genes cu (curled), sr (striped), and e(ebony)
with males that were homozygous for the wild type, cu⫹(straight), sr⫹(not striped),
and e⫹(gray). She then mated (testcrossed) the F1females with homozygous
recessive curled, striped, ebony males.
Here are the phenotypic results of the testcross:
straight, gray, not striped 820
curled, ebony, striped 810
straight, ebony, striped 100
curled, gray, not striped 97
straight, ebony, not striped 80
curled, gray, striped 90
straight, gray, striped 1
curled, ebony, not striped 2
Total 2,000
a. How are the three genes arranged on the chromosomes?
b. What evidence allows you to answer the question in part a?
If one of the genes was not linked, we would expect to see results similar to those
calculated in part b of question 1, where we looked at the results we would get if
c. If any of the genes are linked, how far apart are they on the chromosome? How
can you determine this?
First, look at the double crossovers to determine how the genes are arranged on
the chromosome. The offspring phenotypes that occur in the smallest numbers
are most likely to be the result of double crossovers. They are
102 Activity 15.3
