Notes to Instructors
Chapter 14 Mendel and the Gene Idea
What is the focus of these activities?
Meiosis is the basis for Mendel’s laws of segregation and independent assortment. If
What are the particular activities designed to do?
Activity 14.1 A Genetics Vocabulary Review
Activity 14.2 Modeling meiosis: How can diploid organisms produce haploid
gametes?
Activity 14.3 A Quick Guide to Solving Genetics Problems
Activity 14.4 How can you determine all the possible types of gametes?
The activities for Chapter 14 are designed to help students integrate their understanding
of meiosis and Mendelian genetics using modern terminology. Activity 14.1 provides a
What misconceptions or difficulties can these activities reveal?
Activity 14.2 Modeling Meiosis
Almost all students will quickly discover that this activity is not as easy as it first seems.
They will also discover that they have difficulty translating the information provided in
Notes to Instructors 79
Copyright © 2011 Pearson Education, Inc.
Most students think the Y chromosome is not shaped like an X when duplicated.
In general, students are unaware they have any of these problems until they are asked to
develop a dynamic model of the process.
Activities 14.3 and 14.4
Many introductory students have not developed good strategies for solving genetics
problems involving more than one gene. These activities are designed to help students
understand that if the genes involved are not linked, solving genetics problems one gene
at a time is generally the easiest and most accurate method.
80 Activity 14.1
Copyright © 2011 Pearson Education, Inc.
Answers
Activity 14.1 A Genetics Vocabulary Review
Mendel did not know anything about chromosomes, genes, or DNA. Because modern
genetics uses vocabulary that assumes students today understand these ideas, it’s helpful
to review some key terms.
Match each commonly used genetics term with its appropriate definition or
example.
Terms Definitions and Examples
eheterozygous a. Blue-eyed blonde mates with brown-eyed brunette
bhomozygous b. BB or bb
* Note: Though it is true that a gene can code for a polypeptide, it is important to
remember that not all genes code for polypeptides. Some code for mRNAs that produce
polypeptides, but others code for other forms of RNA—for example, rRNA and tRNA.
Activity 14.2 Modeling meiosis: How can diploid organisms
produce haploid gametes?
Integrate your understanding of meiosis (Chapter 13) and of basic Mendelian principles
Activity 14.1 81
Building the Model
Working in groups of three or four, construct a dynamic (claymation-type) model of
meiosis for the organism described on the next page. You may use the materials provided
in class or devise your own.
What genetic and chromosomal traits does your organism have?
1. Your individual is male/female (choose one). Females are XX and males are XY.
For simplicity, assume that the individual is diploid with 2n⫽6, including the sex
chromosomes. On one pair of autosomes (the nonsex chromosomes), the individual
is heterozygous for hair color (B⫽brown and dominant, b⫽blonde and recessive).
On another pair of autosomes, the organism is heterozygous for hair structure (C⫽
curly and dominant, c⫽straight and recessive). Assume further that the individual’s
mother was homozygous dominant for both traits and the father was homozygous
recessive for both.
a. Is your individual’s hair curly or straight? Brown or blonde?
b. What did the individual’s mother’s hair look like? What did the father’s hair look
like?
The individual’s mother was homozygous dominant and therefore had curly,
c. What chromosomes and alleles were in the egg and the sperm that gave rise to
your individual?
The egg contained an X chromosome, a number 1 chromosome with a brown
What does the nucleus contain?
To answer this question, develop a model of a cell from your individual.
• Use chalk on a tabletop or a marker on a large sheet of paper to draw a cell’s
82 Activity 14.2
When developing and explaining your model, be sure to include definitions or
descriptions of all these terms and structures:
What are the products of meiosis?
2. From a single sex cell going through meiosis, how many daughter cells are
produced?
3. For your model organism or individual (defined in question 1), how many different
kinds of gametes can be produced from a single cell undergoing meiosis? (Assume
no crossing over occurs.)
4. Your individual is heterozygous for two genes on separate pairs of homologous
chromosomes. His/her genotype is CcBb. Given this information alone, how many
Activity 14.2 83
diploid
2n/n
chromosome
chromatid
chromatin
autosome
crossing over
synapsis
recessive allele
dominant allele
spindle fibers
nuclear membrane
nucleolus
phenotype
heterozygous
different kinds of gametes could this individual produce? (Again. assume no
crossing over occurs.)
A heterozygous individual like this could, on average, produce four different kinds
5. Compare your answer to question 4 with your answer to question 3. How do the
numbers of different kinds of gametes in your answers compare? Explain any
difference.
Any single cell going through meiosis (no crossing over) produces only two types of
84 Activity 14.2
14.2 Test Your Understanding
What aspect(s) of meiosis account(s) for:
1. Mendel’s law of segregation?
Mendel’s law of segregation states that although each organism contains two traits
2. Mendel’s law of independent assortment?
Mendel’s law of independent assortment states that the pairs of traits that control
Activity 14.3 A Quick Guide to Solving Genetics Problems
Over the years, rules have been developed for setting up genetics problems and denoting
genes and their alleles in these problems. This activity provides a quick review of some of
these rules. After you have read through all of this material, complete Activities 14.4,
15.1, and 15.2.
Basic Assumptions to Make When Solving Genetics Problems
1. Are the genes linked?
2. Are the genes sex-linked?
Similarly, if the problem does not (a) indicate that the genes are sex-linked (that is, on the
3. Is there a lethal allele?
If a gene is lethal, then you should assume that the offspring that get the lethal allele (if
4. Are the alleles dominant, recessive, or neither?
Unless the problem states otherwise, assume that capital letters (BB, for example) designate
Activity 14.3 85
5. How are genotypes written?
Assume a gene for fur color in hamsters is located on the number 1 pair of homologous
autosomes. Brown fur (B) is dominant over white fur (b). The genotype for fur color can
6. What information do you need to gather before trying to solve a genetics
problem?
Before trying to solve any problem, answer these questions:
a. What information is provided? For example:
• What type of cross is it? Is it a monohybrid or dihybrid cross?
Solving Genetics Problems
1. What is a Punnett square?
Punnett squares are frequently used in solving genetics problems. A Punnett square is a
device that allows you to determine all the possible paired combinations of two sets of
characteristics. For example, if you wanted to determine all the possible combinations of
red, blue, and green shirts with red, blue, and green pants, you could set up this Punnett
square:
Activity 14.3 87
Shirts
Red shirt Blue shirt Green shirt
Similarly, if you wanted to determine the probability of a male (XY) and a female (XX)
having a son or a daughter, you would first determine the possible gametes each could
produce and then set up a Punnett square to look at all the possible combinations of male
and female gametes. Here, meiosis dictates that the female’s gametes get one of her X
chromosomes or the other. In the male, the gametes get either the X chromosome or the
Y. As a result, the Punnett square would look like this:
2. If you know the parents’ genotypes, how can you determine what types of
offspring they will produce?
a. Autosomal genes: For an autosomal gene that has the alleles Aand a, there are
three possible genotypes: AA, Aa, and aa.
88 Activity 14.3
AA
X
Aa
AA Aa
A
AaAaAa
AA Aa
A
A
a
a
a
AA Aa
Aa aa
Aa aa
Aa aa
Aa
X
Aa aa
X
Aa
**
All possible combinations of matings and offspring for two individuals carrying the
autosomal gene with alleles Aand aare shown in the figure below.
If you know how to solve these six crosses you can solve any problem involving one or
more autosomal genes.
* Note: If you take sex into account there are actually nine possible combinations of
matings:
Female genotypes
Male genotypes AA Aa aa
b. Sex-linked genes: For sex-linked genes that have two alleles, e.g., w+ and w,
females have three possible genotypes: Xw+Xw+, Xw+Xw, and XwXw. Males have
only two possible genotypes: Xw+Y and XwY. All the possible combinations of
matings and offspring for a sex-linked trait are listed in the next figure. If you
All possible combinations of matings for two individuals with a sex-linked gene are
shown in the figure below. Fill in the Punnet squares to determine all possible
combinations of offspring.
c. Multiple genes: Remember, if genes are on separate chromosomes, then they
assort independently in meiosis. Therefore, to solve a genetics problem involving
multiple genes, where each gene is on a separate pair of homologous
chromosomes:
Example:
What is the probability that two individuals of the genotype AaBb and aaBb will have any
aabb offspring?
To answer this, solve for each gene separately.
Activity 14.3 89
X
w+
X
w
X
X
w+
YX
w+
X
w
X
X
w
Y
X
w
X
w
X
X
w+
Y
X
w+
X
w+
X
w+
X
w
X
w+
X
w+
X
w
X
w+
X
w
X
w
X
w
X
w+
YX
w
Y
Y
X
w
Y
X
w+
Y
X
w+
X
w
X
w
X
w
X
w+
YX
w
Y
X
w+
X
w
X
w+
X
w
X
w
YX
w
Y
Ova
Sperm
Activity 14.4 How can you determine all the possible types of
gametes?
To solve genetics problems in which genotypes are given, you must first know what types
of gametes each organism can produce.
1. How many different kinds of gametes can individuals with each of the following
genotypes produce?
a. AA 1 kind of gamete ⫽A
b. aa 1 kind of gamete ⫽a
2. Based on your answer in question 1, propose a general rule for determining the
number of different gametes organisms like those described in question 1 can
produce.
Number of different kinds of gametes ⫽2n, where n⫽number of heterozygous
90 Activity 14.4
3. Two individuals have the genotypes AaBbCcDd.
a. How many different types of gametes can each produce?
2n⫽24⫽16 different kinds of gametes
Activity 14.4 91
b. What are these gametes?
One way of figuring this out is to take two genes at a time.
c. You set up a Punnett square using all the possible gametes for both individuals.
How many “offspring squares” are in this Punnett square?
If you used 16 gametes across the top of the Punnett square and 16 down the
side, you have 256 offspring possibilities in this table.
d. If you completed this Punnett square, how easy would it be to find all the
“offspring squares” that contain the genotype AaBBccDd?
e. Given that the genes are all on separate pairs of homologous chromosomes, what
other method(s) could you use to determine the probability of these individuals
having any offspring with the genotype AaBbccDd?
You could handle each gene pair as a separate cross. For example, the cross