STUDY GUIDE
Meiosis and Punnett Squares
Heredity:
Heredity is the study of traits, or features, which are produced by the
action of genes and can be passed on from parents to offspring. An organism’s set
of traits is its phenotype. An organism’s set of genes is its genotype.
Study Question 1:
Sylvia has brown eyes. Is this part of her GENOTYPE or her
PHENOTYPE? (Circle one)
Study Question 2:
Lu inherited a nonfunctional copy of a growth factor gene from his father.
Is this part of his GENOTYPE or his PHENOTYPE? (Circle one)
The Genome:
A person inherits one set of the 23 human chromosomes from each parent at
fertilization, when the sperm and egg combine their chromosomes, making a
total of 46 chromosomes per cell. This total set of chromosomes is called the
genome. Taken together, the version of a chromosome from the father and the
version of the chromosome from the mother are called a homologous pair of
chromosomes. Homologous pairs of chromosomes generally carry the same
genes (pieces of genetic information that can be passed down), but may have
different versions of those genes, called alleles.
Study Question 3:
In class, we described the human genome as a shelf of books. In this
analogy, what do the books represent? How many of these "books" are there in
most of your cells? How many "books" would there be in a sperm cell? What is
represented by an individual sentence in one of those books?
STUDY GUIDE
Meiosis and Punnett Squares
Meiosis:
Like mitosis, meiosis is a form of eukaryotic cell division. Unlike mitosis,
meiosis is when when a single eukaryotic cell divides into four daughter cells
with half the number of chromosomes that the original cell had. These daughter
cells are called gametes, or sex cells, and become the sperm or egg.
During meiosis I, the first division, the homologous pairs of chromosomes
separate. This results in two daughter cells, each with 23 chromosomes. Each of
these chromosomes is from either the mother's set or the father's set; both
possibilities are equally likely.
During meiosis II, the sister chromatids of these 23 chromosomes
separate and the two daughter cells divide again, resulting in a total of four cells
at the end of meiosis.
Study Question 4:
Suppose a certain plant has 6 chromosomes, 3 from each parent. Draw one
of its cells in interphase before meiosis, the two daughter cells produced after
meiosis I, and the four daughter cells produced after meiosis II. Show
chromosomes clearly.
Fertilization and Punnett Squares:
Since a sperm cell and an egg cell each have 23 chromosomes, when they
combine, the resulting cell has 46 chromosomes. This is fertilization. Suppose
that the ability to taste a bitter chemical is controlled by a gene with two alleles, T
and t. People with at least one T allele can taste this chemical. If, upon
fertilization, an egg contributes a chromosome carrying the T allele and a sperm
contributes a chromosome carrying the t allele, the resulting offspring will have
both alleles and the genotype Tt. They would be able to taste the chemical.
To visualize the possible combinations of sperm, egg, and offspring
genotypes, we use a Punnett Square. Punnett squares are shown in your text in
chapter 9, but we will practice them here.
The steps to making a Punnett square are as follows:
1.
Labeling the rows and columns: Figure out what sperm and eggs the
parents might produce. For example, if the father has chromosomes with T and t
STUDY GUIDE
Meiosis and Punnett Squares
alleles, he could make sperm carrying either of those alleles. Label the columns
and rows with the sperm and egg genotypes.
Example: Mating of woman with genotype TT and man with genotype Tt:
Possible
Sperm
T
t
Possible
Eggs
T
T
If multiple chromosomes are involved, the problem is more complicated.
Suppose we're discussing two genes. One controls whether or not a person can
roll their tongue (alleles R and r). The other is the gene for tasting the bitter
chemical (alleles T and t). What if we have a man with genotype RrTt and a
woman with the genotype Rrtt?
Because meiosis produces daughter cells with only one version of each
chromosome, the man cannot pass down Tt sperm. His sperm must have either
the T allele or the t allele. This is called the law of segregation. The same goes for
the R and r alleles. So there are four genotypes of sperm the man can make: RT,
Rt, rT, and rt.
There is a 50/50 chance of passing down allele R, and a 50% chance of
passing down r. This chance is independent of the chance of passing down T or t,
since they're on separate chromosomes. This is called the law of independent
assortment.
What are the two possible egg genotypes the woman can make here? Make
a guess before reading on. Remember that she can pass down R or r, but always
passes down t, since her genotype is Rrtt. She doesn't have a T allele to pass.
Here are the rows and columns for a Punnett square for this mating:
RT
Possible Sperm
Rt
rT
rt
Possible
Eggs
Rt
Rt
rt
rt
(Note: When there are fewer possible egg/sperm for one parent than the
other, we repeat the genotypes to fill space. Here, we list each possible egg twice
(Rt, Rt, rt, rt), since there are 2 possibilities for the mother, but 4 for the father.)
Study Question 5:
Would the law of independent assortment still apply here if the R and T
genes were on the same chromosome? Why or why not?
STUDY GUIDE
Meiosis and Punnett Squares
2.
Fertilizing eggs with sperm: To simulate fertilization, combine the egg
and sperm genotypes to create offspring. For example, if a square is in a row
labeled with the genotype Ab, and a column with the genotype ab, the resulting
child would have the genotype Aabb; it would inherit a chromosome with the A
allele from the egg, a homologous chromosome with the a allele from the sperm,
a chromosome with the b allele from the egg, and a chromosome with the b allele
from the sperm. Note that we keep alleles on homologous chromosomes paired
up together. We write Aabb, not Abab.
Study Question 6:
Let's try this. Here's the first mating, with the genotypes from two
fertilizations (offspring) filled in. Fill in the other two offspring genotypes.
Possible
Sperm
T
t
Possible
Eggs
T
T
Tt
TT
Study Question 7:
Now we'll try the second mating. Some squares are filled in for you:
RT
Possible
Eggs
Rt
Rt
rt
rt
Possible Sperm
Rt
rT
rt
RRtt
rrTt
RrTt
rrtt
Dominant and Recessive Alleles/Traits:
If a person inherits the same allele of a gene from both parents, that person is
homozygous at that gene1. If a person inherits one allele from their father and a
different allele from their mother, they are heterozygous. For example, if a
person inherits a T from their mother and a t allele from their father, they are Tt
heterozygous. If, on the other hand, they inherit two t alleles, they are tt
homozygous.
A trait is recessive if it requires two copies of a certain allele in order to be
expressed. In other words, one cannot express the trait unless one is homozygous
1
Technically, a geneticist would say a person is homozygous at that locus, or location on the chromosome.
But we won’t go into too much detail there.
STUDY GUIDE
Meiosis and Punnett Squares
for the allele. Being unable to taste the bitter chemical is a recessive trait; you
need two copies of the t allele to be unable to taste it. If you have even one T
allele, you can taste it. This means that being able to taste the chemical is a
dominant trait. A trait is dominant if you only need one copy of an allele in order
to express that trait.
The dominant allele is the allele that leads to the dominant trait (in this case,
T). The recessive allele is the allele that leads to the recessive trait (in this case,
t). There are further subtleties of definition, but I will ignore them.
Study Question 7:
Three siblings have the following genotypes and phenotypes for the “tonguerolling” gene:
Genotype
Phenotype
Alice:
Rr
Can roll tongue
Bob:
rr
Can’t roll tongue
Carol:
RR
Can roll tongue
Is being able to roll your tongue a DOMINANT or a RECESSIVE trait? Is
being unable to roll your tongue a DOMINANT or a RECESSIVE trait?
Study Question 8:
Go back to the Punnett square you completed for the second mating (the
4x4 square). What fraction of the possible offspring have the following
phenotypes? (You made 16 possibilities, so the answer should be a fraction out of
16). I've given you two examples to start with:
Phenotype
Can roll tongue, can taste bitter
chemical
Can roll tongue, can't taste bitter
chemical
Can't roll tongue, can taste bitter
chemical
Can't roll tongue, can't taste bitter
chemical
Fraction of Possible
Offspring
6/16
2/16
In other words, the probability of a child from this mating having the
ability to both roll their tongue and taste the chemical is 6/16 = 37.5%, and so on.
This does not mean that if the couple somehow bore 16 children, 6 would have
both abilities. The Punnett square gives us probabilities, not the number of
fertilization events that actually occur!