AP Biology: Notes and Practice
Fertilization and Inheritance
Scientists have discovered a very simple eukaryotic organism that reproduces by sexual reproduction. Its cells
contain only 2 chromosomes: 1 from its mother (solid line) and 1 from its father (dotted line). The
chromosomes are homologous to one another and contain the gene for cell color, either pink (P) or purple (p).
Using the diagrams below, we will predict what type of offspring two parents will produce, based on the alleles
contained on their chromosomes.
Male
Female
Cell from the testes
P
Cell from the ovaries
p
P
p
1. Draw the cell (or cells) that will be present at the following points of meiosis, showing the correct
arrangement of chromosomes. Continue to represent chromosomes from the mothers as solid lines and
chromosomes from the fathers as dotted lines.
Male
Female
After S phase
After S phase
During metaphase of Meiosis I
During metaphase of Meiosis I
During metaphase of Meiosis II
During metaphase of Meiosis II
After cytokinesis of Meiosis II
After cytokinesis of Meiosis II
AP Biology: Notes and Practice
2. The cells that result from meiosis (the cells you drew in your “after cytokinesis of Meiosis II” box) are
called gametes; male gametes mature into sperm while female gametes mature into ovum (egg). One
Sperm cell joins with one egg cell during sexual reproduction in a process called fertilization to form a
zygote.
a. Are the sperm and egg cells that you drew in question 1 haploid or diploid? Explain your answer.
b. What is the ploidy (haploid or diploid) of the zygote? Explain your answer.
c. What would the ploidy of the zygote be if egg and sperm were produced by mitosis rather than
meiosis?
d. Different zygotes can form depending on which sperm fertilizes which egg. If the male and female
described in question 1 mated to produce an offspring, what are the possible zygotes that could form
from their mating? Draw these below. (Hint! There are three)
3. The genotype of an organism is the combination of alleles contained in its cell for a specific trait. In the
case of our parent cells in question 1, their genotype would be written as “Pp” because they each contain
one allele for pink color and one allele for purple color. This genotype is referred to as heterozygous,
because the alleles in the cell are different (hetero-) from one another.
a. Determine the genotype for the zygotes that you drew in question 2d.
b. Determine whether each of the genotypes is heterozygous or not. If they are not, use your
knowledge of prefixes to determine the term for a genotype of a cell that contains two of the same
alleles rather than two different ones.
AP Biology: Notes and Practice
4. The phenotype of an organism is the trait that is expressed, based on the genotype.
a. What are the two phenotypes for cell color in this problem?
b. To determine the phenotype of an organism, you must refer to the genotype. If an organism in this
problem had a genotype “PP” its phenotype would be pink; if it had a genotype “pp” its phenotype
would be purple. In the case of a heterozygous genotype (“Pp”), the dominant allele masks the
recessive allele. The dominant allele is usually indicated by a capital letter, while the recessive allele
is indicated by a lowercase letter. Based on this information, which allele is dominant and which is
recessive in this example?
c. Using your answer to 4b, determine the phenotype of the parent cells AND each of the zygotes in
question 2d.
5. Though many different zygotes can form, only one can result from a mating event. The laws of statistics
can help us predict the chances of certain genotypes and phenotypes appearing in the zygote.
a. Based on your “After cytokinesis of Meiosis II” drawing for the male cell in question 1, what is the
probability that a sperm with a dominant allele for color will be used to create a zygote?
b. What is the probability that a sperm with a recessive allele for color will be used to create a zygote?
c. Based on your “After cytokinesis of Meiosis II” drawing for the female cell in question 1, what is
the probability that an egg with a dominant allele for color will be used to create a zygote?
d. What is the probability that an egg with a recessive allele for color will be used to create a zygote?
6. Use the values identified in question 5 to calculate the following. Show your work.
a. What is the probability that a sperm with a dominant allele AND an egg with a dominant allele will
join to form a zygote (PP)?
b. What is the probability that a sperm with a recessive allele AND an egg with a recessive allele will
join to form a zygote (pp)?
AP Biology: Notes and Practice
c. What is the probability that a sperm with a dominant allele joins with an egg with a recessive allele
OR a sperm with a recessive allele joins with an egg with a dominant allele to form a zygote (Pp)?
7. Use the values identified in question 6 to calculate the following.
a. What is the probability that the zygote will have a pink phenotype? Explain your reasoning.
b. What is the probability that the zygote with have a purple phenotype? Explain your reasoning.
c. Based on the genotypes of the two parents in this problem, which phenotype is the zygote most
likely to be? Explain your reasoning.
PRACTICE: For each of the following examples, determine the genotypic and phenotypic probabilities.
Example 1:
A homozygous dominant brown mouse is crossed with a heterozygous brown mouse (tan is the recessive color).
Example 2:
Two heterozygous white (brown fur is recessive) rabbits are crossed.
AP Biology: Notes and Practice
Example 3:
In purple people eaters, one-horn is dominant and no horns is recessive. A heterozygous purple people eater is
crossed with a purple people eater that does not have horns
Example 4
In humans, the brown-eye (B) allele is dominant to the blue-eye allele (b). Two heterozygotes mate.
Example 5:
In humans the allele for albinism is recessive to the allele for normal skin pigmentation. Two heterozygotes
mate.
AP Biology: Notes and Practice
CHALLENGE!
If the child in example 5 is normal, what is the chance that it is a carrier (heterozygous) for the albino allele?
Explain your reasoning.
For examples 6 and 7, determine the most likely genotypes of the parental organisms
Example 6:
In pea plants, the trait for tall plants is dominant to the trait for short plants. Two plants are testcrossed and 296
of the plants are tall while 104 of the plants are short. What are the most likely genotypes of the parent plants?
Example 7:
In guinea pigs, the trait for curly hair is dominant to the trait for straight hair. Two guinea pigs mate for life and
23 of their offspring have curly hair while 27 of their offspring have straight hair. What are the most likely
genotypes of the parent guinea pigs?