BIOL 150 Introduction to Biological Sciences I
Peer-Led Team Learning Module 8
The Chromosomal Theory of Inheritance
Module
8
Meiosis takes place in all organisms that reproduce sexually. Meiosis consists of two nuclear divisions: meiosis I and meiosis
II. Separating these nuclear divisions is an atypical interphase in which the cells do not grow and do not synthesize DNA.
This means that the final products of meiosis are four cells, each containing half the number of chromosomes (one from each
homologous pair) from each parent cell. Cells that contain only one of each homologous pair of chromosomes are haploid (n),
while cells that contain a homologous pair of chromosomes are diploid (2n). One advantage of meiosis in sexually reproducing
organism is that it prevents the chromosomes number from doubling with every generation when fertilization occurs.
Activity 8.1- Drawing Models of Chromosomes
Drawing models of chromosomes can help you to visualize how chromosomes change during cell division. There are many
ways to make simple drawings of chromosomes. Here are three of the most common:
1. Draw a model to represent a cell with two different chromosomes before and after the chromosomes are replicated. Use a
circle to represent the cell and one of the models above to represent the chromosomes. In your illustration label the sister
chromatids, location of a centromere, and the chromosomes.
2. Now use your model from above to represent two different homologous pairs of chromosomes before and after the
chromosomes are replicated. Label the homologous pairs, sister chromatids, non-sister chromatids.
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Activity 8.2- Illustrating the Process of Meiosis
The fruit fly, Drosophila melanogaster, has a total of eight chromosomes (four pairs) in each of its somatic cells. Somatic cells are
all cells of the body except those that will divide to form the gametes (ova or sperm). Review the events that occur in the
various stages of meiosis. Keep in mind that the stages of cell division were first recognized from an examination of fixed
slides of tissues undergoing division. On fixed slides, cells are captured or frozen at particular points in the division cycle.
Using these static slides, early microscopists identified specific arrangements or patterns of chromosomes that occurred at
various stages of the cycle and gave these stages names (interphase, prophase, and so on). Later work using time-lapse
photography made it clear that meiosis is a continuous process. Once division begins, the chromosomes move fluidly from
one phase to the next.
1. Assume you are a microscopist viewing fruit-fly cells that are undergoing meiosis. In each of the circles, draw what you
would expect to see if you were looking at a cell in the stage of meiosis indicated. If no circle is present, draw what you
would expect to see at the given stage. You must show two crossing-over events in your drawings and label the ploidy
level at each stage.
Meiosis I
Interphase
Metaphase I
Early Prophase I
Late Prophase I
Anaphase I
Cytokinesis
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Telophase I
Meiosis II
Prophase II
Metaphase II
Telophase II
Anaphase II
Cytokinesis
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Activity 8.3- Comparison Between Mitosis and Meiosis
1. Complete the following table by listing the events during each phase of mitosis and meiosis.
Interphase
Prophase
Metaphase
Anaphase
Duplicated
chromosomes line
up independently
along the
metaphase plate
Mitosis
Meiosis I
Meiosis II
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Telophase
and
Cytokinesis
Activity 8.4- Chromosomes Contain Genes
In sexually reproducing animals, genetic information is passed from the parents to offspring by means of haploid
gametes (egg and sperm) which, in most organisms, unite to form a diploid zygote. The zygote receives half of its
chromosomes from one parent and half from the other. The alleles that the parents pass on determine not only the
genotype of the offspring, but also its observable traits -- its phenotype. For example, a child could have the blood
type A, B, AB, or O depending on what alleles were inherited from the parents. Understanding the patterns of
hereditary transmission is the general objective of genetic studies.
It is often important to summarize what alleles are present in a cell by noting its genotype. This notation will also
be the genotype for the multicellular organism to which the cell belongs because all the body (somatic) cells of one
individual have the same alleles on their chromosomes. We represent the genotype by:
• Including a letter for each allele that is present on the chromosomes.
• Writing those alleles in alphabetical order.
• Placing the upper case (capital) letter first when two different alleles for a gene are present on homologous
chromosomes (ex., AaBb).
K
i
K
i
K
i
i
K
Use the figure above to answer the following questions:
1. What alleles are present?
H, h, K, i, E, e
2. Write the correct genotype for this cell.
HhKKIiEe
3. For each gene, determine if the alleles are homozygous or heterozygous.
4. Based on this information is it possible to determine which allele is dominant?
capital
Using the following genotype, illustrate a pair of homologous chromosomes. Use the figure above as an
example. You can place the gene loci in any position.
AAccEeGGjj
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Activity 8.5- Understanding Phenotypes
The central dogma of biology indicates that genes determine the physical characteristics we can observe through the
production of a gene product (an RNA or a protein). Just as Mendel did, we can trace inheritance using these
physical characteristics or phenotypes.
In pea plants, yellow seed color is dominant to green seed color. For each of the genotypes, indicate its
corresponding phenotype.
YY yellow
_______________
green
yy _______________
yellow
Yy _______________
In pea plants, purple flower color is dominant to white flower color. For each of the genotypes, indicate its
corresponding phenotype.
purple
Pp _______________
purple
PP ___________________
white
pp ___________________
Activity 8.6- Patterns of Inheritance (Mendelian Genetics)
Complete each of the following monohybrid crosses. Draw out the Punnett for the F1 generation and the F2
generation.
1:1 genotype
1. When Mendel crossed a homozygous dominant purple flower plant with a homozygous recessive white flower
plant, he observed the following:
a. phenotypic ratio in the F1
b. phenotypic ratio in the F2
c. genotypic ratio in the F1
d. genotypic ratio in the F2
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2. When Mendel crossed a homozygous recessive green seed pea plant with a homozygous dominant yellow seed
pea plant, he observed the following:
a. phenotypic ratio in the F1
b. phenotypic ratio in the F2
c. genotypic ratio in the F1
d. genotypic ratio in the F2
Complete each of the following monohybrid crosses. Draw out the Punnett for the offspring of the cross.
1. Sickle-cell disease is an autosomal recessive genetic disorder. Consider a couple where one individual is a carrier
for the sickle cell allele and the other is homozygous dominant. What are the expected genotypic and
phenotypic ratios of their offspring?
2. Huntington's disease is caused by the allele H, which is completely dominant to the normal recessive allele h.
Consider a couple where the man is heterozygous, and the woman is homozygous recessive. What are the
expected genotypic and phenotypic ratios of their offspring?
3. Tay-Sachs is an autosomal recessive genetic disorder. Consider a couple where both individuals are carriers of
the Tay-Sachs allele. What are the expected genotypic and phenotypic ratios of their offspring?
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