Section 6.6 Meiosis and Genetic Variation
Vocabulary
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Crossing over
Recombination
Independent assortment
Genetic linkage
Review Questions
1. What terms describe the exchange of chromosome segments during prophase 1? Please explain
how the exchange works.
Crossing over – two homologous chromosomes pair up during prophase 1 of meiosis 1. Since they
are close to one another, corresponding segments of DNA are swapped by the homologous pair of
chromosomes (the segments break off their “parent chromosome” and reattach on the other
chromosome in the homologous pair).
2. In what type of cell does crossing over occur?
Any germ cell ready to divide.
3. Briefly explain how sexual reproduction generates new allele combinations in offspring.
The new allele combinations occur because of independent assortment and crossing over during
meiosis (making the gametes) in combination with random fertilization that occurs when the
sperm and egg fuse to become a zygote (with 1 homologous chromosome coming from each
parent). These three things can create unique gene (allele – which is a specific version of a gene)
combinations and genetic diversity.
4. How does crossing over contribute to genetic diversity?
Crossing over’s contribution to genetic diversity is that it creates new gene combinations through
the swapping of corresponding pieces of chromosome between homologous chromosomes. The
gametes that are created are unique and when fertilization occurs, they will combine with genes
from the other individual creating a very unique individual.
5. What role do independent assortment and fertilization play in creating and maintaining genetic
diversity in sexually reproducing organisms?
Independent assortment means that parental chromosomes are not assorted due to parental
(whether they came from mom or dad) origin; therefore, gametes are going to contain a mix of
chromosomes. Random fertilization will result in gametes forming unique gene combinations.
6. You get half your DNA from your mom and half from your dad; does this mean you get one-quarter
of your DNA from each of your grandparents? Please explain your reasoning.
The answer is yes. Remember, you get 50% of your genes from each of your parents and they get
50% of their genes from their parents, your grandparents. So, .5 * .5 = .25 – or 25%. With each
step from you, the relatedness to you is cut in half. Your grandparents are two steps, or two
generations in this case, from you and thus you share only 25% of your genes with them.
7. Thinking along the lines of the previous question, how closely related are you, in percentage of DNA,
to your cousins?
Your relatedness to your cousins is only 12.5% (you have only 12.5% of your genes in common)
because you are three steps -distant from them. You are 25% related to blood aunts and uncles
(two steps from you) and thus half of 25% is 12.5%.
8. What can you say about two genes that are linked?
Linked genes are found close together on chromosomes, tend to go through crossing over
together, and thus are inherited together.
9. Suppose two genes are very close together on a chromosome. Are the genes likely to be separated
by crossing over? Explain.
No, because of their proximity, when crossing over occurs they are likely to be on the same length
of DNA that is “crossed over;” therefore, they more often than not are inherited together.
10. Suppose you know two genes exist on the same chromosome. How could you determine whether
they are located close to each other?
The easiest way is to see if they cross over together. If they do, then we know they are on the
same chromosome, they are found very close together, and they are linked
11. (Challenge question – 2 points extra credit) Fruit fly gametes each have four chromosomes
representing 2 4 or 16 possible chromosome combinations. How many chromosome combinations
could result from fertilization between a fruit fly egg and a sperm cell?
There are 256 different combinations (16 X 16 = 256). The sperm carries 16 possible combinations
and the egg carries 16 possible combinations.
Figure 1: Crossing over and recombination