Download Crossing-over and Independent Assortment

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Recombination Review: Crossing-over and Independent Assortment
Crossing-over Reading – Underline key terms and highlight important concepts in each
paragraph.
Introduction
Recall that chromosomes come in pairs. Each chromosome pair has the same set of genes, but
those genes may be different alleles. There can be many genes on a single chromosome. Pairs of
chromosomes are called homologous chromosomes.
This is a picture of a human karyotype, which is all the
chromosomes present in a single cell. Humans have 23
homologous chromosomes, or a total of 46 chromosomes.
For sexual reproduction to happen, each parent must
make gametes (eggs or sperm) that contain half the number of
chromosomes. That way, when the gametes come together
during fertilization, the resulting baby will have the same
number of chromosomes as the parent. The formation of
gametes is called meiosis.
Meiosis is a type of cell division in which the number of chromosomes in the daughter cells is
reduced by half. It occurs only in certain special cells of sexually reproducing organisms. Two cell
divisions occur during meiosis, and a total of four daughter cells are produced. Those daughter
cells become eggs (in females) or sperm (in males). Genetic recombination can happen during
meiosis because of the way that chromosomes line up and then separate into daughter
cells. There are two ways genetic recombination occurs, crossing-over and independent
assortment.
Crossing Over
Crossing-over is the exchange of genetic material between homologous chromosomes. It results in
new combinations of genes on each chromosome. Crossing-over happens during meiosis when
homologous chromosomes line up in pairs before being separated into different gametes. The
chromosomes come very close to each other and swap segments of DNA. That is, the material
breaks off and reattaches at the same position on the homologous chromosome. This exchange of
genetic material can happen many times within
the same pair of homologous chromosomes,
creating unique combinations of genes.
Crossing-over occurs when homologous
chromosomes pair up during meiosis. The
chromosomes can exchange pieces of DNA or
whole genes.
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Crossing-over Practice
In the circles below, which represent the 4 gametes that will be produced from meiosis, draw (including gene labels) the
chromosomes that will end up in each gamete.
Use information from the reading to help you with your diagram. You can also look at the animations at these websites:
 http://www.biostudio.com/d_%20Meiotic%20Recombination%20Between%20Linked%20Genes.htm
 http://highered.mcgraw-hill.com/sites/dl/free/0072835125/126997/animation5.html
Homologous Chromsomes Not Crossed Over
Homologous Chromsomes Crossed-Over
1.
How does crossing-over increase variation in gametes?
2.
Mechanisms of variation can either produce brand new genetic material (new alleles), or new combinations of
existing alleles. Which category does crossing-over fall into, and why?
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Independent Assortment Reading – Underline key terms and highlight important
concepts in each paragraph.
When cells divide during meiosis, homologous chromosomes are randomly distributed to
daughter cells. Before they separate into daughter cells, homologous chromosomes line up
in the middle of the cell (similar to metaphase in mitosis). But the order in which they line
up is completely random. This is called independent assortment. It results in gametes that
have unique combinations of chromosomes.
Notice how the two different line-ups of chromosomes could result in different gametes. This is
called independent assortment.
In humans, there are over 8 million ways in which the chromosomes can line up during
metaphase I of meiosis. This independent assortment, in which the chromosome inherited
from either the father or mother can sort into any gamete, produces the potential for
tremendous genetic variation.
Genetic recombination can also occur during fertilization. In sexual reproduction, two
gametes unite to produce an offspring. But which two of the millions of possible gametes
will it be? This is likely to be a matter of chance. It is obviously another source of genetic
variation in offspring. This is known as random fertilization.
Because of these three genetic recombination processes, more possibilities for genetic
variation exist between any two people than the number of individuals alive today. Sexual
reproduction is the random fertilization of a gamete from the female using a gamete from
the male. In humans, over 8 million (223) chromosome combinations exist in the production
of gametes in both the male and female. A sperm cell, with over 8 million chromosome
combinations, fertilizes an egg cell, which also has over 8 million chromosome
combinations. That is over 64 trillion unique combinations, not counting the unique
combinations produced by crossing-over. In other words, each human couple could
produce a child with over 64 trillion unique chromosome combinations!
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Independent Assortment Practice
In the blank cell in the Metaphase I section, diagram another way the homologous chromsomes can line up in Metaphase I that
will lead to genetic variation In gametes.
In the four cells beneath each metaphase I cell that represent gametes after meiosis, draw (including gene labels) the
chromosomes that will end up in each gamete
1.
How does independent assortment increase variation in gametes?
2.
Mechanisms of variation can either produce brand new genetic material (new alleles), or new combinations of
existing alleles. Which category does independent assortment fall into, and why?
3.
How is independent assortment represented in:
a. Punnet Squares?
b.
Ugly Baby?
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Extension: Mutation Outcomes
Go to http://learn.genetics.utah.edu/content/variation/outcomes/
This website shows some examples of how mutations can lead to new or different traits in organisms. Choose 3 of
the following to describe using the diagrams below: peas, cattle, cats, curly hair, or red hair. Read each tab
carefully!
What does the protein from this DNA do?
What trait do we see as a result?
What does the protein from this DNA do?
What trait do we see as a result?
What does the protein from this DNA do?
What trait do we see as a result?
Original
DNA
Mutated DNA:
Original
DNA
Mutated DNA:
Original
DNA
Mutated DNA:
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