Meiosis and Genetic Variation
Section 6.6
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Objectives
• SWBAT describe how sexual reproduction
creates unique gene combinations.
• SWBAT explain how crossing over during
meiosis increases genetic diversity.
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Vocabulary
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Crossing over
Recombination
Genetic linkage
Independent assortment
Genotype
Phenotype
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Sexual Reproduction and Unique Gene
Combinations
• Main advantage of sexual reproduction – it gives
rise to genetic variation within a species.
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Meiosis and Genetic Variation
• Variation results from:
– The independent assortment of chromosomes
during meiosis.
– The random fertilization of gametes.
• Independent Assortment: Since homologous
chromosomes pair up randomly along the cell
equator during meiosis 1, it’s a matter of
chance which of the two chromosomes from
any homologous pair ends up in a given
gamete.
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Meiosis and Genetic Variation
• In humans, there are 23 pairs of chromosomes
and each pair lines up independently during
meiosis 1.
– As a result, in one human sex cell there are
approximately 8 million different possible
combinations of chromosomes (223).
– Sexual reproduction, fertilization, produces offspring
from the random combination of two gametes. In
humans, the total number of possible chromosome
combinations is more than 70 trillion (223 X 223).
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Meiosis and Genetic Variation
• The possible number of chromosome
combinations varies by species.
– For example, fruit flies have only 4 chromosomes
– or 16 possible chromosome combinations in a
sex cell.
• Since each egg and sperm has 16 possible chromosome
combinations, the total number of possible
combinations is 256 - 16 X 16.
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Sexual Reproduction
• As the two examples (human and fruit fly)
show, sexual reproduction creates unique
combinations of genes.
– This results in organisms with unique phenotypes
(an organisms physical characteristics).
• The offspring of sexual reproduction has a
mixture of traits from both parents.
– Returning to our wolf example, one wolf sibling
can be black while the other is gray.
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Unique phenotypes from Sexual
Reproduction
Behavioral characteristics, not just
appearance, are also part of an
individual wolf’s phenotype.
Possible behavioral characteristics for
a wolf might be speed, ability to spot
weakened prey; more or less
aggressive, etc.
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Crossing Over
• Independent assortment creates a lot of
variation within a species.
• However, crossing over and recombination
create even greater variation to that created
by independent assortment.
– Creates new combinations of genes.
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Crossing Over
• Crossing over - is the exchange of chromosome
segments between homologous chromosomes during
prophase 1 of meiosis 1.
• results in new combinations of genes
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Crossing Over
• Once homologous chromosomes have paired
with one another:
– Some of the chromatids are very clos to each other.
– One chromatid from each chromosome breaks off and
reattaches to the other chromosome (there is a swap
of DNA between chromatids).
– Crossing over (the swap of DNA) can occur multiple
times within the same pair of homologous
chromosomes.
• Crossing over happens any time there is a germ
cell dividing.
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Recombination
• The term recombination refers to any mixing
of parental alleles.
– Therefore, crossing over is considered a
recombination event.
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Genetic Linkage
• Each gene has a specific location on a
chromosome (we call this a gene’s locus).
• Some genes are close together (in terms of
location) and will tend to be inherited
together – this is known as Genetic Linkage.
– They will remain together during crossing over.
• Conversely, genes that are located far apart
(or at least farther apart) are more likely to be
separated when crossing over happens.
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Genetic Linkage
Genetic linkage allows scientists to measure the
physical distance between two genes to be calculated.
This has been important to creating genetic maps of
many species.
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