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Meiosis and Genetic Variation
Section 6.6
• SWBAT describe how sexual reproduction
creates unique gene combinations.
• SWBAT explain how crossing over during
meiosis increases genetic diversity.
Independent assortment
Crossing over
Genetic linkage
Sexual Reproduction and Unique Gene
• Main advantage of sexual reproduction – it gives
rise to genetic variation within a species.
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
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).
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.
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.
Unique phenotypes from Sexual
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.
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.
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
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
• Crossing over happens any time there is a germ
cell dividing.
Genetic Recombination
• The term recombination refers to any mixing
of parental alleles.
– Therefore, crossing over is considered a
recombination event.
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.
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.