9.6 Meiosis increases genetic
variation among offspring
• Objectives
• Describe how chromosome assortment
during meiosis contributes to genetic
variation.
• Explain how crossing over contributes to
genetic variation.
• Compare and contrast mitosis and meiosis.
• Key Terms
• crossing over
• genetic recombination
• Genetic variety in offspring is the raw
material for natural selection
• Assortments of Chromosomes
Meiosis contributes to genetic variety.
• How the chromosomes in each homologous
pair (tetrads) line up and separate at
metaphase I is a matter of chance, like the
flip of a coin.
• So, the assortment of chromosomes that end
up in the resulting cells occurs randomly. In
this example, four combinations are
possible
•
• If you know the haploid number for an
organism, you can calculate the number of
possible combinations in the gametes
• . The possible combinations are equal to 2n,
where n is the haploid number.
• For the organism in Figure 9-18, n = 2, so
the number of chromosome combinations is
22, or 4. For a human, n = 23, so there are
223, or about 8 million, possible
chromosome combinations!
• Crossing Over
The number of different chromosome
combinations in gametes is one factor that
contributes to genetic variation.
• A second factor is crossing over—the
exchange of genetic material between
homologous chromosomes. This exchange
occurs during prophase I of meiosis.
• When crossing over begins, homologous
chromosomes are closely paired all along
their lengths. There is a precise gene-bygene alignment between adjacent
chromatids of the two chromosomes.
• Segments of the two chromatids can be
exchanged at one or more sites
•
• Crossing over can produce a single
chromosome that contains a new
combination of genetic information from
different parents, a result called genetic
recombination.
• Review: Comparison of Mitosis and
Meiosis
You have now learned about two versions of
cell reproduction in eukaryotic organisms.
• Mitosis, which provides for growth, repair,
and asexual reproduction, produces
daughter cells that are genetically identical
to the parent cell..
• Meiosis, which takes place in a subset of
specialized cells in sexually reproducing
organisms, yields four haploid daughter
cells with only one set of homologous
chromosomes. This set consists of one
member of each homologous pair
• In both mitosis and meiosis, the
chromosomes duplicate only once, in the
preceding interphase.
• Mitosis involves one division of the genetic
material in the nucleus, and it is usually
accompanied by cytokinesis, producing two
diploid cells.
• Meiosis involves two nuclear divisions,
yielding four haploid cells.
• The key events that distinguish meiosis
from mitosis occur during the stages of
meiosis I..
• Mitosis and meiosis both make it possible
for cells to inherit genetic information in the
form of chromosome copies
•