Biology Reference
In-Depth Information
1.24 Independent Assortment and Recombination during Sexual
Reproduction
For organisms to survive and evolve with changing environmental conditions,
they need to be able to generate genetic variability. Mutations are one source
of genetic variability and thus are not always undesirable. Another source of
genetic variability is the result of sexual reproduction.
In sexually reproducing organisms, the progeny produced by parents that
have different versions of genes (different alleles, AA or aa) will have a differ-
ent combination of alleles (Aa). This shuffling of the genetic information during
sexual reproduction is due to the
independent assortment
of chromosomes into
the gametes during
meiosis
. Thus, an individual of genotype Aa Bb, in which
the genes A and B are located on different chromosomes, will produce equal
numbers of four different types of gametes: AB, Ab, aB, or ab.
Crossing over
also leads to recombination between DNA molecules. Crossing
over occurs between homologous chromosomes during the production of eggs
or sperm in meiosis I, and results in an exchange of genetic material. Crossing
over allows new combinations of different genes that are linked (located on the
same chromosome). Thus, if a parent has one chromosome containing with A
and B, and the homologous chromosome has a and b alleles, a physical exchange
between the chromatids during meiosis I can lead to gametes that have the fol-
lowing combinations: A and B, A and b, a and B, and a and b. Nonhomologous
recombination, crossing over between DNA lacking sequence homology, also may
occur. Meiosis and mitosis are described further in Chapter 3.
General References
Brown, T.A., (2011).
Introduction to Genetics: A Molecular Approach
. Garland Science, New York.
Carlson, E.A., (2004).
Mendel's Legacy. The Origin of Classical Genetics
. Cold Spring Harbor
Laboratory Press, Cold Spring Harbor, NY.
Cooper, G.M., and Hausman, R.E., (2009).
The Cell. A Molecular Approach
, 5th ed. ASM Press,
Washington, DC.
Deamer, D., and Szostak, J.W., (2010).
The Origins of Life
. Cold Spring Harbor Laboratory Press,
Cold Spring Harbor, NY.
DePamphilis, M.L., and Bell, S.D., (2010).
Genome Duplication: Concepts, Mechanisms, Evolution,
and Disease
. Garland Science, New York.
Friedberg., E.C., (1997).
Correcting the Blueprint of Life: An Historical Account of the Discovery of
DNA Repair Mechanisms
. Cold Spring Harbor Laboratory Press, Cold Spring Harbor, NY.
Haga, S.B., (2006). Teaching resources for genetics.
Nat. Rev. Genet.
7
: 223-229.
Hartl, D.L., and Jones, E.W., (1998).
Genetics: Principles and Analysis
. Jones and Bartlett, Sudbury,
MA.
Micklos, D.A., and Freyer, G., (2010).
DNA Science: A First Course
, 2nd ed. Cold Spring Harbor
Laboratory Press, Cold Spring Harbor, NY.
