Crossing
over is the exchange of corresponding segments between two homologous
chromosomes. During prophase 1 of meiosis, chiasma is formed. It is X-shaped
region where two homologous non-sister chromatids are attached to each other.
Crossing
over can take place because during synapsis homologous chromosomes pair up very
closely, when homologous chromosomes exchange segments, the genetic variability
increases.
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| How Crossing Over Leads to Genetic Recombination |
Figure
illustrates how crossing over can produce new combinations of genes, using as
examples the Mouse genes. Letter C represents the gene for agouti, the brownish
coat color. Letter c represents the
gene for white (albino) coat color. Likewise gene E, for black eyes is at the
same locus as e for pink eyes. The
process begins during prophase l of
meiosis. At the top of the figure is a tetrad with coat-color (C, c) and
eye-color (E, e) genes labeled. In step (1), a chromatid from each homologous chromosome
breaks in two; notice that the two chromatids break at corresponding points. (2) Next, the two broken chromatids join together in a
new way the result is a chiasma. (3) When the
homologous chromosomes separate in anaphase l,
the joined homologous chromatids come completely apart. However, as the colors
indicate, crossing over has changed the content of these two chromatids. A
segment of one chromatid has changed place with the equivalent segment of its
homologue. (4) Finally, in meiosis II, the sister chromatids
separate, each going to a different gamete.
In
this example, if there were no crossing over, meiosis could produce only two
genetic types of gametes. These would be the ones with the “parental"
types of chromosomes shown at the bottom in the figure, carrying either genes C and E (agouti coat, black eyes) or genes c and e (white coat, pink
eyes). With crossing over, two other types of gametes can result. One of these
carries genes C and e (agouti coat, pink eyes), and the
other carries genes c and E (white coat, black eyes). The
chromosomes carried by these gametes are called “recombinant" because they
result from genetic recombination, the production of gene combinations
different from those carried by the original chromosomes.
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| Fruit Fly experiment demonstrating the role of crossing over in inheritance |
Recombination Frequencies
Figure
shows one of Morgan's experiments, a cross between a wild-type fruit fly (gray
body and long wings) and a fly with a black body and undeveloped, or vestigial
wings. Morgan knew the genotypes of these flies from previous studies. In the
figure here, we use the following gene symbols:
G =
gray body (dominant), g = black
body (recessive)
L =
long wings (dominant), l = vestigial wings
(recessive)
In
mating a gray fly with long wings (genotype GgLl) with a black fly
with vestigial wings (genotype ggll), Morgan performed a testcross. If the
genes had not been linked, then independent assortment would have produced
offspring in a phenotypic ratio of 1:1:1:1 (1/4
gray body, long wings: 1/4 black body, vestigial wings: 1/4 gray body,
vestigial wings, and 1/4 black body, long wings). But because these genes
were linked, Morgan obtained the results shown in figure. Most of the offspring
had parental phenotypes, but 17% of the offspring flies were recombinants. The
percentage of recombinants is called recombination frequency.
Geneticists Use Crossover Data to Map Genes
One
of Sturtevant's major contributions to genetics was an approach for using
crossover data to map gene loci. Sturtevant began using recombination data from
fruit-fly crosses to assign to genes relative positions on chromosomes that is,
to map genes.
Figure
here represents a part of the chromosome that carries the linked genes for
black body g and vestigial wings l.
This same chromosome also carries a gene that has a recessive allele c
determining cinnabar eye color, a red much brighter than the wild-type color.
The diagram shows the actual crossover (recombination frequencies between these
alleles, taken two at a time: 17% between the g and l
alleles, 9% between g and c, and 9.5% between l and
c. Sturtevant reasoned that these values represent the relative distances
between the genes. Because the crossover frequencies between g and c and
between l and
c are approximately half that between g and l,
gene c must lie roughly midway between g and l.
Thus, the sequence of these genes on one of the fruit-fly chromosomes must
between g and l.
More recently, geneticists have actually been able to measure the distances (in
nanometers) between genes on chromosomes. Their results, establishing absolute
locations of genes on chromosomes.
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| Mapping genes from crossing over data in Drosophila |
