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Cytological
Basis
of
Crossing
Over
Between
Genes - Morgan first proposed crossing over to explain the formation of recombinant combinations of genes that were shown to be linked by genetic data.
He proposed that this linkage was the result of the location of these genes on the same chromosomes.
If crossing over occurs, one might expect to be able to observe it under the microscope. In fact crossing over was first detected in amphibians by F. Jannsens.
Stern and Creinghton, and McClintock are two classic works in genetics.
They provided confirmation of Morgan's hypothesis that crossing over involves the interchange of pans of homologous chromosomes, they also provided strong evidence that indicated that genes are indeed located on the chromosome.
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Direct cytological evidence that homologous chromosomes exchange pans during crossing over was first obtained in 1931 by C. Stern. Normally, the two chromosomes of any homologous pair are morphologically indistinguishable, that is they were not entirely homologous.
The chromosome pairs studied by these workers were homologous along most of their length, such that they paired and segregated normally during meiosis. The homologues differed at their end having distinct morphological features that could be recognized by microscopy.
Stem studied two X chromosomes that differed from the normal X chromosome of Drosophila. One X chromosome had a part of a Y chromosome attached to one end. The second X chromosome was shorter than the normal X chromosome.
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This chromosome has a recessive gene car (produces carnation eye colour) and a dominant gene B (Bar eye shape), whereas the first chromosome had the dominant gene car (produces red colour) and the recessive gene B+ (round eye shape).
Crosses were done to produce female flies heterozygous for these two morphologically distinguishable X chromosomes. These heterozygous females with cis configuration were crossed to males with car/+ (carnation eye colour, round eye shaped).
As
expected, the following four types of flies were recovered in the test cross progeny car+/+ (red eye colour, round eye shape), car/B (carnation eye colour, bar eye shape), car/B+ (carnation eye colour, round eye shape) and car/B (red eye colour, bar eye shape).
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As two out of these four phenotypes, carnation individuals are expected to carry one short X chromosome, while the red, normal flies would have one long X chromosome with an attached Y segment, whereas the two phenotypes, viz. red bar and a carnation, normal are crossovers.
Hence, carnation, normal (car/B) flies are expected to have a normal or long X chromosome without the attached Y segment. In contrast red, bar (car/B) individuals will have one short X chromosome with the attached Y segment.
Stem determined the genotypes of the progeny both by chi-square method and by cytological observations. The results were precisely those predicted. Therefore he concluded that during meiosis, there is exchange of precisely homologous chromatin segments between homologous chromosomes (crossing over) and that crossing over is responsible for recombination between linked genes.
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Barbara McClintock and Harriet Creighton provided a direct physical demonstration of recombination in 1931. By examining maize chromosomes microscopically, they could detect recombinations between two easily identifiable features of a particular .chromosome both by physical observation as well as genetic recombinants.
Creighton and McClintock worked with a strain of maize which contains an abnormal chromosome especially at the end.
One end had a knob and the other had an added piece of chromatin from another chromosome.
This knobbed chromosome was thus clearly different from its normal homologue. It also connects the dominant coloured (c) allele and the recessive waxy texture (wx) allele. After mapping studies showed that c was very close to the knob and wx was close to the added piece of chromatin.
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Dihybrid plant with heteromorphic chromosome was crossed with the normal homomorphic plant (colourless and non-waxy phenotype).
If a crossover occurred during meiosis in the dihybrid in the region between c and wx, a physical crossover, visible cytological by should also occur causing the knob to become associated with an otherwise normal chromosome and the extra piece of chromosome to be associated with a knobless chromosome.
Four types of gametes would result. Out of the off spring examined, one offspring had coloured waxy phenotype, all had a knobbed interchange chromosome as well as a normal homologue.
Those with the colourless, waxy phenotype had a knob less interchange chromosome.
All of the coloured, non waxy phenotypes had a knobbed normal chromosome. Of those that contained only normal chromosomes, some were wxwx and some were heterozygotes.
Of those containing interchange chromosomes, two were heterozygous and two were homozygous.
These represent a crossover in the region between the waxy locus and the extra piece and chromatin, producing a knobless c wx extra piece chromosome.
Crieghton and McClintock concluded that pairing chromosomes, heteromorphic in two regions, have been shown to exchange parents at the same time they exchange genes assigned to these regions.
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