Cytological Basis of Crossing over PDF

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Cytological Basis of Crossing over...

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CYTOLOGICAL BASIS OF CROSSING OVER The cytological basis of crossing over showing physical exchange of parts between homologous chromosomes resulting in intrachromosomal recombination was exhibited by the work of Harriet Creighton and Barbara McClintock in 1931. In other words, their work established the fact that chiasmata are the visible manifestations of the crossing over during meiosis. They discovered a strain of corn, Zea mays that had an abnormal chromosome 9, containing a densely staining knob at one end and a small piece of another chromosome attached to the other end (Figure 9). This aberrant chromosome enabled them to visually distinguish the two members of a homologous pair. They studied the inheritance of two linked genes on chromosome 9. At one locus, the alleles colorless (c) and colored (C) control the endosperm coloration. At the other locus, the alleles starchy (Wx) and waxy (wx) control the carbohydrate characteristics of the endosperm. A heterozygous plant with both loci in repulsion phase was obtained, with the alleles for colored and waxy on the abnormal chromosome and the alleles for colorless and starchy on a normal chromosome. They crossed this heterozygous plant with a plant that was homozygous for colorless and heterozygous for waxy. Different combinations of traits in the progeny were produced, but the only way that colorless and waxy progeny could arise was through crossing over in the doubly heterozygous parent. They examined for the presence of the cytological markers for the colorless and waxy phenotype. They thought that if genetic crossing over was accompanied by a physical exchange between homologs, the translocated chromosome would still be present, but the knob would not. As expected they got the same result, confirming the physical exchange of chromosmome parts during crossing over.

Figure : Chromosome 9 of Zea mays (The aberrant chromosome bears knob at one end and extra piece at the other end) Source: Self made

Figure : Creighton and McClintock’s experiment in maize showing establishing the fact that crossing over involves an actual exchange of chromosome arms....