Wx

Wx wx crossover progeny

C wx crossover progeny

C wx

FIGURE 1-5 Demonstration of physical exchanges between homologous chromosomes. In most organisms, pairi of homologous chromosomes have identical shapes. Occasionally, however, the two members of a pair are not identical, one is marked by the presence of extrachromosofrwt materia) or comparted regions ihet leprodudbly farm knob-like structures. McGintock and Creighton found ore such pair arid used it to show that crossing over involves actual physical exchanges between the paired chromosomes, in the experiment shown here, the homozygous c, wx progeny had to arise by crossing over between the C and wx tact. When such c, wx offspring were cytologtcaily examined, knob chromosomes were seen, showing that a knobless Wx region had been physically replaced by a knobbed wx region. The colored box in the ftgure identifies the chromosomes of the homozygous c, wx offspring

12 The Mendehan View of the World

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