13 dalian View of the World cable 10. In the figure in the book showing Barbara McCliptock and Harriet Creighton's experiment with conm nt the e pro dis one of the crossover progenies highlighted in blue? why is this progeny more useful in this expernment tham ail of the other noncrossover and crossover progeny? How else did thev confirm it? cytological demonstration ahd rejoining (Fig. 1-5). two chromatids bend across each other CHROMOSOME MAPPING Thomas Hunt Morgan and his students, however, did not await formal cyto- logical proof of crossing over before exploiting the implication of Janssens's hypothesis. They reasoned that genes located close together on a chromo- some would assort with one another much more regularly (close linkage) than genes located far apart on a chromosome. They immediately saw this as a way to locate (map) the relative positions of genes on chromosomes and thus to produce a genetic map. The way they used the frequencies of the various recombinant classes is very straightforward. Consider the segre- gation of three genes all located on the same chromosome. The arrangement of the genes can be determined by means of three crosses, in each of which two genes are followed (two-factor crosses). A cross between AB and ab vields four progeny types: the two parental genotypes (AB and ab) and two recombinant genotypes (Ab and aB). A cross between AC and ac similarly gives two parental combinations as well as the Ac and aC each chromatid breaks at point of contact and fuses with a portion of the other FIGURE 1-4 Janssens's hypothesis of crossing over. parental genotypes extrachromosomal material Wx Wx FIGURE 1-5 Demonstration of phys- ical exchanges between homologous chromosomes. In most organisms, pairs of homologous chromosomes have identi- cal shapes. Occasionally, however, the two members of a pair are not identical; one is marked by the presence of extrachromo- somal material or compacted regions that reproducibly form knob-like structures. Mc- Clintock and Creighton found one such pair and 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 loci. When such c, wx offspring were cytologically examined, knob chromo- somes were seen, showing that a knobless Wx region had been physically replaced by a knobbed wx region. The colored box in the figure identifies the chromosomes of the homozygousc, Wx offspring. WX WX knob crossover progeny noncrossover progeny WX Wx Wx Wx Wx Wx Wx WX WX WX WX Wx Wx WX