By Stanton A. Cook (auth.)
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Third, since there are hundreds of genes and many fewer chromosomes, the genes must be linked in groups on them. Fourth, crossing-over occurs, and furthermore the farther apart genes are on chromosomes, the more likely they are to recombine by crossing-over. This explains the existence of differences in frequency of recombination between genes within one linkage group. So it appears that genes are arranged in linear sequence in chromosomes. It took years of research to discover this physical basis of inheritance.
The presence of a loop reduces crossing-over within its region, with the result that the inverted segment of chromosome becomes in- 38 REPRODUCTION, HEREDITY, AND SEXUALITY herited as a block of genes. Thus, inversion can be a mechanism favoring coherence of genetic factors. The mechanism (or mechanisms) whereby inversions reduce crossing-over is complicated and still a subject of investigation. Some evidence indicates that the loop itself reduces crossing-over, perhaps by impairing synapsis. In the main, however, the reduction seems to come about by elimination of the chromatids that experience crossingover.
For example, assume that species A has some desirable trait and species B some other. It may be possible to cross the species, but the hybrid formed is sterile because the chromosomes of A and B will not synapse in meiosis. Failure of synapsis results in some daughter cells' getting too many and others' getting too few chromosomes or, in other words, abnormal chromosome complements. The gametes formed from these cells may themselves die or the zygote they produce dies; thus sterility results. When, however, the entire chromosome complement of each species is present in the hybrid (as happens in fusion of diploid gam- 36 REPRODUCTION, HEREDITY, AND SEXUALITY etes), the chromosomes of A will synapse among themselves, and B chromosomes will do likewise.