markers does not permit us to be more precise. In the left arm of chromosome II again this maximum concentration is within 18 units from the centromere, and preliminary evidence suggests that the same is true for the right arm.

Clearly, we cannot yet generalise but there does not seem to be the overwhelming concentration of mitotic crossing over very near the centromere which Whittinghill ( 1955) found for spermatogonial and oögonial crossing over induced by X-rays in Drosophila.

It is necessary here to stress again the following incidental point. That mitotic crossing over in Aspergillus gives origin to reciprocal products of exchange is unquestionable: it has been ascertained both by isolating and testing the genotypes of both members of what corresponds to a "twin spot" in Drosophila and, more crucial, by recovering the two complementary products of one exchange within one nucleus ( Roper and Pritchard, 1955). The recovery of the complementary products in one nucleus is based on 2+3 ÷ 1+4 segregation (see Table 14).


CONCLUSIONS

The most important result of these investigations is that mapping by means of mitotic recombination is not only feasible, but easy. Indeed, because of the rarity of multiple exchanges in mitotic crossing over and the occurrence of haploidisation without crossing over, it is qualitatively more dependable than meiotic mapping.

Mitotic recombination has already been used for mapping in organisms in which the sexual cycle does not occur like Aspergillus niger ( Pontecorvo, Roper and Forbes, 1953; Hutchinson, 1958), Aspergillus sojae ( Ishitaniet al., 1957), Penicillium chrysogenum ( Sermonti, 1955). It can be used, of course, for "breeding" of industrial strains with improved qualities.

There is no reason in principle, though there is probably great technical difficulty, why it should not be used for the formal genetics of higher organisms, including man, in tissue or organ cultures, as suggested by Pontecorvo and Klifer ( 1954; 1956; 1958). Even if mitotic crossing over did not occur in higher organisms, processes

-112-

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Trends in Genetic Analysis
Table of contents

Table of contents

  • Title Page iii
  • Contents vii
  • Tables ix
  • Contents x
  • Introduction 1
  • Chapter I - Genetic Analysis and Its Resolving Power 8
  • Chapter II - Allelism 28
  • Chapter III - Structure and Function of the Genetic Material 54
  • Conclusions 67
  • Chapter IV - Recombination 72
  • Chapter V - Mapping Chromosomes Via Mitotic Recombination 101
  • Conclusions 112
  • Chapter VI - Novel Genetic Systems 114
  • Conclusions 128
  • Works Cited 135
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