A Genetic Study of Segregation in a Translocation Heterozygote in Drosophila.

URING meiosis, homologous chromosomes normally pair with each other and separate whereas the members of pairs of non-homologous chromosomes assort independently of one another. This behavior is disturbed, however, in individuals carrying a translocation. It is well known that a t the first meiotic division separation of the four components of a translocation configuration may take place in a number of different ways and that each type of first division separation gives rise to two kinds of gametes, each the complement of the other. It is ordinarily expected that the two complementary products from any one type of separation will be produced equally frequently. Certain cases in Drosophila melanogaster which did not meet the expectation of equal recovery were reported by GLASS (1934, 1935) who found that from crosses of females, heterozygous for a translocation of the type diagrammed in figure 1, by heterozygous translocation males, female gametes with 2s + 3’ were recovered twice as frequently as those with 2’L’ + 3 (see the legend to figure 1 for explanation of symbols). The results from other kinds of tests established beyond any doubt that this discrepancy could not be accounted for on the basis of a difference in the viability between these types. NOVITSKI (1951) showed that from a Drosophila female carrying two homologous X-chromosomes differing in length, the shorter of the two is recovered about twice as often as the longer. This phenomenon of non-random disjunction depends upon the occurrence of an exchange between the two homologs, with the formation of a dyad composed of two structurally dissimilar chromatids. He suggested that the inequality of the frequencies of the two types of gametes from the translocation heterozygote described above might also be a manifestation of non-random disjunction. The experiments to be described below were designed to test the validity of this explanation for the unusual behavior of the translocation heterozygote, as well as to test certain other possibilities.