Data are presented which show that, where a large portion of the third chromosome ofDrosophila is attached to the second chromosome, the directions of migration of the free fragment of the third chromosome and of the homologons third chromosome of normal structure are decidedly influenced by or correlated with the position and direction of migration of the second chromosome bearing the translocated piece of the third, with the result that nearly twice (1.7 times) as many gametes are formed bearing normally proportioned (“balanced”) chromosome combinations as those bearing disproportioned (“unbalanced”) combinations. The assortment thus tends to be determinate rather than random.
This shows that linkage between chromosomes, tending usually to result in the formation of “old-combination” (or, more correctly, normally proportioned) types of gametes, can occur somewhat as Cleland postulated forOenothera, but that such linkage follows as a consequence of previous interchange of material between non-homologons chromosomes in the manner indicated by Darlington and by Blakeslee and Belling. The present finding, therefore, lends genetic evidence in support of Darlington’s theory of theOenothera phenomena.
The latter theory, in its turn, is considered as including a confirmation and extension of the writer’s theory of the origin of the spurious mutants inOenothera by means of the rare formation of new combinations between heterozygous visible genes (or groups of genes) and balanced lethals that were linked with them. The fact; that the linkage is now found to be partly or wholly inter-instead, of intra-chromosomal does not destroy the validity of this general mechanism. It means rather, as Darlington has pointed out, that the balanced lethal alignment is probably of positive value from its effect in enforcing heterozygosis on other loci besides those of these lethals themselves; and the translocations which caused the inter-chromosomal linkage tended to survive because they permitted an extension of this temporarily advantageous effect to previously disjoined portions of the chromatin. Even without a positive survival value, however, such translocations would tend to accumulate in a race already possessing balanced, lethals, because they would not appreciably lower productivity so long as they determined the direction of chromosome segregation.