Genetical and embryological studies of a dominant mutant, the "new additional crescent" of the silkworm, Bombyx mori L. Report I.

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(1) In the spring culture of 1940, two male larvae of a dominant mutation appeared among the offspring of an F1 hybrid between Sanminsiro and Chinese 108 (Fig. 1-b, c). Genetical studies showed that the multiple effects of the mutant gene, which is denoted by N, greatly resembled those of the "Additional Crescent" gene Ca, described by Suzuki and Ota, in 1930, and by Hasimoto, in 1940. The gene effects of N, however, differ considerably from those of Ca in the form of the homozygous embryo, which both die in the late embryonal stage. (2) In the homozygote, the mutant embryo developed both thoracic and abdominal appendages, while that of the "Additional Crescent" developed only thoracic legs (Fig. 2-A, B). (3) An interesting fact is that one-fourth of the eggs of every batch of the cross between Ca and N die in the embryonal stage, the same as in the case of the homozygote of Ca or N, the embryonal form of Ca dominating that of N in the CaN embryo. From the remaining three-fourths of the eggs of the same batch, healthy larvae hatched out, segregating into two kinds of larvae, additional crescent and normal, in the ratio of 2 : 1. (4) The N gene, which has the same locus as the Kp in the chromosome, belongs to the Kp-lk linkage group, constituting one of the Kp-multiple alleles.
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