Inheritance of the"Additional Crescent" Mutation in the Silkworm
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概要
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The normal silkworm larva is furnished with a pair of large"crescent" marking ("lunules") on the second abdominal segment, and a pair of small"star"marking on the fifth segment.A single"additional crescent"larva appeared as a mutation in the F1 generation from the cross Japanese race No. 109 _??_×Chinese race No. 102 _??_ reared in 1926. This individual had two pairs of large "crescent"marking, one on the second abdominal segment, as usual, and another pair on the third segment, while the"star"marking was entirely absent from the fifth segment.The new type has been preserved and propagated by crossing the mutant to the normal strain, but the characteristic has never bred true, segregating the normal type in a number of about one third of the total offspring. This fact can be explained as due to the lethal function of the "additional crescent"gene, which may be represented by the symbol Ca. About one fourth of the embryos produced by mating of"additional crescent"males and females are killed just before the hatching. The killed embryos were undoubtedly homozygous with respect to the Ca gene. This is why the"additional crescent"does not breed true. The same result is obtained in absence of P gene (normal gene allelomorphic to the"plain") and presence of Ca, where is no"additional crescent" marking visible.In short, Ca is a dominant mutation which is lethal when homozygous, regardless of that the"additional crescent"character is externally visible or not.
- 日本遺伝学会の論文