HOW THE NUCLEUS ACTS 



The same principle applies to the sex determination of the midge 

 Sciara. Some females produce only male offspring, and the rest only 

 female. The females are again of the two types: those like the mother 

 and those like the father's mother. 



Sometimes we can see that the delay in the time at which a gene 

 becomes apparent depends on the amount of store, or the degree 

 of multiplication, of the substance it produces in the cytoplasm. 

 The disappearance of the old gene's products and the accumulation 

 of the new gene's products may be proportionately slow. Thus, with 

 the moth Ephestia, Kiihn (1937) found that, in the progeny of pig- 

 mentless aa mothers (crossed with Aa), the pigmentless a^eyes were 

 at once distinguishable from the pigmented Aa eyes. But in the 

 young progeny of pigmented Aa mothers the distinction was 

 smothered. The pigment determinants in the cytoplasm of the egg 

 provided full coloration until a late larval instar, so that only then 

 did the segregation appear. These determinants, by the way, were 

 diffusible, for transplanted Aa testes could darken the eyes of an aa 

 host and of its young aa offspring. 



The extreme slowness in action, or in accumulation for action, 

 is attained by the gene ' grandchildless" in Drosophila suhohscura. 

 Females homozygous for this gene are fertile; but no matter to 

 what male they are mated, their offspring are sterile. They can have 

 children but no grandchildren. This gene's normal allelomorph must 

 be producing something necessary for fertility, but producing it so 

 slowly that its absence is not felt until a generation has elapsed. 



Many genes will obviously depend for their manifestation not 

 merely on a threshold value of their product, but also on the oppor- 

 tunity for its giving a different result from that of its allelomorph. 

 The direction of coihng in the snail is determined at the second 

 cleavage division of the fertihzed egg. If this opportunity is missed, 

 it will not recur for a whole lifetime. All we can know of the 

 gene's lag, therefore, is that it is somewhere between two cell genera- 

 tions and a whole life-cycle in length. Since a gene's opportunity 

 IS itself determined by the totahty of genes, it is easy to see how 

 each gene depends for its expression on its fellows. 



LUimnts nfOcfuUics 



193 N 



