GENES 699 



character to which they correspond : it will only appear if other 

 conditions are right. In other words, there is at the nuclear 

 as well as at the more obvious organismal level, a balance 

 between heredity and environment. Height in man, for instance, 

 is undoubtedly determined by a group of genes, but they can 

 produce their full effect only when nutrition is adequate ; the 

 recessive gene cubitus interruphis in the fruit-fly causes a break 

 in one of the wing veins, but while at 19° C. all homozygotes 

 are affected, at 25° C. half of them are normal. In these examples 

 the genes are interacting with the external environment, but in 

 others the interaction is with the internal environment of the 

 animal's body. The colour of genetically dominant black rabbits 

 can only be produced in parts of the skin to which a specific 

 colour precursor, or chromogen, is supplied by the blood. If, 

 as is probable, the presence of this chromogen is itself determined 

 by a gene, we may speak of the final colour as being dependent 

 on gene interaction. Secondly, instead of there being two alter- 

 native characters, there are often several possibilities, each being 

 exclusive of all the others ; these (or their determiners) are known 

 as multiple allelomorphs, and theory requires that only one 

 factor from the whole set can occur in a gamete. A series of coat 

 colours in the rabbit, ranging from natural or agouti to albino, 

 is of this type, and another example is a series for reduction of 

 the wings in the fruit-fly. 



The coiling and uncoiling which go on in chromosomes 

 suggest a parallel with the similar behaviour which is known to 

 occur in a protein molecule, and probably each chromosome 

 does in fact consist basically of a protein thread which changes 

 its state at different times in the mitotic cycle. The genes are 

 best looked on as side chains on this thread, always associated 

 with a substance called nucleic acid, and their multiplication 

 takes place because the thread is able to attract the necessary 

 radicles to form an image of itself alongside. The genes are thus 

 fed from the cytoplasm. In their turn they must liberate substances 

 into the cytoplasm in order that they may have any effect, and 

 it is simplest to regard these substances as enzymes or precursors 

 of enzymes. The gene-product, for instance, which produces 

 the black pigment in rabbits can be simulated by a peroxidase. 

 The simplest difference between a dominant gene and its recessive 

 would be that the latter is simply the absence of the former ; 

 the recessive homozygote would then have no opportunity to 



