194 GENETICS 



In many such cases in which the conspicuous effect of a 

 gene appears at a relatively late stage in development there 

 are less conspicuous effects that indicate that the gene may 

 be in operation throughout development. Thus, individuals 

 with white eyes, or with rudimentary or vestigial wings, are 

 less vigorous than those with normal eyes or wings; they are 

 less resistant and have shorter lives, and this is true for 

 most of the types having recessive characters. Though the 

 conspicuous special effect of the changed gene is late in ap- 

 pearing, the effect on the general constitution indicates that 

 there may be an effect throughout development. 



That this is indeed the case is indicated strongly by certain 

 other facts. Sometimes a gene at a certain locus in the 

 chromosome becomes entirely inoperative; it has either 

 dropped out of the chromosome, or it has totally lost its 

 action. Such cases are commonly known as deficiencies. De- 

 ficiencies are producible by the action of radiation on the 

 chromosomes, so that large numbers of them have been 

 studied. A deficiency is detected in the following manner. A 

 chromosome suspected of being deficient for a certain gene 

 at a certain locus is brought into an individual in which the 

 other member of the pair to which it belongs carries a re- 

 cessive gene at that locus (figure 43). If the chromosome 

 is deficient in the suspected gene, the recessive gene is mani- 

 fested, since there is present no dominant gene of that pair. 

 Thus if an X-chromosome carrying white eye is brought into 

 a female whose other X is deficient for the gene at the 

 white eye locus (the locus 1.5), the female has white eyes. 



In the case of almost any gene, if two chromosomes that 

 are deficient for the same single gene are brought into the 

 same fertilized egg, so that the egg lacks that gene entirely, 

 the egg does not fully develop. In most cases it is not known 

 at just what stage development ceases. But the fact shows 

 that in addition to its special effect on a particular charac- 

 teristic, the gene is necessary if development is to continue 



