406 RADIATION BIOLOGY 



a stock can readily be made, (2) the finding of lethals is subject to little 

 subjective error, and (3) the frequency of origination of new lethals, 

 even without special treatments, is high enough to be dealt with statis- 

 tically. Moreover, evidence was found for regarding most of them as 

 being no different in their manner of origin or type of genetic change from 

 mutant genes in general, including those having a visible expression. 



Since a male carries but one X chromosome, a sex-linked lethal in this 

 chromosome results in his death prior to maturity, whereas in a female 

 such a lethal in one of her two X chromosomes very seldom results in 

 her death because it is strongly dominated over by the normal allele 

 present in her other X chromosome. When she reproduces, half her 

 sons will by Mendelian segregation receive from her an X chromosome 

 with the lethal allele and they will therefore be killed, while the other half 

 of the sons, receiving an X chromosome with the normal allele, will tend 

 to live. All her daughters, however, will tend to survive, since even 

 those getting the lethal will, like their mother, have in addition a dom- 

 inant normal allele in the X chromosome derived from their father. 

 Thus the existence of the lethal in one X chromosome of the mother can 

 usually be recognized from the fact that she has approximately half as 

 many sons as daughters. In Drosophila the criterion of a lethal is 

 ordinarily made more definite than this by having that X chromosome 

 of the mother which contains the lethal differ from her other X chromo- 

 some in regard to some gene or genes, termed markers, which have a con- 

 spicuous visible effect, and also by having crossing over between her two 

 X chromosomes rendered ineffectual by the presence of an inversion in 

 one of them. When such a female carrying a lethal is bred, all her sons 

 inheriting the normal allele of the lethal, i.e., all her sons that survive, 

 will exhibit the given marker characters that were in the X chromosome 

 not having the lethal, whereas, if the female had carried no lethal, 

 approximately half her sons would have these marker characters and the 

 rest would have their alleles, giving the contrasting visible characters. 



Essentially similar but more compUcated techniques are employed for 

 detecting lethals in chromosomes other than the sex chromosomes (the 

 autosomes), but it is then necessary to find out whether or not indi- 

 viduals homozygous for the chromosome under investigation are able to 

 survive. For this purpose a special scheme of inbreeding must be used, 

 and the existence of the lethal, evidenced by the absence of a whole 

 group of individuals of a given expected (marker) phenotype, can be 

 ascertained only in the third generation of descendants. Moreover, the 

 same kind of methods may, though with much more effort, be used for 

 detecting genes which, instead of being fully lethal, are only detrimental. 

 For, although individuals with the markers representing the chromosome 

 in question are not completely absent in such cases, they may be sig- 

 nificantly reduced in numbers, relatively to those of other types. Like- 



