474 Genetics of Sex Determination 



individual cases, and the cytological mechanisms may be of many 

 different kinds. (See the reviews by Vandel, 1931; and Whiting, 1945.) 

 But it is not necessary to go into details, since the result is always 

 controlled in these diploid females and males of whatever origin ( e.g., 

 parthenogenesis with subsequent autogamy or with doubling of the 

 chromosomes, etc.) by the standard 2X-1X mechanism and the F/M 

 balance. Only the haploid parthenogenetic males in Hymenoptera and 

 other groups (haploid arrhenotoky) seem to be unexplainable on the 

 basis of the general theory: their balance F/M must be the same, for 

 example, as in parthenogenetic females produced by whatever kind 

 of doubling of the chromosome set, developed possibly from the same 

 unfertilized egg, which could have developed into a parthenogenetic 

 male. The fertilized eggs are the same eggs which would have pro- 

 duced males if unfertilized. 



A remarkable experimental attack upon this problem has been 

 made by Whiting and his school (quotations in Whiting, 1945). He 

 worked with the ichneumonid wasp, Habrobracon fuglandis, in which 

 all eggs are facultatively parthenogenetic, and, as a rule, unfertilized 

 (and haploid) eggs produce males in the classic way, as one can also 

 make certain by the use of genetic markers. But, if the parents come 

 from the same parental stock, some diploid biparental males (as the 

 markers prove) are also produced. They are of low viabiHty, almost 

 sterile, and not intersexual. Their viable sperm are diploid and, with 

 normal eggs, may produce triploid daughters. The crosses show that a 

 number of X-chromosome types act as multiple alleles in regard to 

 sex determination, though sex-linked traits cross over. One might 

 compare this series of multiple allelic X-chromosomes to the X- and 

 Y-chromosomes of different potency in Lymantria. Whiting found nine 

 such types Xa, Xb, Xc, and so on which, then, may exist as hemizygous 



(haploid) — ..., or as compounds in diploid condition, — ..., 



or as diploid homozygotes — .... The hemizygotes are always 



Xa 



males, the homozygotes are diploid males, and the diploid compounds 



are females. This is explained by the assumption that the so-called 



multiple alleles, Xa, Xb, . . . , are in fact chromosomal sections in 



which no crossing over occurs and in which a dominant female sex 



determiner is always present. Fa Fb, . . . , and a number of recessive 



male determiners ma, mt, mc, . . . , or their normal alleles +. Thus 



a diploid heterozygote (compound) always has two dominant F, 



which are epistatic to the homo- or heterozygous m. A haploid has 



