468 Genetics of Sex Determination 



could decide which alternative happens on the basis of threshold 

 conditions. 



De Lattin finds in his own material other difficulties for Vandel's 

 hypothesis. Monogene females may appear among the daughters of 

 an amphogene mother, also amphogene females among the daughters 

 of thelygene mothers; further, nearly 50 per cent of monogene females 

 appear among the offspring of females from amphogene mothers, and 

 fathers of arrhenogene descent. The conclusion which we could draw 

 from such special facts is that within the system of directed meiosis of 

 an XY egg, which accounts for monogenesis, other modifying features 

 must be present affecting the completeness of control of Y movement 

 in one direction, the threshold for working or not working of the 

 control, and other imaginable interferences with the basic mechanism 

 of meiotic control. But de Lattin concludes that we must renounce all 

 explanations on the basis of an XY mechanism, and that only two 

 possibilities remain: either phenotypic or multifactorial sex determina- 

 tion. This is proposed without realizing that the existence of an XY 

 mechanism and its control is not at stake but only the variability of 

 the control mechanism, whatever it is. 



The "polyfactorial" scheme proposed is this. There are realizator 

 complexes F and M which are spread over all the autosomes and are 

 in an approximate equilibrium. But because some of them are still 

 heterozygous, the equilibrium is only relative and thus works only for 

 the average, not for the individual that may digress considerably from 

 equilibrium conditions. (We would have to call this a somewhat 

 labile monoecism.) To explain the monogenic animals, two other pairs 

 of alleles are added: a dominant F^ which is one of the realizators of 

 the F complex and has a predetermining action, assumed to be via 

 a stuff phi which may come from the mother in F^ and f ^f ^ individuals. 

 Then the author adds another gene, I, that does not belong to the 

 realizator complex. It is supposed to increase the feminizing action of 

 F^. When its products are carried over to the next generation, absolute 

 feminization results. Thus animals containing I and F^ (or phi) will 

 "almost always" be females, independent of other realizators over 

 which F^ is epistatic in the presence of I. The gene I may act in the 

 mother with the result that all offspring receiving the intensified phi 

 become females, even if they are themselves f^ f ^ i i. But the I may act 

 only in the zygote if a maternal phi is combined with a paternal I. 

 The constitution of thelygene females is then F^ f^ I I. The pairing of 

 two such individuals will produce only daughters, which are half F^ f\ 

 half f ^ f ^ The first will propagate like their mothers; the second, which 



