SEX 



36' 



Mendelian interpretatioD has been restated as follows. There is :i single 

 factor for sex. In some organisms the presence of two of these factor- 

 is correlated with femaleness and one with maleness (Fig. 111. A I: the 

 male, having only one sex-factor (S), is heterozygous and produces 

 gametes of two kinds, with and without the fact or; the female, having two 

 sex-factors, is homozygous and produces eggs of one kind, with one sex- 

 factor each. Two types of union are here possible, giving males and 

 females with one and two sex-factors respectively. This interpretation 

 is directly applicable to those cases in which the male has one sex-chromo- 

 some and the female two (Ancyracanthus, Ascaris), and also to those 

 having an XY pair in the male and an XX pair in the female (Droso- 

 phila, Lygceus). Each sex-factor is thus thought to be located in an A- 



O0O0 



\ X / 



OQ 



/ \ / \ 



OQD0 



Fig. 144. — Mendelian interpretations of sex-inheritance. Explanation in text. 



chromosome. In other organisms (moths and birds) these conditions 

 are reversed (Fig. 144, C), the presence of two sex-factors being correlated 

 with maleness and one with femaleness. The female is thus heterozygous 

 and produces eggs of two kinds, with and without the factor. In the 

 next generation the male receives two sex-factors (in the Z-chromosomes 

 of the egg and sperm) and the female one (in the Z-chromosome of the 

 sperm; the W-chromosome of the egg carries no sex-factors). In view of 

 these two contrasted conditions as regards the quantitative relationship 

 between factors and sex, it is probable that the sex-factors carried by the 

 X-chromosomes are in some manner different from those in the /-chromo- 

 somes. It has been suggested that in some cases ( Fig. 144, B) the male 

 may have no sex-factors at all, the heterozygous female thus having one 

 more sex-factor than the male, as in the homozygous females of Fig. 

 144, A. 



Experimental Alteration of the Sex Ratio. Although most organisms 

 approximate closely the I : 1 sex ratio called for on the basis of the 



