CYTOLOGY AND MEN DELI AN HEREDITY 189 



notable feature is the frequent absence of crossing over between the 

 X and Y. 



The significance of all these facts is by no means well understood, but 

 there is a tendenc}^ to associate them with the advantages of dioecism. 

 It has been pointed out above that the sex chromosome mechanism 

 results in the production of males and females in equal numbers, even 

 though this ratio may be disturbed by other causes. The 1 : 1 ratio is 

 expected not only for sex but for all allelic character pairs in each genera- 

 tion in monoploid organisms. In diploid organisms the sexes also 

 appear 1 : 1 in each generation, even though most of the other character 

 pairs show a 1 :0 ratio in the F^ generation and a 3: 1 ratio in F2. This 

 numerical equality of the sexes in spite of inequality in other characters 

 is a significant result of the differentiation of a special sex chromosome 

 pair with unlike members. If X and Y exert different influences upon the 



^^<9. 





Fig. 136. — Sex chromosomes in a liverwort (Sphaerocarpos Donneliii) . a, genome in 

 female gametophyte. b, genome in male gametophyte. c, diploid complement in sporo- 

 phyte. d, first meiotic division in sporocyte. (After G. Lorhccr.) 



type of sex developed, it can be realized that crossing over could impair the 

 distinctness of the two sex differentiating processes and lead to deleterious 

 sexual states; hence the value of the lack of crossing over between these 

 chromosomes, at least in some organisms. Also suggestive is the further 

 fact that heteropyknosis evidently interferes Avith or prevents crossing 

 over. 



It is to be emphasized that the chromosomes are not alone responsible 

 for the type of sex developed. As in the case of all other characters, the 

 sex expressed is a result of the interaction of genetical and environmental 

 factors. The significance of the sex chromosomes lies in the fact that 

 they automatically maintain two genotypes in the species, these resulting 

 in the development of two types of organism, male and female, under 

 the range of environmental conditions normally encountered. This is 

 all that nature requires for the successful operation of the mechanism: 

 absolute distinctness between males and females in all cases and under 

 all possible conditions is not to be expected. The fact that abnormal 

 environments may sometimes alter the sex is accordingly no argument 

 against the sex-determining role of the chromosomes as properly con- 

 ceived. Under normal conditions the genotype is the sufficiently decisive 



