8o GENETICS 



Different Conditions as to Sex in Different Organisms 



There are many other conditions with respect to sex 

 among different organisms. It will not be possible to deal 

 with all these, but it will be worth while to enumerate some 

 of the more important conditions.^ 



(a) Organisms with the two sexes in separate individ- 

 uals, the sex being determined by differences in the chromo- 

 somes, as by the presence of one X or of two. It is mainly 

 such organisms that have been dealt with in the foregoing 

 pages. As we have seen, there are among these two groups, 

 having diverse relations of chromosomes to sex. In Group I, 

 the female has two X-chromosomes, the male but one; in 

 Group II the reverse condition is found. 



(b) Organisms having both sexes united in one individ- 

 ual (hermaphrodites). The same individual produces both 

 ova and sperms. This condition exists in many organisms, 

 as in common snails. The relation of this condition to chro- 

 mosomes is not clear. 



(c) Organisms in which the individuals are male during 

 one period of their lives, female during another. Certain 

 fish show this condition; it is indeed found in many animals. 



(d) Organisms that are diverse as to sex in successive 

 generations. Among these are to be distinguished several 

 different types, as follows: 



(i) Some organisms are hermaphrodites in one genera- 

 tion, but have separate sexes, male and female, in a later 

 generation. Certain thread worms (Nematodes) show this 

 condition. 



(2) Some have no sex In one generation, reproducing 

 without sperm and ova, but in the next generation have the 

 two sexes, or are hermaphrodites. This is the condition 

 found in plants. 



(3) Some are exclusively female in certain generations, 

 reproducing by parthenogenesis, the eggs not requiring 



