vi CHROMOSOMES AND SEGREGATION 171 



The zygote AAAa produces gametes A A and Aa, and therefore if 

 bred with its like will produce among its offspring no pure aaaa. 



The zygote AAaa produces four kinds of gametes, AA, Aa, Aa and 

 aa, and therefore will produce one pure aaaa among every sixteen off- 

 spring, or i : 15. 



The zygote Aaaa produces gametes Aa and aa, and therefore we will 

 find one aaaa among every four offspring, or i : 3. 



Similarly, if crossed with a pure aaaa plant (producing gametes aa) 

 the first kind of heterozygote (AAAa) will produce no aaaa among its 

 offspring, the second kind (AAaa) will produce one to every three others, 

 and the third (Aaaa) equal numbers of aaaa and Aaaa. 



The last two of the three types of heterozygotes described above can 

 be identified among the tetraploid Primulas. Thus, taking the charac- 

 teristics green style (dominant) and red style (recessive) , one heterozygous 

 plant, self- fertilized, gave forty-four green and two red, i.e. approxi- 

 mately 15 : i. It was therefore an AAaa plant. Ten others gave ninety- 

 nine green and thirty-four red, i.e. 3:1. They were therefore Aaaa 

 plants. 



Again, taking the characteristics short style (dominant) and long 

 style (recessive), one heterozygous short styled plant crossed with a 

 long styled plant (i.e. pure recessive, aaaa) gave thirty-seven short and 

 fifteen long, or approximately 3:1; i.e. this heterozygote was AAaa. 

 Another plant gave, with the same cross, forty-nine short and forty-seven 

 long approximate equality, so that this plant was Aaaa. 



Thus the quadruple set of chromosomes is shown to be associated with 

 a quadruple set of hereditary factors. 



(2) Segregation and Parthenogenesis 



If the segregation of characteristics which is found in inheritance is 

 due to the separation of homologous chromosomes in gametogenesis, 

 it is clear that there should be no such segregation in obligatory 

 parthenogenetic reproduction, since all the offspring contain the same 

 chromosomes as their parent. Many species of the plant genus Hieracium 

 produce some egg cells in which meiosis takes place (and therefore, 

 having the reduced number of chromosomes, are capable of fertilization), 

 and also others in which reduction does not take place and which develop 

 parthenogenetically. In many species the latter egg cells are much more 

 numerous and hence reproduction is mainly parthenogenetic ; the 

 occurrence, however, of a certain number of haploid egg cells permits the 

 possibility of sexual reproduction and hence allows of crossing between 

 different species. It is found that the hybrids so formed are constant 

 under parthenogenetic reproduction that is to say, the parental char- 



