264 Practical Plant Biology. 



by the respective parents. In the cell-divisions which convert the 

 oosperm into the embryo each resulting cell has both kinds of 

 determinants distributed to it ; but inasmuch as the determinant 

 of roundness is dominant the embryo develops round. Originating 

 from gametes carrying alternative determinants this individual is 

 called a " Heterozygote ". To account for the observed offspring 

 of this hybrid plant, Mendel assumed that at the formation of the 

 gametes a separation and sorting of the determinants of the pairs 

 of allelomorphs take place, so that each gamete receives the de- 

 terminant of roundness or the determinant of angularity, but no 

 gamete ever receives the two determinants of a pair. Since large 

 numbers of gametes are formed and there is an equal number of 

 those containing the determinant of roundness as of those with 

 that of angularity, it is evident that there is an equal chance for 

 the unions RR, R<z, #R, and aa (indicating the gametes by the 

 initial letter of the allelomorph, the determinant of which they 

 contain, and putting a capital letter for the dominant and a small 

 one for the recessive determinant.) In the diagram above the 

 enclosed small circles and squares represent the constitution of 

 the gametes. 



We have now strong evidence to compel us to believe that this 

 segregation of determinants, postulated long ago by Mendel on 

 theoretical grounds, may be actually observed in the reducing 

 mitosis of the spore-mother-cells and in other tetrad mitoses. 



The characters of roundness and angularity in the embryo are 

 only one pair of many allelomorphic pairs in peas investigated by 

 Mendel. Another one was also a character of the embryo and 

 recognisable as soon as the seed is mature. It was that of green- 

 ness and yellowness. In some varieties the embryo is green in the 

 ripe seed, while in others it is yellow. Mendel showed that this 

 character is transmitted according to exactly the same scheme as 

 that which defines the transmission of roundness and angularity. 

 In this case yellowness is dominant and greenness is recessive. So 

 that the hybrid formed is yellow and when self-pollinated it bears 

 75 per cent, yellow and 25 per cent, green embryos. As before 

 when the individuals of this F 2 generation are self-pollinated it is 

 found that one third of the yellow produce nothing but yellow 

 embryos, and two thirds of the yellow have a mixed progeny. The 

 green when selfed produce green only. 



In these two examples complete dominance of one determinant 

 over the other of the pair is illustrated. This complete dominance 

 appears to be the general rule. However, some cases are known 

 where the heterozygote, or hybrid, resulting from the fusion of two 



