HYBRIDS, SEGREGATION 



385 



the latter bears only one allelomorph. It is only on this basis 

 that the reappearance of a latent character in the second genera- 

 tion can be explained. If the dominant and recessive characters 

 of an aUelomorphic pair are represented by A and a respectively, 

 then the fertilised eggs produced by crossing will all have the 

 constitution Aa, and the resulting plants will all show the 

 dominant character only ; such individuals are spoken of as 

 heterozygotes. It is believed that in these hybrids, during the" 

 formation of each tetrad of spores, and in conjunction with the 

 reduction division (see p. 306), the characters A, a become 

 separated, so that each spore, and consequently each of the 



Fig. 224. — Geum rivals (A), Geuni urbanum (C), and the hybrid between 

 them, Geum intermedium (B), In each case the receptacle and calyx 

 arc shown, together with an enlargement of a single carpel. 



gametes, to which it ultimately gives rise, bear either A or a. 

 It is probable, moreover, that the determinants for these charac- 

 ters are contained within the nuclei of the sexual cells (p. 390). 

 During self-fertilisation, and assuming the gametes to meet 

 according to the laws of chance, there are four possible com- 

 binations : viz. (i) a male gamete bearing A and a female bearing 

 A; (2) a male bearing A and a female bearing a; (3) a male 

 bearing a and a female bearing A ; and (4) a male bearing a and 

 a female bearing a. That is to say, among each four fertilised 

 eggs that result, there will probably be i AA, 2 Aa, and i aa. 

 Seeing that A is invariably dominant, there will in the second 

 generation be three individuals with the dominant to one indi-' 

 25 



