FUNCTIONAL HYBRIDITY 137 



properties which are seen, first, in the relation of the corresponding 

 genetic elements at meiosis and, secondly, in the differences between 

 their offspring. The study of meiosis in non-hybrid forms and 

 the variations already seen at mitosis enable one to classify hybrids 

 and predict the kind of chromosome pairing that will be found in 

 each class, as follows. (The term " hybrid " is used here for the 

 broader classes and the term " heterozygote " for the individual 

 categories discussed later). 



(i) Numerical Hybrids. The zygote is derived from the union 

 of gametes dissimilar in regard to the number of their chromosomes. 

 Trisomic and triploid organisms which owe their origin simply to 

 the reduplication of one or more chromosomes in one of the gametes 

 are the simplest of this kind. It is irrelevant to the consideration 

 of their properties at meiosis whether they result from mitotic or 

 meiotic irregularities. The behaviour of these hybrids at meiosis 

 has already been considered. 



(ii) Structural Hybrids. The zygote is derived from the 

 union of gametes dissimilar in regard to the structure of their 

 chromosomes, i.e., the linear arrangement of their chromomeres or 

 genes. The change in structure may also involve a change in 

 number, as in fragmentation. From what we know of the pairing 

 of chromosomes in polyploids we can predict that the chromosomes 

 of these hybrids will pair according to the relationships of their 

 parts, subject to these parts being long enough to allow of their 

 occasionally pairing at pachytene and forming a chiasma in the 

 length paired. 



(iii) Undefined Structural Hybrids. The zygote is derived 

 from the union of gametes dissimilar as a result of changes which 

 cannot be dehned. In this class may be considered those whose 

 parental gametes either had the same number of chromosomes or a 

 different number, through a change that cannot be identified. 

 These hybrids may be said to be " undefined " simply because the 

 structural differences between their chromosomes are too slight or 

 too numerous to be readily detected. We then find three types 

 of behaviour in this kind of hybrid at the first metaphase of meiosis, 

 according to the frequency of differences between the chromosomes : 

 (i) Normal behaviour owing to the differences being too slight to 



