CHAPTER XV 

 CYTOLOGICAL ASPECTS OF HYBRIDITY 



The traditional conception of a hybrid was that it was the offspring of 

 parents belonging to different species. From a practical point of view, a 

 hybrid was an individual manifesting a combination or blend of characters 

 from those species. The advances in modern genetics and cytology have 

 led to an interpretation that is at once broader in its basis and more 

 specific in its designation of what constitutes hybridity. It is now usually- 

 stated that a hybrid is the product of the union of two genetically unlike 

 gametes, whatever their source. They may come from two species, 

 varieties, or inbred lines, or even from the same heterozygous bisexual 

 individual. They may or may not manifest combinations or blends of 

 characters, for dominance often renders them indistinguishable from one 

 parent. Hence, in the modern view, the essence of hybridity lies in the 

 genetical constitution of the individual itself rather than in the taxonomic 

 relationship of the parents which contributed to this constitution, 

 although it is of course realized as fully as ever that the crossing of indi- 

 viduals with greater than vaz'ietal differences is of special importance in 

 the evolution of natural types. 



The essential constitutional state of an ordinary diploid hybrid lies in 

 the unlikeness of its two genomes. The smallest degree of difference 

 between them is seen in an individual heterozygous for only one gene. 

 (It may be pointed out in passing that this condition may also arise by 

 mutation in the individual, rather than by gametic union.) Heterozj^- 

 gosity in its many possible degrees thus constitutes hybridity in modern 

 genetics. The two genomes may also differ in the arrangement of the 

 genes in regions of certain chromosomes, as in the plants heterozygous 

 for translocations and inversions described in Chap. XIII. Such plants 

 are referred to as structural hybrids, even though the}" may conceivably' 

 contain no heterozygous pairs of genes. Furthermore, one genome may 

 have genes with no counterpart in the other, as in plants with a deletion 

 in one genome, or in a hybrid between two rather distantly related types. 

 The number of chromosomes may even differ in the two genomes. The 

 present tendency is, therefore, to include a wider variety of cases under 

 the heading of hybridity, but to state more specifically what is common to 

 all of them: a dissimilarity in the genomes responsible for their cytogenet- 

 ical behavior. 



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