224 



FUNDAMENTALS OF CYTOLOGY 



genomes differing in genie constitution and therefore in influence upon 

 characters. This difference arises in practically all cases from the union 

 of nuclei from two sources, each nucleus in the individual having a 

 "hybrid" chromosome complement when the two sources contribute 

 unlike sets of genes. This is evident enough in the ordinary diploid 

 hybrid employed in most cytogenetical researches, but because there 

 are other ways in which unlike outfits of genes may be associated in an 

 individual, it will be well at this point to pass them in brief review, 

 beginning with the ordinary intraspecific hybrid (see Fig. 166). 



5 6 7 3 



Fig. 166. — Diagram illustrating various ways in which unlike genetical elements 



may be combined in an organism. Rectangles and circles represent cells and nuclei, 

 respectively. Explanation in text. 



(1) Two similar genomes with one or more genes in the heterozygous 

 state. (2) Two genomes similar in their genes but differing in the 

 arrangement of these in the chromosomes: "structural hybrids" with 

 chromosomal regions translocated or inverted in one of the genomes. 



(3) Two genomes with \vider genie differences and often differing in 

 chromosome number: diploid interspecific hybrids with synapsis and 

 reassortment of specific characters, or with asynapsis and sterility. 



(4) More than two genomes from two species ^^dth regular or irregular 

 cytogenetical behavior: polyploid hybrids, most of whose gametes are 

 themselves "hybrid" in constitution. (5) Two unfused monoploid 

 nuclei carrying different genomes: dikaryotic hybrids in certain fungi 

 (page 163). (6) One genome only, this being composed of chromosomes 

 and genes from different sources: "monoi^loid hybrids," or "ha])l()micts," 



