During conjugation of germ 

 cells, the two members of 

 each pair of chromosomes be- 

 come intimately intertwined. 

 When they separate again, 

 portions of the two chromo- 

 somes seem to have become 

 interchanged. The occasional 

 failure of linkage would seem 

 to be due to the occasional 

 interchange of chromosome 

 segments between the pater- 

 nal and maternal chromo- 

 somes of a particular pair 



CROSSING OVER 



are black, as the photograph shows; yet they are distinct in appearance — 

 and distinct in their hereditary or breeding behavior. When these two 

 types are mated, there appears only the "Blended Crossfox" type. When 

 these hybrids are mated, their offspring divide into nine easily recognized 

 types, which are shown in the picture. That is, the hybrids are hetero- 

 zygous with respect to some of the genes, or factors, that determine the 

 coat characteristics. This is, of course, what we should expect on a simple 

 Mendelian interpretation. But further study shows that the situation is 

 not simple. Among the offspring of these hybrids 25 per cent, on an 

 average, are of the parental hybrid type, the crossfox; but there are also 

 four other "hybrid types" — 12^ per cent of each. And finally, there are 

 four types that are "pure" — two like the black grandparents, as we might 

 expect, and two quite different. These two are the so-called "double black", 

 which is quite new, and the "red" fox — the original wild type. 



An analysis of these experiments indicates that there are probably two 

 pairs of genes that account for the facts. The types shown in the four 

 corners of the illustration all breed true; that is, each of the genes in ques- 

 tion occurs in a homozygous state — altogether dominant or altogether 

 recessive. This is represented by the symbols AABB, AAbb, aaBB, and 

 aabb. We can check this idea by working out (1) the result of inbreeding 

 any of the hybrids; and (2) the result of mating any two of the hybrid 

 types, using the Punnett squares.^ 



Each character of the organism, each part, perhaps even each gene in the 

 chromatin of a cell, influences the whole body. And each part or process is 

 influenced by all the others. The organism continues as a unity. 



Our method of study makes it necessary to analyze. We analyze the or- 



^See No. Ab, p. 504. 

 494 



