54 INTROGRESSIVE HYBRIDIZATION 



only 42 in 10,368 which are more extreme recombinations 

 than any of those in the F2. 



Up to this point we have considered merely the effect of 

 linkage in any one chromosome. Actually, of course, the 

 recombination of any 2 multiple-factor characters will de- 

 pend on how many genes are concerned, how they are dis- 

 tributed through the chromosomes, and the chromosome 

 number. As an instructive limiting case, let us consider the 

 recombinations of 2 multiple-factor characters, each due to a 

 large number of genes more or less evenly distributed be- 

 tween the chromosomes. Let us suppose that there was only 

 1 pair of chromosomes and complete linkage. In the F2 we 

 would have only 3 types of individuals — those with both 

 chromosomes from one parent, those with 1 of each, and those 

 with both of the other. Our recombination spindle would be 

 a line reaching from one parental corner to the other with fre- 

 quencies of 1 at each end and of 2 in the middle. With 2 

 pairs of chromosomes and with the other conditions re- 

 maining the same, we have 5 possible types of F2 in- 

 dividuals, with frequencies of 1-4-6-4-1. Again, as a 

 recombination spindle, they would be restricted to an 

 absolute line running from one corner of our figure, to 

 the F' position in the center, to the opposite diagonal 

 corner. 



With more and more chromosomes, as long as the genes 

 for the 2 characters were many and were distributed at 

 random, we would still have an absolutely attentuated re- 

 combination spindle consisting of a mere diagonal line across 

 the square representing all the possible recombinations. The 

 larger the number of chromosomes, the greater would be 

 the chance of achieving F2 recombinations very similar to 

 the Fi, and the slighter would be that of recombinations 

 similar to one parent or the other. With a large number of 

 chromosomes there might be many possible genotypes, but 

 they would all go in a graded series from one parental ex- 

 treme, to the Fi, to the other parental extreme, and increase 

 in one character in the direction of one of the parents would 



