GENETIC BASIS 27 



genes such as would be found in a species cross, and with no 

 further compHcating factors, the F2 would be composed of 

 individuals no two of which would be exactly alike but most 

 of which would be intermediate between the two parents. 

 Recombinations somewhat resembling either parent would 

 be very much in the minority. In a similar way with a large 

 number of independent factors all the backcrosses would 

 tend to be different from each other and for the most part 

 intermediate between the Fi and the recurrent parent. In- 

 dividuals closely resembling this parent (as in the F2) would 

 be in the minority but not so strikingly as in the F2. 



If we study the curve (1:2:1)'' we find that, with an in- 

 creasing number of independent genes responsible for the 

 differences between the two species, there is a great increase 

 in the proportion of the F2 plants that are about as inter- 

 mediate as was the Fi. At the same time the number of dif- 

 ferent genotypes that can produce this intermediate condi- 

 tion also rises enormously. With a very large number of 

 independent genes we expect an F2 that phenotypically is 

 not very different from the Fi yet that genotypically is tre- 

 mendously variable from plant to plant. 



So far we have considered the kinds of results that would 

 be obtained by many independent genes all affecting the 

 same character. Actually, of course, such results are ab- 

 solutely impossible in any plant or animal known to science. 

 The germplasm is not made up of tiny independent units. 

 It is organized into chromosomes — long, narrow, threadlike 

 protein aggregations with longitudinal differentiation of the 

 germinal material. The genes in any one chromosome are not 

 free to assort at random with each other. A certain amount 

 of recombination is possible, the exact amount depending 

 on how much crossing over takes place at the reduction di- 

 vision and on the extent to which crossovers tend to be 

 localized. In any case, however, the gene recombinations 

 that can be achieved within a chromosome pair are an al- 

 most infinitesimal fraction of what could be obtained with 

 the same number of completely independent genes. 



