GENE RECOMBINATION AND HETEROSIS 



315 



chromosomes and diflferent numbers of genes, combinations of genes in the 

 parents, and number of linkage groups would not alter the conclusions to be 

 drawn. In the illustration chosen only two linkage groups are shown and each 

 has three pairs of genes. Also, the top row of genes represents the gamete from 

 one parent and the lower row of genes the gamete from the other parent. In 

 all three assumed cases, 3 plus and 3 minus genes entered the cross from 

 each parent. 



It is evident that innumerable plausible cases could be assumed, but the 

 fundamental principles derived from a consideration of the theoretical values 

 given in the table would not be altered. One further assumption should be 



TABLE 19.9 



THEORETICAL NUMBER OF INDIVIDUALS IN THE F2 POPU- 

 LATION THAT CARRY 12 PLUS (+) GENES WHEN THE PAR- 

 ENTS ARE DIFFERENTIATED BY 6 PAIRS OF GENES, EACH 

 OF 2 CHROMOSOME PAIRS CARRYING 3 PAIRS OF GENES* 



* The crossover values for each section of the chromosome being equal and of the magni- 

 tude shown. 



mentioned. In every case the plus genes are assumed to give an increase in 

 some desirable quantitative character and, comparatively, the minus genes 

 a decrease. Finally, in the table two extreme situations are shown, namely 

 that in which there is no crossing over and that in which the two sections 

 of the chromosome between adjacent genes show 50 per cent of crossing over. 



The data in the second column apply to that situation in which all of the 

 plus genes occur in one member of the homologous chromosomes in each of 

 the two pairs of chromosomes depicted. In the case of 50 per cent of crossing 

 over or independent inheritance, only 244 individuals in a million of the F2 

 population possess all twelve plus genes. The number of such individuals 

 among a million F2 individuals increases with a decrease in the percentage 

 of crossing over until with no crossing over 62,500 individuals in a million 

 possess all six pairs of the plus genes in the homozygous condition. 



The data in the third column apply to that situation in which two plus 

 genes are linked with one minus gene in one member of a chromosome pair 

 and two minus genes with one plus gene in the other member of the same 



