GENETIC BASIS 



29 



the chromosomes is behaving hke a giant gene of 5 units of 

 value. The results to be expected by these two hypothetical 

 cases are compared in Fig. 1. It will be seen that they are 

 exactly the same general type of curve. The effects of link- 

 age are greatly to increase the chances of getting F2 recom- 

 binations very similar to the parental species and greatly 

 to decrease the percentage of segregants more or less similar 

 to the Fi. 



With linkage there is one chance in a thousand of obtain- 

 ing an F2 individual with the same combination of genes as 

 one of the parents. Without linkage, for the same number 

 of genes the chances would be only one in a million, million, 

 million, million, million (10~^^). In other words, if we grew 

 several hundred F2 plants of each of these two hypothetical 



Table 2 



s 

 o 



S| 



1- is 



o — 



C s 



1 



2 

 3 

 4 



N 



o ^«^ 



1:2:1 



1:4:6:4:1 



1:6:15:20:15:6:1 



1:8:28:56:70:56:28:8:1 



(1:2:1)^ 



o 3 "S 



S. 73 3 



o S " 

 i: P aj 



Me 



K- 



,iV 



cases, for those with the genes in 10 chromosomes we would 

 expect several plants closely resembling each parent, and 

 there is a very slight chance we might get one exactly like 

 one of the parents. In the second case the chances of any 

 such recombination (10~^^) are too remote for most human 

 minds to grasp. We could not possibly expect, among our 

 sample of a few hundred individuals, any recombination 

 resembling either parent at all closely. 



