F2 RATIOS MODIFIED BY LINKAGE 



121 



It is evident that linkage will cause modification of the tj'pical dihybrid 

 ratio, 9:3:3:1, since the four possible classes of gametes formed by Fi 

 individuals will not all be equally numerous. Accordingly the stronger 

 the linkage, the greater will be the modification of the tj^pical ratio. Con- 

 versely, we may estimate the strength oj the linkage by the observed depart- 

 ure from the 9:3:3:1 ratio. 



In so doing, tables 10 and 11 may be found useful, in which the expected 

 modification of the 9:3:3:1 F2 ratio is given for various integral ratios of 



TABLE 11 



The F2 Ratio, 9:3:3:1, as Affected by Repulsion (Negative Linkage), 

 A AND B Entering the Fi Zygote in Different Gametes 



3 No repulsion. 



* Not distinguishable from the case in which A and B are allelomorphs. 



gametes showing the 'parental combinations, to gametes not showing them. 

 Morgan calls the gametes which show novel combinations crossover 

 gametes and those which show the original combinations non-crossover 

 gametes. If the latter are two, three, four, etc., times as numerous as the 

 former, then we get the modified F2 ratios shown in the tables, where also 

 formulae are given for extending the tables to any desired extent. In 

 making use of these tables, it is necessary only to reduce to the basis of a 

 common total the observed F2 zygotic series and any series of the table 

 with which a comparison is desired. This will be facilitated by consulting 

 Table 11a, in which each zygotic class of Tables 10 and 11 is expressed as 

 a percentage of the total population. 



