THE FUNDAMENTALS OF MENDELISM 47 



two poles of the meiotic spindle. Direct evidence of this independence 

 was found by Carothers^ in the Orthopteran Trimerotropis where there 

 was a second unequal pair in addition to an unequal sex pair. In nearly 



Fig. 13. Random Assortment of Chromo- 

 somes. — In animals with a heteromorphic 

 pair of chromosomes (/j) either the / or the 

 7-shaped chromosome may go to the same 

 pole as the unpaired X in the reduction 

 division, and these two alternatives happen 

 with equal frequencies. 



(After Carothers.) 



1 j 

 I - J 



exactly half the cases (51-3 per cent) the larger number of the unequal 

 pair segregated to the same pole as the X, as would be expected if the 

 chromosomes were independent. 



Independence of characters is not foimd in the exceptional cases 



Fig. ^U. Linkage. — The gametes produced by a heterozygote are best identified 

 by crossing the heterozygote to a double recessive (the "backcross"). 



PI. AA66 X aabb 



(gametes AB) | (gametes ah) 



F1. 



gametes 



-AB . ab cross with aabb 



I 

 gametes 



—(1 - x) AB (1 - x) ab xAb xab 



F2. (1 - x) Aabb (1 - x) aabb xAabb xaaBb \ ab 



When there is no linkage, x = ^ and AB, Ab, aB and ab gametes are formed In 

 equal numbers, which gives independent assortment. 



When linkage is complete, x = and only AB and ab gametes are formed. 



When there is some linkage, x is between and ^ and more gametes are formed 

 of the original kinds AB and ab than of the new combinations Ab and 06. 



in which the chromosomes do not behave independently of one another 

 (cf. ring formation, p. 109.) 



Limited independence of factors, or Hnkage, was at first explained as 

 a consequence of differential multipUcation of the cells containing the 

 different combinations of factors, which were supposed to be segregated 

 ^ Cf. Carothers 1926, 



