156 LIFE: ITS NATURE AND ORIGIN 



Mendel's Second Law: Independent Assortment 



When peas having yellow and round seeds were crossed with peas 

 having green and wrinkled seeds, Mendel found that all the Fi 

 seeds were yellow and round, these characters being dominant over 

 the recessive alternates, green and wrinkled. But the plants from 

 these Fi seeds, when self-fertilized, gave seeds, and therefore 

 progeny, of jour types: yellow/round; green/wrinkled; green/ 

 round; green/wrinkled, in the ratios 9:3:3:1. To explain these 

 results Mendel assumed that the segregation of the genes of the 

 yellow/green pair takes places independently of the segregation of 

 the round/wrinkled pair; so that there would be four equally 

 numerous kinds of ova and four equally numerous kinds of 

 pollen grains, which, uniting at random during fertilization, 

 would give sixteen different combinations that yielded (bearing in 

 mind the factor of dominance) visible types in the 9:3:3:1 ratio 

 actually found. 



Linkage 



Soon after the resurrection of Mendel's work, Sir William 

 Bateson and R. C. Punnett of Cambridge (England), in making 

 experiments with sweet peas, found that the factor pairs (red 

 vs purple flower) and (long vs round pollen grain) did not assort 

 at random. When (red) and (long) entered a cross together, they 

 tended to remain together in subsequent generations to a greater 

 extent than warranted by random assortment. This failure of 

 random assortment is understandable on the assumption that the 

 genes of the two pairs involved act as discrete units or gene blocks 

 in the chromosomes. Chromosomes are microscopically resolvable 

 bodies in the cell; they contain invisible genes arranged in linear 

 order. Chromosomes were well known to cytologists, but their 

 importance in genetics became evident in 1907 when Frances E. 

 Lutz, working with the evening primrose, Oenothera (the experi- 

 mental plant used by H. de Vries), found a great variation in the 

 the number of chromosomes in the various varieties, as follows: 

 O. lamarckiana, 14 (its variety gigas has 28); O. lata and O. albida 

 have each 15 chromosomes. Gates independently published some 

 of these results a few months later. 



In 1901 Charles E. McClung of Princeton discovered unequal 

 chromosomes in the two sexes, a fact brought out in Figure 18, 

 indicating how the chromosomes behave in the formation of sperm 



