470 Orgams?HS Are Products of Heredity and E7iviron?nent unit ix 



Fig. 419 The guinea pigs at the left were the parents of those at the right. Were the 

 parents pure black or hybrid? Explain, (castle's genetics and eugenics) 



Mendel's law of segregation is truly a 

 law, for it holds for all of the plants and 

 animals which have been studied. There 

 are many. Among others tobacco, corn, 

 Jimson weed, squash, and snapdragon 

 have received much attention. Among 

 animals studied extensively are rats, mice, 

 cattle, rabbits, fowl, dogs, cats, horses, 

 insects, fishes, and man himself. But the 

 animal with which most experiments 

 have been performed is the fruit fly 

 (Drosophila melanogaster). 



The famous fruit fly. The fruit fly is a 

 tiny fly frequently found near ripe fruit. 

 You can show by diagrams the results of 

 one crossing of the fruit fly. See Exer- 

 cise 13. Its heredity has been studied in 

 laboratories all over the world because 

 it is particularly suitable for such work. 

 It is small, needs little food, and is easy 

 to raise. Its life cycle takes only two 

 weeks, so 25 generations can be studied 

 in one year. There are about 400 ofi^- 

 spring from one mating; these large 

 numbers give us reliable ratios. Then, 

 too, it has a relatively small number of 

 chromosomes, only four pairs, each pair 

 difi^ering from the others in size and 

 shape. Biologists probably know more 

 about the heredity of the fruit fly than 

 of any animal or plant. Morgan and 

 his students and fellow workers suc- 

 ceeded in developing a map of its chro- 



mosomes, showing at which point each 

 kind of gene must lie. 



Innumerable experiments with the 

 fruit fly have shown that in the inher- 

 itance of some characters there is domi- 

 nance, in others there is blending inher- 

 itance. But no matter which character is 

 studied, the law of segregation holds. 

 Can you explain the results indicated in 

 Figure 422? See also Figure 420. 



(Optional) Two pairs of characters. 

 Mendel asked himself this question: If 

 a tall plant with yellow seeds is crossed 

 with a short plant bearing green seeds, 

 will the tall plants always have yellow 

 seeds in later generations or will all or 

 some of them have green seeds? Experi- 

 ment showed that in the Fo some tall 

 plants had green seeds and some short 

 plants had yellow^ seeds. He thus dis- 

 cov^ered that the characters of size and 

 seed color are inherited independently 

 of one another. All the other pairs of 

 characters he studied were also inherited 

 independently of one another. Mendel 

 therefore formulated the bw of bide- 

 peiidevt assortment of characters or the 

 law of unit characters., meaning that 

 every character is passed on to the off^- 

 spring independently of every other. 

 This is not now accepted as a law be- 

 cause there are important exceptions, 

 as you will see later. 



