PROBLEM 2. How New Hereditary Characters Appear 



Fig. 431 Dr. H. J. Midler in 

 his laboratory x-raying flics. 

 What nmtatioTis i/iight oc- 

 cur? See Figure 42^ for pos- 

 sible results, (life) 



481 



The rate of mutation may be increased. 



Some years ago Professor Hermann J. 

 iMuller, an American biologist, succeeded 

 in increasing the rate of mutation in the 

 fruit fly by the use of x-rays. He counted 

 the number of mutations that occurred 

 among flies that had been x-rayed and 

 among flies that had not been exposed 

 and found that far more mutations oc- 

 curred in the x-rayed flies. Similar re- 

 sults have been obtained in experiments 

 with plants. 



Changes in chromosome number. Biol- 

 ogists have examined the chromosomes 

 of the organisms in which new characters 

 appeared. Sometimes they do not have 

 the chromosome number normal to the 

 species. The plant or animal may have 

 half the number (haploid); more often 

 it has one whole set of chromosomes 

 beyond the normal number, or two ex- 

 tra sets, or even more. Mitosis or matu- 

 ration may be irregular and cause changes 



Fig. 432 The ]imson weed {Datura) on the 

 left has the half (haploid) number of chromo- 

 somes in its cells; the one on the right has the 

 norvial (diploid) number. What differences do 

 you note besides height of the plants? (blakes- 



LEE — AMERICAN GENETICS ASSOCIATION) 



