3l8 GENETICS 



been found in many different animals and plants. Such 

 changes in the numbers of chromosomes alter the develop- 

 ment, and so yield Individuals having different character- 

 istics from those which have all the chromosomes in pairs. 

 The differences result mainly from differences in the balance 



A 



B 



D 



Figure 64. Diagram to illustrate the formation of trisomies (C), 

 and zygotes with but one chromosome in one of the pairs (F), The 

 basic number of chromosome pairs is here four. At A, a germ cell 

 with two chromosomes of the second pair (from the left) unites with 

 a normal germ cell B to form the trisomic C, which has three 

 chromosomes for the second pair. At D, a germ cell lacking a chro- 

 mosome of the second pair unites with a normal germ cell E to 

 form the zygote F, having but one chromosome of the second pair. 



among the genes and chromosomes (compare page 67). A 

 trisomic individual differs in certain respects from the typ- 

 ical individuals; an individual with but one chromosome in 

 place of two differs in another way. These matters have 

 been most thoroughly studied in certain plants, particularly 



