176 ORIGIN OF NON-SEXUAL CHARACTERS 



were due to the loss of one factor affecting the eye, 

 the heterozygote carrying the normal factor from 

 the mother only might very well develop a some- 

 what imperfect eye. 



Morgan arranges the numerous mutations observed 

 in Drosophila in four groups, corresponding in his 

 opinion to the four pairs of chromosomes occurring 

 in the cells of the insect. After the meiotic or reduc- 

 tion divisions each gamete of course contains in 

 its nucleus four single chromosomes. One of the 

 four pairs consists of the sex-chromosomes. All the 

 factors of one group are contained in one chromo- 

 some, and it is found in experiments that the members 

 of each group tend to be inherited together — that is 

 to say, if two or more enter a cross together, in other 

 words, if a specimen possessing two or more muta- 

 tions is crossed with another in which they are 

 absent, they tend to segregate as though they were 

 a single factor. This fact agrees with the hypo- 

 thesis that the factors in such a case are contained 

 in a single chromosome which segregates from the 

 fellow of its pair in the reduction divisions. Excep- 

 tions may occur, however, and these are explained 

 by what is called ' crossing over.' When one 

 chromosome of a pair, instead of being parallel to 

 the other in the gametocyte, crosses it at a point of 

 contact, then when the chromosomes separate, part of 

 one chromosome remains connected with the part of 

 the other on the same side and the two parts separate 

 as a new chromosome, so that two factors originally 

 in the same chromosome may thus come to lie in 

 different chromosomes. In consequence of this, two or 

 more factors which are usually ' coupled ' or inherited 

 together may come to appear in different individuals. 



