178 



Heredity and Environment 



FIG. 59. DIAGRAM SHOWING UNION OF FACTORS IN FERTILIZATION AND 

 THEIR SEGREGATION IN THE FORMATION OF GERM CELLS. With 4 pairs of fac- 

 tors (Aa, Bb, Cc, Dd) 16 types of gametes are possible as shown in the 

 two series of small circles at the right. (After Wilson.) 



factors. The absence of a whole chromosome is plainly visible 

 under the microscope, whereas the absence of a single factor or 

 gene from a chromosome would never have been seen. These 

 fortunate cases in which the male lacks a whole chromosome seem 

 almost to have been intended to give ocular proof of the chromo- 

 somal theory of heredity ; they are to biology what the rings of 

 Saturn are to astronomy, a visible confirmation of a great 

 theory. 



We have in these facts a remarkable correlation between the 

 distribution of the chromosomes and the occurrence of certain 

 characters of the adult animal. The association of maternal and 

 paternal chromosomes in fertilization and their segregation in 

 the maturation of the germ cells is parallel to the association of 

 Mendelian characters in the zygote and their segregation in the 

 gametes ; if the distribution of chromosomes in cleavage is ab- 



