RELATION OF GENES TO CHARACTERISTICS 1 99 



type of twins, the two are derived from separate eggs; such 

 dizygotic twins have diverse gene combinations. 



By comparing the similarities and differences of identical 

 twins with those in other individuals, and particularly with 

 those of twins of the other type, it is possible to find out 

 what characteristics are affected ( i ) by having all the genes 

 alike, (2) by having different combinations of genes. 



"Identical twins," or offspring all having the same genes, 

 occur in many organisms. In lower animals (Protozoa, 

 plants. Hydra, worms, etc.,) a single individual often divides 

 into two or more, by "vegetative" reproduction. All these 

 individuals have the same sets of genes, since in vegetative 

 reproduction every chromosome and every gene divides. 

 Such individuals are very exactly alike. They have been 

 studied a great deal. In such a Protozoan as Parmecium, a 

 great number of individuals are produced in this way, all 

 closely alike. But when conjugation takes place, this gives 

 new gene combinations, and the individuals after they have 

 conjugated are very different. 



In one of the vertebrates, the armadillo, identical twins 

 are formed in essentially the same way that they are in 

 man, but are formed regularly.^ A single fertilized egg, after 

 developing some distance, divides into four, so that four 

 "identical twins," or quadruplets, are produced at each 

 birth. The four are extraordinarily alike even in fine details. 

 But twins belonging to different sets (and so having differ- 

 ent gene combinations) are often very different. 



Identical Twins in Man 



Identical twins are produced in man by the division of a 

 single egg into two, so that they may be called monozygotic 

 or "one-egg" twins. There occur also dizygotic or "two- 

 egg" twins, produced from two separately fertilized eggs; 



