HEREDITY 



597 



Figure 422. A pair of strikingly similar identical twins. Identical twins are usually very much 

 alike in physical, mental, and personality traits. (Courtesy of The Toni Company.) 



ences between them are due to environ- 

 mental influences. 



In twins, if the embryo division is not 

 complete the result may be conjoined twins; 

 these are popularly known as Siamese twins 

 because the most widely publicized pair 

 came from Siam (Fig. 423). 



Quadruplets and quintuplets may include 

 fraternals and identicals. At this date, the 

 famed Dionne quintuplets of Canada, and 

 the Diligentes "quints," born in Argentina, 

 are the only groups of 5 which are known to 

 have survived infancy. The Dionne girls 

 were identical quintuplets. 



False ideas about heredity 



Telegony 



This is the idea that one mating leaves 

 an impression on the mother which affects 

 later births. The belief is widespread, but, 

 obviously, this is impossible, since the sperm 

 that effects the union with the egg is the 



only one that contributes to the offspring 

 and all others of that mating die. Thus 

 there is no way in which they can have an 

 influence in subsequent fertilizations. 



Maternal impressions 



This is another erroneous idea that modifi- 

 cation in the development of the fetus re- 

 sults from experiences that occur to the 

 mother during pregnancy. For example, a 

 prospective mother sees the hand of a small 

 child crushed accidentally, and so her baby 

 is born with a deformed hand. Underlying 

 this false notion is also the belief that the 

 mother contributes to the heredity of her 

 child throughout pregnancy. However, it is 

 true that the health and nutrition of the 

 mother may influence the development of 

 the embryo, but this is a matter of physiol- 

 og}', which is quite another thing. 



Prepotency 



So-called prepotency refers to the sup- 

 posed power of one parent to transmit its 



