GENETICS 223 



in the maturation of the germ cells, but the crossing of unlike individuals 

 demonstrates the nature of much that is invisible. 



Modem Genetics. — The story of the Austrian monk Gregor Mendel 

 as the leading figure in the beginnings of modern genetics has been 

 recounted in the opening chapter (page 18). Before going into the 

 details of hereditary transmission, it will be profitable to indicate briefly 

 wherein his ideas of heredity differed from those which preceded him; 

 for it must be remembered that Mendel was not the first student of 

 heredity. Many before him had tried to solve its mysteries, and the 

 mere fact of resemblance between parents and offspring, or even between 

 more distant relatives, had been recognized from time immemorial. 



One of the chief distinctions of the Mendelian system was the recogni- 

 tion that offspring do not necessarily inherit any particular character 

 of either parent. Not only do the offspring not have to show such a 

 character in themselves, they may even be quite incapable of transmitting 

 it to subsequent generations. Prior to Mendel's time there had been a 

 prevalent suspicion that any character which appeared in one or more 

 individuals in a given line of descent might be expected at some future 

 time to appear in any branch of their posterity. No one of the descend- 

 ants was to be regarded as free from the possibility of that character's 

 recurrence. According to this old notion, if in a given line of descent 

 of horses there had once been a chestnut animal, there was a distinct 

 expectation that some time or other the chestnut character would 

 reappear in some individual of any branch of the descending family. 

 According to the IMendelian scheme, it is now clear that this color may 

 be bred entirely out of the descendants. It is almost certain to be 

 bred out of some branches of the general relationship and may be lost 

 to all of them; and chestnut is no more likely to occur after such elimina- 

 tion than it is in a line which never had a chestnut ancestor. Later 

 we shall see why this is true. 



Another distinctive feature of Mendel's contribution to knowledge 

 of heredity was his discovery that characters may be transmitted quite 

 independently of one another. Wing length is one character, eye color 

 another, body color a third, and so on, each having its own inheritance. 

 Because of their separateness, such characters have been spoken of as 

 "unit" characters. Some degree of detachment of traits was, of course, 

 popularly implied when it was pointed out that a child had its mother's 

 eyes, its father's lips, and perhaps its grandfather's wavy hair. But 

 complete scattering of one individual's characters in succeeding genera- 

 tions was not previously thought to take place — certainly not as a regular 

 occurrence. Before Mendel's time there was a strong tendency to think 

 of heredity in terms of the totality of characters exhibited by an indi- 

 vidual; by Mendel himself emphasis was put upon the single characters. 



