2 28 READINGS IN BIOLOGICAL SCIENCE 



SOME BEARINGS OF GENETICS ON 

 HUMAN AFFAIRS * 



OTTO L. MOHR 

 INTERMARRIAGE AND CROSS-BREEDING 



A series of fundamental problems has, thanks to our modern knowledge 

 of heredity, come into an entirely new light. This applies, for instance, to 

 the old question of inbreeding. There is a widespread popular belief that 

 intermarriage, e. g., marriage between first cousins, is to be advised 

 against, since occasionally unfavorable results are seen. On the other hand 

 there is ample evidence, also in human material, that intermarriage has no 

 harmful effects at all. As historical examples of very close inbreeding in 

 man, the brother-sister marriages among the Ptolemaic family of old 

 Egypt as well as among the Incas of Peru and the Aztecs of Mexico may 

 be mentioned. 



The question is of foremost importance, and an enormous amount of 

 experimental work has been devoted to its solution. The results of these 

 investigations may be summarized thus: inbreeding as such has no harmful 

 effects at all. On the contrary, the astounding progress within animal 

 breeding has been mainly based upon close inbreeding among the off- 

 spring of a limited number of prominent sires. In this way we are able to 

 "recapture" as many valuable genes as possible of those carried by the 

 prominent sire in question. Valuable genes, this is the nub of the problem. 

 If an undesirable recessive gene happens to be present within the family 

 strain beforehand, then inbreeding will favor the occurrence of indi- 

 viduals that receive the undesirable gene in double dose, in which case the 

 corresponding harmful character will come to light. The unfortunate re- 

 sults sometimes seen in consanguineous marriages are in other words not 

 due to inbreeding as such, but to the presence of undesirable recessive 

 genes in heterozygous condition in the antecedents of the family. Con- 

 versely, if the hereditary factors in the family are good, then even close 

 inbreeding will give valuable offspring. 



THE BLUE BLOOD 



Some people are proud when they are able to trace their pedigree back 

 to the portrait of a remote ancestor. From a genetic point of view such 

 "pedigrees" are rather comic. Disregarding possible cases of intermar- 

 riage, we have already 64 ancestors in the sixth generation. What does it 

 matter to know one of these, when the rest, the 63 unknown ones, are 

 genetically equally important? If we go the other way, trying to con- 



* Reprinted from Heredity arid Disease by Dr. Otto L. Mohr by permission of 

 W. W. Norton and Co., Inc., New York. Copyright 1934, by the publishers. 



