454 THE MAINTENANCE OF SPECIES 



cannot produce milk but they can sire heifers that do. In these 

 cases, instead of predicting what the offspring will do by observing 

 the parental performance, the offspring themselves are taken to show 

 what their parents can do in producing desirable progeny. Mendelism 

 has shown that selection of any kind, in order to be effective, must 

 deal with genotypes rather than phenotypes, and that the material 

 from which selection is made must be hybrid rather than pure in its 

 composition if progress is to result. 



Inbreeding and Cousin Marriage 



Inbreeding in various degrees of consanguinity or blood relationship 

 tends to produce uniformity, or purity, in the hereditary stream. 

 Notwithstanding popular opinion to the contrary, inbreeding in itself 

 is not harmful. It simply tends, in the case of hybrids, to bring 

 recessive traits out into the open, and these are in many instances 

 less desirable than dominant characters. Cousin marriage in highly 

 hybridized human stocks is a potent way of unearthing "skeletons in 

 the closet," for cousins, being of approximately the same hereditary 

 make-up, are apt to carry concealed the same recessive characters, 

 which thus have a Mendelian chance of getting together and becom- 

 ing somatically visible. On the other hand, when people not closely 

 related are mated together, their undesirable recessive traits, being 

 different in each 'parent, are likely to remain concealed or covered up 

 by corresponding dominants contributed by the other parent. For 

 example, \i Aa and Aa represent two similar related individuals of 

 the same make-up so far as the characteristics A and a are concerned, 

 there is one chance in four, according to the Mendelian monohybrid 

 ratio, that the undesirable combination of aa will appear in the off- 

 spring. If, however, two unrelated individuals, Aa and Bh, carry 

 undesirable gametes represented by the small letters a and h, there 

 is only one dihybrid chance in sixteen that the individual showing the 

 undesirable recessive combination aahh, with no concealing dominant 

 to interfere, will appear, and there are only three additional chances 

 each out of sixteen that either the aa or the hb recessive characteristic 

 will come to light. (See checkerboard on page 451.) 



In nature there are many instances where inbreeding is enforced. 

 Wheat, and cereals generally, as well as the legumes to which Mendel's 

 peas belong, are habitually self-fertilized, and this is even closer 

 inbreeding than brother and sister mating, to say nothing of the 

 pairing of cousins. 



