G£N£r/CS 581 



of inbreeding itself. It is, in fact, one of the standard procedures used 

 by commercial breeders to improve strains of cattle, corn, cats and canta- 

 loupes. It is not necessarily a bad practice in the human species. In all 

 anmials or plants it simply tends to make the strain homozygous. All 

 natural populations of individuals are heterozygous for many traits; 

 some of the hidden recessive genes are for desirable traits, others are 

 for undesirable ones, inbreeding will simply permit these genes to be- 

 come homozygous and lead to the unmasking of the good or bad traits. 

 If a stock is good, inbreeding will improve it; but if a stock has many 

 undesirable recessive traits, inbreeding will lead to their phenotypic 

 expression. 



The crossing of two completely unrelated strains, called outbreed- 

 ing, is another widely used genetic maneuver. It is frequently found 

 that the offspring of such a mating are much larger, stronger and 

 healthier than either parent. Much of the corn grown in the United 

 States is a special hybrid variety developed by the United States Depart- 

 ment of Agriculture from a mating of four different inbred strains. 

 Each year, the seed to grow this uniformly fine hybrid corn is obtained 

 by mating the original inbred lines. If the hybrid corn were used in 

 mating it would give rise to many different kinds of corn, since it is 

 heterozygous for many different traits. The mule, the hybrid offspring of 

 the mating of a horse and donkey, is a strong, sturdy animal, better 

 adapted for many kinds of work than either of its parents. This phe- 

 nomenon of hybrid vigor, or heterosis, does not result from the act of 

 outbreeding itself, but from the heterozygous nature of the F^ organisms 

 which result from outbreeding. Each of the parental strains is homo- 

 zygous for certain undesirable recessive traits, but the two strains are 

 homozygous for different traits, and each one has dominant genes to 

 mask the undesirable recessive genes of the other. As a concrete ex- 

 ample, let us suppose that there are four pairs of genes. A, B, C and D; 

 the capital letters represent the dominant gene for some desirable trait 

 and the lower case letters represent the recessive gene for its unde- 

 sirable allele. If one parental strain is then AAbbCCdd and the other 

 aaBBccDD the offspring will all be AaBbCcDd and have all of the de- 

 sirable and none of the undesirable traits. The actual situation in any 

 given cross is undoubtedly much more complex and involves many 

 pairs of genes. 



286. Population Genetics 



A question that appears to trouble many new students of genetics 

 is why, if the gene for brown eyes is dominant to the gene for blue 

 eyes, are there any blue eye genes left? The answer lies partly in the 

 fact that a recessive gene, such as the one for blue eyes, is not altered 

 in any w^ay by existing for a generation in a heterozygote next to a 

 brown eye gene. The rest of the explanation follows from the fact that 

 as long as there is no selection for either eye color, as long as people 

 with blue eyes are just as likely to marry and to have as many children 



