HEREDITY 



595 



tion and distributed to offspring, just as are 

 normal genes. The sex-linked recessive gene 

 for one type of hemophilia is located in the 

 X chromosome; males containing such a 

 chromosome and females with two X chro- 

 mosomes bearing this gene are hemophilic. 

 Females with only one gene for hemophilia 

 do not exhibit this condition. If a recessive 

 gene for a defect is borne by an autosome, 

 two allelic genes must be present in both 

 males or females before the defect appears. 

 Usually it is impossible to determine when 

 an individual has only one recessive gene 

 for a defect; hence it is often impossible to 

 weed out defectives other than those that 

 have two defective recessive genes, that is, 

 those who actually exhibit the defects as a 

 trait. 



What is the effect 

 of inbreeding? 



Harmful genes are, for the most part, 

 recessive. This fact has an important bear- 

 ing on the results of inbreeding, that is, 

 mating of closely related individuals. It is 

 obvious that there may be various degrees 

 of inbreeding, depending on the closeness 

 of the relationship, varying from self-fertil- 

 ization, through brother-sister matings, to 

 the mating of distant cousins. It is the 

 popular belief that the children of parents, 

 closely related, for example, brother-sister 

 marriages, are likely to be defective. Hence 

 human laws nearly everywhere forbid 

 brother-sister marriages and even first cousin 

 marriages. Experiments with many plants 

 and animals prove that in most instances, 

 inbreeding weakens the race and outbreed- 

 ing strengthens it. These results in man, in 

 other animals, and in plants, are due to the 

 fact that inbreeding tends to increase rap- 

 idly the homozygosity of a population, that 

 is, progeny in whom pairs of allelic recessive 

 genes are present. These individuals, since 

 defects are usually recessive, will exhibit 

 more defects. If, however, inbreeding is 

 carried on so as to preserve advantageous 



traits, stocks may be secured that are much 

 superior to the original and will remain so. 



Thus we see that inbreeding in itself does 

 not create weaknesses, but it does increase 

 the opportunity for harmful recessive genes 

 to come to expression. 



Outbreeding tends to produce heterozy- 

 gous dominants in which the defective genes 

 will have normal dominant alleles and the 

 defects will not appear. Hybrid corn is one 

 of the many examples of the benefits of out- 

 breeding to agriculture. 



Origin of hereditary 

 differences— mutations 



The character of the genetic constitution 

 of an animal depends not only on the dis- 

 tribution of genes during maturation and 

 fertilization, but also on changes that may 

 take place in the nature of the genes them- 

 selves. If these changes are heritable, they 

 are known as mutations. A mutation may be 

 defined as a sudden change in a gene result- 

 ing in a new hereditary trait (Fig. 439). 

 Formerly, it was supposed that mutations 

 were sudden, conspicuous, heritable modifi- 

 cations; but at present the heritability of the 

 change and not its extent is considered im- 

 portant. It is apparent that mutations occur 

 in the germ cell before the traits affected 

 are exhibited by the adult organism; and 

 that recessive mutations, which are the more 

 frequent, will become visible only when the 

 individual is homozygous for the mutant 

 gene. Although we know that mutations oc- 

 cur in the germ cell, we do not know how 

 those that occur spontaneously in nature 

 originate. The great majority of mutations 

 are undesirable; therefore, their production 

 in man should, insofar as possible, be 

 avoided. 



Origin of artificially 

 produced mutations 



It has been proved that radiation can pro- 

 duce mutations in the germ plasm. When 



