340 ELEMENTS OF BIOLOGY 



inherent capacity for developing a somatic property and the prop- 

 erty itself. Particularly is this true in the recognition of the nature 

 of the source of skill or mental properties and of the source of 

 disease. It has been demonstrated that high susceptibility to certain 

 diseases (for example, cancer and tuberculosis) are genetic and 

 transmitted according to Mendelian principles. But the disease itself 

 is not transmitted; it is a somatic character that may or may not 

 appear, depending on environmental conditions. Similarly, capacity 

 to develop skill, or facility with instruments, or for acquiring knowl- 

 edge, may be genetic, but whether or not the individual ever 

 realizes such capacities depends on the opportunities of the 

 environment. 



Inbreeding. Mating between closely related individuals is 

 known as inbreeding. In most human societies it is forbidden by 

 statute, but is a common procedure in breeding domestic animals 

 and plants and occurs widely in nature. In the behavior of the genes 

 of a monohybrid cross between Tall and Dwarf (p. 322), we may 

 substitute for one of this pair of allelomorphic genes another which, 

 for the purposes of an illustration, is an undesirable character or 

 defect of some sort. Further, suppose that this gene is recessive to 

 a normal, or desirable character. Then in brother-sister matings in 

 the Fi generation the segregation of this character results in the 

 appearance of one-fourth the F2 generation as pure for this unde- 

 sirable, with one-half the progeny heterozygous and capable of 

 transmitting the character. Thus inbreeding tends to segregate 

 characters as homozygous in the next generation. If, as in the 

 example given, the character is an undesirable, inbreeding is unde- 

 sirable. If, on the other hand, it is a desirable character, inbreeding 

 is also desirable. But due to the tendency of undesirable characters 

 to accumulate in homozygous form, it is a common practice to 

 break them up again by occasional outbreeding with an unrelated 

 line. There is no reason for regarding human inheritance as an 

 exception to these laws. 



