GENERAL RELATIONS IN INHERITANCE 287 



do not change as generations pass. This is the case in re- 

 production by fission in Protozoa and many other animals, 

 and in reproduction by cuttings, bulbs and the like in plants. 

 What Is essentially this same process of vegetative reproduc- 

 tion occurs in man in the cases In which a single embryo 

 divides to produce Identical twins. The identity In character- 

 istics of such twins Is a striking illustration of the fact that 

 In such reproduction the characteristics are not changed. 

 This is the usual situation in any type of reproduction from 

 a single parent. 



Of course in such reproduction, diversities of environ- 

 mental conditions may cause different characteristics to be 

 produced in different individuals. Further, In some of the 

 lower organisms there occur at times nuclear changes which 

 may alter the characteristics. And genetic variations, such 

 as are dealt with in later chapters, may occur in organisms 

 that are multiplying by uniparental reproduction. 



2. Homozygous organisms, uniform in their genes. — 

 Even in biparental inheritance there may occur situations in 

 which there is no change In the combinations of genes and 

 characteristics as generations pass. This will be the case in 

 stocks In which all the members are alike and homozygotic in 

 all their genes. 



In such a uniform and homozygotic stock, all the indi- 

 viduals show combinations of genes that could be repre- 

 sented as AAbbCCDDeeffGG and so on. Then all the germ 

 cells will have the same genes AbCDefG, etc. When any two 

 of these unite they of course restore the original combina- 

 tion that was present in the parents, and this will continue 

 in later generations. Thus all individuals of the stock will 

 have the same characteristics, except in so far as modified 

 by diversities in environment. 



Such uniform and homozygotic stocks are rare in or- 

 ganisms bred by man, though they perhaps occur in nature. 

 Such a stock does not remain In this uniform condition, be- 



