286 GENETICS 



to go together to the same one of the offspring. But this is 

 not absolute; genes that are side by side in the series may 

 separate into different offspring. 



Thus there are great numbers of possible different com- 

 binations of genes from one parent, any one of which may 

 pass into any one of the offspring. There it may meet any 

 one of an equally great number of possible different combina- 

 tions from the other parent. The result is that the different 

 offspring from a single pair of parents receive different com- 

 binations of genes. Thus the number of different types of 

 offspring (with different gene combinations) that may be 

 produced from a single pair of parents may be almost inex- 

 pressibly great; a few of such are illustrated in figure 59. 



The characteristics that appear in the different offspring 

 depend, we know, on what combination of genes Is present. 

 Altering a single gene may alter one or several character- 

 istics, and may affect the developmental vigor of the entire 

 organism. Any characteristic may be changed by alteration 

 of one or more of many different genes. Thus the result of 

 biparental reproduction is that the individuals produced have 

 different characteristics. And the combinations of hereditary 

 characteristics, like the combinations of genes, are con- 

 tinually changing as generations pass. 



We may now pass in review some of the chief types of 

 results that are thus produced in the passage of generations. 



/. Uniparental reproduction; no change in inherited 

 characteristics. — We may begin with the limiting cases, in 

 which no such changes in gene combinations and in char- 

 acteristics are produced at reproduction. This is the situa- 

 tion in vegetative reproduction from a single parent, such 

 as occurs in many lower animals, and in plants. In such re- 

 production each gene divides and one of its halves passes 

 to each of the offspring, so that all the offspring receive the 

 same combination of genes that was carried by the parent. 

 The result is that in such reproduction the characteristics 



