CHAPTER XXVIII 

 MENDEL'S LAWS 



Variation. — Children never resemble either parent 

 exactly in all particulars and it is very doubtful whether 

 any single character is an exact duplicate of that of either 

 parent. Such " differ ences among individuals related by 

 descent '' are called variations by the biologist. The 

 student should note that the differences between an oak 

 and a man are not included in this definition because 

 these two organisms are not nearly related by descent. 



Pure Lines and Hybrids. — In preceding chapters 

 the student has learned that there are two sorts of repro- 

 duction found among both plants and animals; viz., sexual 

 and asexual. In all asexual methods there is only one 

 parent and the successive generations are all alike 

 (Chap. XXVII). Moreover, when the two parents are 

 germinally alike or when the sperm and egg are regularly 

 produced by the same individual, it is clear that sexual 

 methods will produce pure lines. But when the two 

 parents belong to different species, races, or varieties the 

 offspring will receive a different contribution from each 

 and consequently will not be pure bred and will not be- 

 long to a pure line. The breeding together of such dif- 

 ferent parents (indicated in this book by X) is called a 

 cross and the offspring is said to b e hybrid . 



Modifications. — In any pure line there is always 

 some variation among the various individuals. For ex- 

 ample, two beans from the same pod belong to the same 

 pure line because beans are ordinarily self-pollinated. 

 Nevertheless they would scarcely ever weigh precisely 

 the same nor when planted produce plants exactly alike. 



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