444 ELEMENTARY BIOLOGY 



Most people have the impression that where individuals with 

 differing characters are mated, the offspring will show characters 

 somewhere between the characters of the parents. The reason 

 for this common belief lies in the fact that in our everyday 

 experience we notice that cJiildren resemble both parents ; but 

 most of us have failed to notice further that this resemblance 

 to both parents consists not merely in Jiaving many characters 

 Jialfivay bctzveen the corresponding characters of the parents, 

 but also in having some characters just like those of the mother 

 dnd other characters J7ist like those of the father. 



470. The Law of Dominance. Mendel found that this com- 

 plete resemblance of the offspring to one of the parents (in 

 regard to a particular character) was quite the rule with each 

 of the other pairs of characters. Thus, the offspring of a tall- 

 and-short cross were all tall. The offspring of a smooth-and- 

 wrinkled-seeded cross were all smooth, and so on. This fact is 

 called dominance, the idea being that where two characters of 

 a pair meet in an individual one of them masks or dominates 

 over the other, called recessive. The latter is not destroyed, as 

 we shall see. Of course, we cannot tell which of the two char- 

 acters in a pair will appear in the offspring before trying them 

 out. In the tables on page 445 are given the dominants and their 

 alternatives for a number of characters in plants and animals. 

 For many other characters it has been found that the offspring 

 does not show complete dominance of one alternative. 



471. The Law of Segregation. The yellow seeds of the 

 hybrid ^ pea plant are hardly distinguishable from the pure yellow 

 seeds of one parent. With plants grown from the hybrid yellow 

 seeds Mendel brought about three classes of cross-pollenation. 



1 The word hybrid was formerly applied to the offspring of two parents of 

 different species or races, as, for example, the mule, or a mulatto, or the off- 

 spring of a Caucasian and an Indian. It is now used quite generally among 

 biologists, horticulturists, animal breeders, etc. to mean the offspring of two 

 parents that differ with respect to any particular character. For example, a 

 seedsman might speak of a hybrid tomato, meaning a plant resulting from 

 a cross between two varieties of tomatoes, and so on. 



