44 



ZOOLOGY 



Discovery 

 of the 



three-to-one 

 ratio 



Allelomor- 

 phic or 

 alternative 

 characters 



all the plants resulting from a cross were alike in this 

 respect, and it made no difference which was the seed 

 and which the pollen parent. Thus, on crossing 

 smooth-seeded with wrinkled-seeded varieties, only 

 smooth-seeded plants were produced. Plants from 

 green seeds crossed with those from yellow seeds gave 

 only plants with yellow seeds. Tall with dwarf gave 

 only tall. The character which thus appeared Mendel 

 called dominant; the other, which disappeared, reces- 

 sive. 



5. When the plants resulting from such crosses were 

 crossed together, or produced seeds by self-fertilization, 

 the next generation showed both the dominant and re- 

 cessive characters, without any intermediates. After 

 elaborate statistical studies, Mendel discovered that 

 the numerical relations between the two sorts in the 

 grandchildren of the original cross were substantially 

 constant, following what appeared to be a definite law. 

 Of every four individuals, on the average, three showed 

 the dominant character and one the recessive. Al- 

 though the immediate parents had exhibited no trace 

 of the recessive character, it reappeared in apparently 

 pure and uncontaminated form in one fourth of their 

 offspring. 



6. Mendel did not stop here, but continued his ex- 

 periments, breeding together the plants obtained as just 

 described. He found that when the extracted recessives 

 (as they are called) were bred together, they gave only 

 plants showing the recessive character, no matter how 

 many generations were produced. With the dominants 

 it was different ; some gave only dominants, and others 

 again split up into dominants and recessives, in the pro- 

 portion of three to one. It was eventually determined 

 that of the three dominants, one came pure, while the 



