THE ESSENCE OF MENDELISM 



joiix Belling, Washington, D. C. 



MIoNDI'vL discovered the facts 

 rclaliiiK' to the progeny of 

 hybrids in rather complex 

 cases. His method of exposi- 

 tion, beginning with more or less 

 complex examples, has been followed 

 to some extent inmost text-books. But 

 it is an advantage for the student to 

 begin with the simplest case, as is shown 

 below. 



1. If we cross a jjurple-flowered ])ea 

 with a white-flowered one, both belong- 

 ing to true-breeding races, all the 

 progeny (hybrids) no matter which 

 way we make the cross, have ])ur]jle 

 flowers. 



2. When we fertilize the white-flow- 

 ered parent with hybrid pollen, on the 

 average half the progeny have purple 

 flowers and half have white flowers. 

 Hence only half the hybrid pollen-grains 

 carry something which makes the i)ur]jle 

 color appear. 



3. When we fertilize the hybrid with 

 pollen from the white-flowered plant, 

 we get again on the average half purple 

 and half white-flowered plants. Hence 

 half the egg-cells of the hybrid carry 

 something which makes the inir])le 

 appear. 



4. In cases 2 and 3 the ])urple jjrogeny 

 are found, as we would ex])ect, to be 

 all hybrids, i. c, they are not constant 

 in their own ])rogeny but give both 

 pur])le- and white-flowered ]jlants. 



5. When we cross a jjurple-flowering 

 pea with green pods with a white- 

 flowering jjea with yellow pods, all the 

 hybrids ha\'e jjurjjle flowers and green 

 pods. 



6. When we fertilize the white-flow- 

 ered ])lant with hybrid jKjllen we get, 

 on the average, jjlants in ihcse propor- 

 tions: 



14, puri)lc' flowers and f^rccn |)ods. 

 14, purple flowers and yellow pods. 

 \4, while flowers and ^recn pods. 

 J^ white flowers and yellow pods. 



Hence, one-fourth of the hybrid 

 pollen grains carry something (which we 

 call for convenience u factor) for ])urple 



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and green, one-fourth factors for purple 

 and yellow, one-fourth factors for white 

 and green and one-fourth factors for 

 white and yellow. This is shown by 

 testing the self-fertilized progenies, when 

 it is found that all jnirjjles or greens 

 are inconstant (hybrid), while all whites 

 and yellows are constant. It follows: 

 (a) that the factors for purple flower 

 and white flower are independent of 

 the factors for green ])od and yellow pod ; 

 and (/') that it is the combination of 

 factors which are h\'brid, and not the 

 whole plant. 



Generally, if .4, B, C. D, E, . . . 

 etc., are s>TTibols for n independent 

 dominant factors (which show in the 

 hybrid) and a, b, c, d. e, . . . etc., 

 are s>Tnbols for the corresponding 

 recessive factors in a second plant, then 

 on crossing the hybrids and the reces.sive 

 jjarent we get 2" different combinations 

 in equal numbers on the average, 

 showing that the hybrids have 2" 

 different kinds of pollen-grains and 

 egg-cells. 



Thus the hybrid AaBbCcDd, crossed 

 with the recessive aobiddi gives, on 

 the average : 



7. If we were to ferliUze a hyl)rid 

 plant with its own pollen, it is readih- 

 seen that the result would be the same, 

 as regards one pair of factors, as if we 

 used equal amounts of the pollen of 

 its two parents. One-quarter of the 

 l)rogeny would be constant dominant, 

 one-quarter constant recessive, and 

 one-half hyV)rid. Similarly, the com- 

 plicated formulas given by Mendel for 

 two or more i)airs of factors can l)e 

 deduced withotU finMhi-r ex])eriment. 



