282 THE CONTINUITY OF THE RACE 



5. Since half of the female gametes are D and half d and since half of 

 the pollen grains are D and half d, there are four possible ways in which 

 they can combine: (1) pollen D with ovum D — DD; (2) pollen D with 

 ovum d — Dd; (3) pollen d with ovum D — dD; and (4) pollen d with ovum 

 d — dd. 



6. According to the laws of chance, the resulting combinations should 

 be in the following proportions: \^DD, J+Dd, and ^dd. Since, as we have 

 seen in paragraphs 1 and 3, above, both DD and Dd produce an equal 

 degree of tallness in the individual, % of the F 2 should be tall and 34 

 dwarf. 



7. It is also evident that all the dwarf members of the F 2 should breed 

 true but that only one-third (the DD individuals) of the tails should 

 breed true; the other two-thirds of the tails (the Dd individuals) should 

 produce a Y^DD, %Dd, and }/^dd ratio when allowed to self-fertilize. 



Here, then, is a hypothesis that explains all Mendel's monohybrid 

 crosses. If we make exactly the same assumptions and diagram the cross 

 between yellow-seeded (GG) and green-seeded (gg) peas, or the cross be- 

 tween round-seeded (WW) and wrinkled-seeded (to) peas, we see how 

 it comes about that they produce the same numerical ratios in the F\, F 2 , 

 and subsequent generations. 



MENDEL'S FIRST LAW: GAMETIC PURITY 



The principle of inheritance that has just been discussed is now known 

 as Mendel's first law and is often stated as follows: "Inherited pairs of 

 factors segregate at germ-cell formation and recombine at fertilization." 

 The first law is often referred to as the principle of gametic purity, i.e., 

 that a gamete can carry but one of any two alternative characters (for 

 example, D or d) and hence can never be hybrid. It is also important 

 to note that neither the D nor the d produced by the hybrid F x generation 

 is in any way contaminated by their existence together in the Fi genera- 

 tion and that each is just as "pure" as if derived from pure-breeding tall 

 and dwarf individuals, respectively. 



SOME NECESSARY TERMINOLOGY 



In order to follow Mendel's further work, as well as post-Mendelian 

 genetics, it is necessary to learn a number of definitions and timesaving 

 genetic conventions and symbols. These not only make for greater clarity 

 and preciseness but save much time in writing and thinking about genetic 

 concepts. 



gene (factor or determiner). An unknown entity carried in the germ cells that 

 (under proper conditions) results in the development of a definite quality by 

 the zygote. (The term gene is generally preferable to the synonymous terms 

 factor and determiner.) 



