336 CIVIC BIOLOGY 



Gregor Johann Mendel, discovered a law of heredity, claimed 

 to be equal, for biology, to the law of gravitation in physics 

 or to that of atomic equivalents in chemistry. 



Mendel's law. Characters are represented in germ cells by 

 units which tend to segregate or combine in definite propor- 

 tions, the result of mating together first crosses falling in the 

 ratios 1DD + 2DR + 1RR for characters D ami R. 



Illustration. A tall and a short pea are crossed. The seeds resulting 

 from the cross produce only tall plants. When the seeds (self-fertilized) 

 of these plants are grown, they are found to produce 75 per cent tall 

 plants and 25 per cent short, or 3 tall to 1 short. Here tallness is 

 dominant (character D) and shortness recessive (character R). A 

 dominant character dominates the outward form of the plant or animal 

 body, while a recessive character has its units persisting unchanged in 

 the germ cells. When male and female germs again combine, they do 

 so according to the law of chance (like dice, or any other free units) 

 and so fall out 1DD + 2DR + 1RR. Since we cannot distinguish the 

 DD plants from the DR plants, except by planting the seeds and analy/- 

 ing the progeny, we have 3D to 1R. All the RR plants are found to be 

 as pure and to breed as true as if they had never been crossed, and so 

 are all the DD plants when we propagate them. The DR plants will 

 continue forever to produce 1DD -I- 2DR + 1RR. A hybrid can never 

 be fixed so as to breed true. 



The above is the law for monohybrids forms in which a single 

 character or pair of characters is involved, and instead of assuming 

 the presence of a unit (determiner) for a character (for example, 

 "shortness), the tendency is to assume merely the absence of the germi- 

 nal determiner for tallness. In cases of two characters being involved 

 in each parent, that is, in dihybrids (characters Dd and Rr), there is 

 IDd-Dd and IRr-Rr, that is, 1 pure dominant and 1 pure recessive in 

 10. In case of trihybrids only 1 offspring in 64 is pure dominant or 

 pure recessive. If ten characters are involved, the offspring of the sec- 

 ond generation would fall into 1,048,576 different kinds, of which only 

 1 would be pure for each set of characters. 



When we consider that this law of inheritance applies to fixation of 

 all kinds of characters, from tallness of peas to tallness of men, from 

 rust resistance in \vheat, egg production in poultry, or milk production 

 of cows to feeble-mindedness or normal intelligence in men, we begin 



