In each generation, then, the descendants of hybrids will behave in three 

 possible ways with respect to a particular characteristic: (1) the recessives will 

 remain pure, or capable of reproducing the recessive trait; (2) one out of 

 every three dominants will turn out to be a pure dominant; (3) two out of 

 the three seemingly dominant plants will behave like hybrids and split up 

 again when they reproduce. 



Combinations of Characters^ We know that every organism consists of 

 not one, but many characters. Mendel also experimented on the results of cross- 

 ing peas with different combinations of characters. Two plants, for example, 

 differ not only as to the color of the seed but also as to tallncss. What happens 

 when they are crossed? Mendel crossed tall green-seeded plants with short 

 yellow-seeded ones. All the next generation were dominant for size (tall), 

 and dominant for seed-color (yellow). The hybrids resembled one parent 

 altogether in one character, and the other parent entirely in the other charac- 

 ter (see illustration, p. 478). In the following generation the offspring of such 

 hybrids appeared in four types: tall-yellow, short-yellow, tall-green, short- 

 green. That is, there was "segregation" for each pair of characters. 



Experiments of this kind have since been repeated by the thousand. From 

 them we conclude that each pair of alternative characters behaves according 

 to the first two laws {dominance and segregatio?2), regardless of the other char- 

 acters present. This general fact is called the law of independent assortment, 

 or the law of unit "characters" (see illustration, p. 479). 



This principle of independent characters may help us understand how 



When hybrids of two pure strains (which are all dominant in appearance) are mated — 



with pure dominants, like their 



parents, all the ofFspring are 



dominant, as we should expect 



Hybrid Dominant 



All dominant 



with pure recessives, like their 

 parents, half the progeny is 

 dominant and half recessive 



with similar hybrids, the offspring 



resemble dominant and recessive 



grandparents in the ratio 3 : 1 



■ijWilWBM 



mim- 



Hybrid Recessive Hybrid Hybrid 



^ r 



Two 

 dominant 



Two 

 recessive 



Three One 



dominant recessive 



HYBRIDS IN THE SECOND GENERATION 



iSee No. 4. p. 503. 

 477 



