. Biology and man. Biology; Human beings. 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
. Biology and man. Biology; Human beings. 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 â 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
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