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When these heterozygous plants self-pollinated, their offspring had an equal chance of receiving either two recessive factors, two dominant factors, or a dominant and a recessive factor. One quarter of these offspring were homozygous recessive, one quarter were homozygous dominant, one-half, were heterozygotes. Except for the one quarter that were homozygous recessives, the rest had at least one dominant factor and showed the dominant form of the trait. This explained Mendel's observation that three of every four plants showed the dominant form, and one in four the recessive.

Mendel also allowed the offspring of the heterozygous plants to self-pollinate. When he let plants with recessive features self-pollinate, only recessive features developed in their descendants, supporting the theory that they all contained only recessive factors.

When he let plants with dominant features self-pollinate, one-third gave rise to descendants that exhibited only dominant features. The other two-thirds gave rise to progeny with both dominant and recessive features, and therefore had to contain both dominant and recessive factors. Mendel tested six generations of plants and got similar results for each generation. Each generation of self-pollinating heterozygotes bore offspring, of which half were heterozygotes and half were homozygotes.

Mendel also did reciprocal crosses for each of the seven traits, switching the egg-bearing and the pollen-bearing variety to transmit the dominant and recessive features. The same ratio—three plants with dominant features for every one with recessive features—emerged from all the reciprocal crosses. Mendel concluded that a descendant had an equal chance of heterozygous character- getting a dominant or a recessive factor (now called alleles) from either het-

ized by possession of erozvgous parent, regardless of sex. two different forms (alle-

les) of a particular gene

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