Genetics: A Short History

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two varieties of garden peas from the genus Pisum, one with a yellow seed and one with a green seed. First he crossed a parent purebred for yellow seeds with a parent purebred for green seeds. All of the offspring gave yellow seeds. (This first cross is called the F1, or filial cross, and the offspring are called F1s.) This observation was nothing new. Many traits in plants and animals seem to be swamped by other traits. But Mendel somehow reasoned that the green trait did not get altered—it was just hidden.

So he took the F1s and crossed them, probably reasoning that the green trait would come back. He counted the peas in all the offspring (called F2s) and observed that 6,022 F2s were yellow and 2,001 F2s were green. Do the math, and you, like Mendel, will recognize a 3:1 ratio.

This observation by itself would not have meant much, but Mendel's scientific thinking led him to do several other crosses, with these results:

Table 1: The Results of Mendel’s Crossing ...

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