This is a diagram of an experiment in cross fertilizing a thoroughbred long-sized pea plant with a thoroughbred short-sized pea plant. (Thoroughbred plants are acquired by repeatedly crossing long plants with long plants, till no short ones are among the descendants. The same for shortsized plants.) The first generation of descendants, which is called F1, surprisingly exists purely of long plants. Cross fertilizing the plants of this F1 generation gives a F2 generation consisting of long and short plants in the proportion 3 : 1 (approximately). It was the great achievement of Mendel that he could explain this little miracle.

Mendel supposed every individual to own every hereditary trait twice: one from the side of the mother and one from the father. If A represents the trait 'long-sized', then AA represents a thoroughbred long-sized pea plant. In the same way de combination aa stands for a thoroughbred short pea, when a represents the trait 'short-sized'. See the picture above. During the reproduction these characters split up again and then the characters of the father and the mother are being combined. In the F1-generation the combination Aa comes four times into existence. Mendel posed (rightly) that 'long' is dominant over 'short'. The trait 'short' is called recessive. The combination Aa represents thus a long-sized plant. That explains why the F1 consists completely of long plants. The lower part of the diagram shows which combinations appear in the case of further breeding with F1 plants. This gives the combinations AA, Aa, aA and aa. Because A is dominant over a, three of the four possibilities are long plants. Only the thoroughbred aa is a short plant. This explains the proportion 3 : 1 in the F2-generation. Know more? Mendel's discoveries Pea experiment

Evolution 9

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