Mendel's Laws

Mendel's first experiments

One of the reasons for Mendel's success was the material chosen for his research: the pea of the species Pisum sativum. This vegetable has a number of advantages:

• It is easy to grow.
• It produces many seeds and, consequently, a large number of offspring.
• The flower is hermaphrodite and reproduces by self-fertilization.
• Cross-fertilization can be achieved by making a flower cross with another flower on another pea plant.

In addition to these advantages, the pea has a number of simple and contrasting characteristics:

• The seed color is yellow or green, with no intermediate shades.
• The seed shape is smooth or wrinkled.
• Either the plant was too tall or too short.

The fact that Mendel analyzed one characteristic at a time, without worrying about the others, contributed to the success of his research.

The first crosses

In one of the experiments, he tried to cross yellow-seeded plants with green-seeded plants. Before, however, he tried to select pure plants. To do this, he chose an individual and observed the results of self-fertilization over six generations. In each generation, he analyzed large numbers of descendants and, if none of them produced seeds of a color different from the color of the initial individual, he concluded that it was a pure plant.

With pure peas, Mendel made the cross using the male part of a yellow seed plant and the female part of a green seed. This first generation of the cross is now called the parental or P generation. In the next generation, called the first generation or F1, all the peas were yellow.

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Mendel then self-fertilized one of these yellow-seeded hybrids and analyzed the resulting plants, that is, the second generation or F2: about 75% (3/4) yellow seeds and 25% (1/4) of green seeds. The appearance of green seeds allowed Mendel to conclude that the green factor had not been destroyed, it just did not manifest itself in the presence of the factor for the yellow color.

Mendel's results can be explained with the following hypotheses:

• Each organism has a pair of factors responsible for the appearance of a certain characteristic.
• These factors are received from the paternal and maternal individuals; each contributes only one factor from each pair.
• When an organism has two different factors, it may happen that only one of the characteristics appears (dominant), and the other does not appear (recessive).
• The factors of a contrasting pair do not mix. During the formation of gametes, the factors appear in simple doses, that is, each gamete has only one factor.

This last conclusion became known as Mendel's first law, law of segregation of a pair of factors or law of monohybridism.

"Each character is conditioned by a couple of factors that separate in the formation of gametes, in which they occur in a simple way".

Mendel's second law

Mendel crossed pure peas for yellow seed and smooth surface (dominant characters) with peas for green seed and rough surface (recessive characters). When causing the fertilization of an F1 individual, he observed that the F2 generation was composed of four types of seeds: yellow and smooth, 9/16; yellow and rough, 3/16; green and smooth, 3/16; green and wrinkled, 1/16.

Mendel's second law, also called the law of recombination or law of independent segregation:

"In a cross in which two or more characters are involved, the factors that determine each separate (segregate) independently during gamete formation, recombine at random, and form possible combinations."

References:

LINHARES, Sergio; GEWANDSZNAJDER, Fernando. Biology Today: Genetics, Evolution, Ecology. 1st Edition. São Paulo: Editora Ática, 2012.