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3.2.3. Mendel's third law

Mendel's third law or Law of the independence of characters

In the transmission of two or more characters, each pair of alleles that controls one character is transmitted independently of any other pair of alleles that control another character.

As we have seen, Mendel's first two laws are based on the study of the behavior of hereditary factors for a character. But Mendel wanted to see if two non-antagonistic characters were transmitted independently. The genes that determine each trait are passed on independently.

To understand the influence that two or more characters can have on each other when inherited, he devised the following experiment.

He selected purebreds for two different characters, which he had already studied how they were inherited, but separately. Now he would study them together. He first made the following crossing:

P : Yellow and smooth        Green seed and rough seed 

1 : 100% Yellow and smooth seed.

Then he verified that for two characters his first law was also valid, since all the descendants were equal to each other, and equal to one of their parents.

Later, he crossed two dihybrid individuals of F1 or self-fertilized, and obtained a second filial generation (F2) composed of 556 seeds with the following characteristics:

  • 315 seeds yellow and smooth.
  • 108 yellow and rough seeds.
  • 101 seeds green and smooth.
  • 32 green and rough seeds.

These results correspond to approximate relative ratios of 9:3:3:1. That is, out of 16 individuals, 9 would be obtained with dominant dihomozygous, with the two dominant characteristics, 3 with a dominant character and a recessive one, another 3 with the other dominant character and the other recessive, and one dihomozygous, doubly recessive for both characters.

Thus, Mendel proposed his third law or the independent inheritance of non-antagonistic characters, which states that different characters are inherited independently of each other.

With these results he observed something that did not happen when he studied a single character:

  • Individuals other than the parents appeared, such as pea plants with green and rough seeds and yellow and smooth seeds .
  • Proportions 9:3:3:1 were obtained.

Mendel concluded that just as hereditary factors are independent, the characters are also independent, so they can be combined in all possible ways, resulting in combinations that did not exist before.

We have to bear in mind that Mendel's findings occurred without him having the knowledge that we have of DNA.