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Mendel's third law
According to the Mendel's first law,
when a purebred dominant parent (AA) is crossed with another purebred recessive (aa),
all offspring are equal hybrids (Aa) and make up the first filial generation or F1.
In the Mendel's second law, a parent hybrid (Aa) crosses with another hybrid (Aa).
As a result, 25% of the offspring have an AA genotype and a yellow phenotype,
50% have an Aa genotype and a yellow phenotype,
and 25% have an aa genotype and a green phenotype.
Therefore, from every four resulting plants,
three of them have yellow seeds and one has green seeds,
representing a phenotypic ratio of 3:1.
Let's now study Mendel's third law,
also known as the principle of independent assortment of alleles.
This principle states that each allele is distributed independently when forming the gametes.
In his third hybridization experiment,
Mendel investigated the simultaneous inheritance of two characters.
He crossed purebred plants that produced yellow smooth seeds (AABB),
with others that produced green rough seeds (aabb).
As a result, in the first filial generation or F1,
all plants produced yellow smooth seeds: AaBb.
That is, only dominant characters were expressed.
Then, Mendel allowed the self-crossing of F1 generation.
Namely, he performed the cross of a F1 plant with another F1: AaBb x AaBb.
As a result, Mendel obtained in F2 a phenotypic ratio of 9:3:3:1.
This pattern of inheritance underlies the third principle of Mendelian inheritance:
each allele is distributed independently when forming the gametes.
Graphical representation of Mendel's third law
Mendel's third law can be represented graphically in a Punnett square as follows:
1. Draw a table of five columns by five rows.
2. In the gray vertical boxes, write the alleles of the male gametes.
3. In the gray horizontal boxes, write the alleles of the female gametes.
4. White boxes correspond to the zygotes resulting from the fusion of a male gamete with a female gamete.
Since there are 16 white boxes, each of them is equivalent to 1/16.
This process is as follows:
From every 16 resulting plants, 9 of them have yellow smooth seeds,
3 have green smooth seeds,
3 have yellow rough seeds and 1 has green rough seeds,
representing a phenotypic ratio of 9:3:3:1.
The genetic studies conducted by Mendel, as well as non-Mendelian genetics,
are clearly, completely, and didactically presented in: Mendoza, Biologia II, Trillas, Mexico
Refer to the webpage http://www.mendoza-sierra.org There you may find several answered questionnaires.