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Mendel's second 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.
Let's now regard Mendel's second law,
also known as the principle of segregation of alleles.
This principle states
that the two alleles of the same gene,
in this case Aa,
come apart when forming the gametes
that is, A and a separate and end up in distinct zygotes.
that is, A and a separate and end up in distinct zygotes.
Mendel arrived at this second principle
by self-crossing plants of F1 generation.
This amounts to cross a F1 with another F1,
which is represented as Aa x Aa.
As a result, he obtained plants known
as the second filial generation or F2.
Here, statistically, from every four resulting plants,
three of them had yellow seeds and one had green seeds.
Namely, a phenotypic ratio of 3:1 was obtained.
Therefore, the final result of F2 generation was:
25% AA genotype, yellow phenotype
50% Aa genotype, yellow phenotype
25% aa genotype, green phenotype
Graphical representation of Mendel's second law
Mendel's second law can be represented graphically
in a Punnett square as follows:
1. Draw a table of three columns by three rows.
2. In the gray vertical boxes, write the alleles of the male gametes,
in this case, A and a.
3. In the gray horizontal boxes,
write the alleles of the female gametes,
in this case, A and a.
4. White boxes correspond to the zygotes resulting
from the fusion of a male gamete and a female gamete.
This process is as follows:
5. Since each white box is equivalent to 25%,
the result of the cross can be written as:
In the Mendel's second law,
from every four resulting plants,
three of them have yellow seeds and one has green seeds,
representing a phenotypic ratio of 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.