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The 3 laws of Mendel and the peas: this is what they teach us

The 3 laws of Mendel and the peas: this is what they teach us

June 24, 2021

It has long been known that inside the cells is the DNA, which contains all the information for the proper development and functioning of an organism. In addition, it is a heritable material, which means that it is transferred from fathers and mothers to sons and daughters. What can now be explained, a time ago had no answer.

Throughout history, different theories have been appearing, some more accurate than others, trying to find logical answers to natural events. In this case, Why does the son have part of the traits of the mother but also part of the father? Or, why does a son have some characteristic of his grandparents? The mystery of the inheritance has had its importance for farmers and farmers who sought to obtain more productive descendants of animals and plants.


The surprising thing is that these doubts were resolved by a priest, Gregor Mendel, who stipulated Mendel's laws and that is currently recognized as the father of genetics. In this article we will see what this theory is about, which along with the contributions of Charles Darwin laid the foundations of biology as we know it.

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Discovering the bases of genetics

This Austro-Hungarian priest during his life in the convent of Brno, became interested in the peas after seeing a possible pattern in their offspring. This is how he began to perform different experiments , which consisted of crossing different types of peas and observing the result in their offspring.


In 1865 he presented his work to the Brno Natural History Society, but they quickly dismissed his proposal, so his conclusions were not published. It took thirty years for these experiments to be recognized and for what were called Mendel's laws to be established today.

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The 3 laws of Mendel

The father of genetics, thanks to his work, came to the conclusion that there are three laws to explain how genetic inheritance works . In some bibliographies there are two, since the first two join them in a third. However, keep in mind that many of the terms I will use here were unknown by Mendel, such as genes, variants of the same gene (allele) or the dominance of genes.

In an attempt to make the explanation more entertaining, the genes and their alleles will be represented by letters (A / a). And remember, the descendant receives an allele from each parent.


1. Principle of uniformity

To explain this first law, Mendel made crosses between peas yellow (AA) with another scarce species of green peas (aa). The result was that in the offspring dominates the yellow color (Aa), without the presence of any green pea.

The explanation of what happened in this first law of Mendel, according to this researcher, is that the allele of yellow color dominates on an allele of green color , it only needs that in one way of life one of the two alleles is yellow to express itself. It must be added that it is fundamental that the parents must be pure races, that is to say, that their genetics be homogeneous (AA or aa) so that this is fulfilled. Consequently, their offspring become 100% heterozygous (Aa)

2. Principle of segregation

Mendel continued crossing pea species, this time the results of his previous experiment, that is, heterozygous yellow peas (Aa). The result surprised him, since 25% of the descendants were green, although their parents were yellow.

In this second law of Mendel what is explained is that if the parents are heterozygous for a gene (Aa), its distribution in offspring will be 50% homozygous (AA and aa) and the other heterozygous half (Aa). This principle explains how a child can have green eyes like his grandmother, if their parents have brown.

3. Principle of independent character segregation

This last Mendel's law is something more complex. To arrive at this conclusion, Mendel crossed smooth yellow pea species (AA BB) with other rough green peas (aa bb). As the above principles are fulfilled, the resulting offspring is heterozygous (Aa Bb), which interweaved it.

The result of two smooth yellow peas (Aa Bb) were 9 smooth yellow peas (A_ B_), 3 smooth green peas (aa B_), 3 rough yellow peas (A_ bb) and 1 rough green pea (aa bb).

This third law of Mendel, which he intends to demonstrate is that the traits are distributed independently and they do not interfere with each other.

Mendelian inheritance

It is true that with these three laws of Mendel can explain much of the cases of genetic inheritance, but manages to capture all the complexity of the mechanisms of inheritance. There are many types of inheritances that do not follow these guidelines, which are known as non-Mendelian inheritances. For example, the inheritance linked to sex, which depends on the X and Y chromosomes; or multiple alleles, that the expression of a gene depends on other genes can not be explained by Mendel's laws.


How Mendel's pea plants helped us understand genetics - Hortensia Jiménez Díaz (June 2021).


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