Mendel's 3 laws and what they teach us about genes

It has long been known that DNA is found inside cells, 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. This, which can now be explained, a while ago had no answer.

Throughout history, different theories have appeared, some more accurate than others, trying to find logical answers to natural events. In this case, Why does the son have part of the mother's traits but also part of the father? Or, why does a son have some characteristics of his grandparents? The mystery of inheritance has been important to ranchers and farmers seeking to obtain more productive offspring 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 he is currently recognized as the father of genetics. In this article we will see what this theory is about, which together 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 Brno convent, became interested in peas after seeing a possible pattern in his 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 rejected his proposal, so his conclusions were not published. It took thirty years for these experiments to be recognized and what are now called Mendel's laws to be established.

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Mendel's 3 Laws

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 that I will use here were unknown to Mendel, such as genes, variants of the same gene (allele) or the dominance of genes.

In an attempt to make the explanation more enjoyable, the genes and their alleles will be represented with letters (A / a). And he remembers, the offspring receives one allele from each parent.

1. Principle of uniformity

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

The explanation of what happened in this first Mendel law, according to this researcher, is that the allele of the color yellow dominates over the allele of the color green, it only needs one of the two alleles to be yellow in a life form to express itself. It should be added that it is essential that the parents must be pure breeds, that is, that their genetics be homogeneous (AA or aa) for this to be fulfilled. Consequently, their offspring become 100% heterozygous (Aa).

2. Segregation principle

Mendel continued to cross pea species, this time those resulting from his previous experiment, that is, heterozygous yellow peas (Aa). The result surprised him, since 25% of the descendants were green, despite their parents being yellow.

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

3. Principle of independent segregation of character

This last Mendel's law is somewhat more complex. To reach this conclusion, Mendel crossed species of smooth yellow peas (AA BB) with other rough green peas (aa bb). As the previous principles are fulfilled, the resulting offspring is heterozygous (Aa Bb), which crossed 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).

What this third law of Mendel tries to demonstrate is that traits are distributed independently and they do not interfere with each other.

Mendelian inheritance

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