Mendel’s laws: teachings and examples

How do you explain that my brother inherits my father’s characteristics and I inherit my mother’s? If this question is difficult today, there is no need to imagine ourselves in the 19th century. We invite you to analyze the explanations proposed by Gregor Mendel

Mendel’s laws constitute a milestone in the history of science. Its author, Gregor Mendel, gave shape to a set of ideas that already lived in the heads of many. How is it possible that children inherit their parents’ traits and not random ones? It is logical, Children look like their parents, but… There must be something involved!

In the 19th century, some concepts of reproduction and genetics were already being used, but there was no material explanation in this regard. Mendel’s contributions on genetic material that is passed from generation to generation go hand in hand with the ideas of Darwin, who postulates that successful human traits are those that triumph in natural selection.

Who was Gregor Mendel?

Gregor Mendel was an Augustinian monk born in the Czech Republic (Austro-Hungarian Empire) in 1822. This illustrious priest was educated at the University of Vienna, where he studied history, botany, physics, chemistry and mathematics. It was during this time at the university where he began to become interested in inherited characteristics in animals.

A man ahead of his time

Since he was a child, he acquired a taste for gardening, learning from his father how to perform grafts and other tasks. It was in the convent gardens where he carried out his meticulous and systematic studies with pea plants..

In 1866, he published his studies now known as Mendel’s Laws, but the scientific community was not prepared for it and ended up ignoring him.

Gregor Mendel Monk

Mendel’s laws

Through the study with peas, Mendel described the phenotypic factors (external characteristics) that were transmitted between generations. This information is what we know today as “gene”.

The most interesting thing about his contributions is that discovered that when there were two different elements (parents), the characteristics inherited in the new generation were distributed proportionally 3 to 1.

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In order to understand this relationship, it is necessary to describe some concepts:

Character. Set of visible characteristics inherited by genetic information.Baseline or pure. When two races of homozygous plants were crossed, their genetic material was mixed, but all the resulting plants inherited the characteristics of a single race since their parents were homozygous dominant (AA) and recessive (aa). And although the result was mixed (Aa), it expressed the characteristics of the dominant gene (A).dominant gene. In the first non-pure generation (Aa), the plants expressed all the characteristics of a parent. Thus, this represented their dominant gene (A), which did not mean that they also had the information from the other plant (a).Recessive gene. In that first generation, the plants also carried information from the recessive parent, despite not expressing its characteristics. This gene is called recessive, since it is not expressed in visible characteristics.Second generation. The incredible thing happened when two plants of the first generation (Aa) crossed each other. In most of the resulting plants, the expressed characteristics were those of the dominant gene, but in another group, the expressed characteristics corresponded to those of the recessive gene. Of four resulting plants, three had the dominant characteristics and one had the recessive characteristics.

How is this information transmitted?

Each plant has a pair of genes that represent a characteristic, for example color, being the one that is expressed and corresponding to the dominant gene.

Thus, when information is passed on, children receive a gene from each parent. That is why they can receive two dominants (AA), one dominant and one recessive (Aa) or two recessives (aa).

3 to 1 distribution

This happens because when two plants that carry both dominant and recessive information are crossed, a small group will inherit the recessive genes of the two parent plants.

Since there is no dominant gene, the characteristics of the recessive gene are expressed. This explains how sometimes we have traits from a grandparent that have not manifested in the parents.

Mendel’s first law: principle of uniformity

The first of Mendel’s laws refers to the first stagewhen there is a pure homozygous race, totally dominant (AA) or totally recessive (aa).

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The resulting heterozygous individuals will all be uniform among themselves (Aa), expressing the same visible characteristics. The result is the first filial generation.

Mendel’s second law. Segregation principle

Individuals are capable of transmitting a character even if it does not manifest itself in them.. For example, if my mother is blonde, but I have my father’s brown hair, that does not mean that I cannot pass on my mother’s information to my children and that one of them has blonde hair color.

The transferred character would be recessive, and therefore, the law refers to the second stage of the process. At this stage, the first generation crosses with each other, being able to pass on both the dominant and recessive genes.

Mendel’s Third Law. Independent combination principle

Characters are transmitted. What does this mean? That a single character or gene does not contain all the information. So hair color, nose size or eye type are transmitted in independent processes. We can portray this process as follows:

Suppose I have inherited my father’s hair color, but my mother’s eye color. This means that the dominant gene of the pair inherited from father and mother, corresponding to hair, is that of my father. Likewise, the gene for eyes will be that of my mother. This can be translated into the expression AaAa, where in the first pair “Aa” corresponding to the hair, the “A” is my father’s color. Mendel refers to independence, since the probability that the dominant genes are expressed before the recessive, it is still 3 to 1 even if there are more characters. This happens because when the information from the parents is crossed, it is only enough for there to be a dominant gene for the characteristic to occur and therefore the expression “aaaa” (all recessive) is less likely.

Variations of Mendel’s laws

We can also find variations of Mendel’s laws or non-Mendelian inheritance, which refer to the existence of inheritance patterns that were not taken into account in Mendel’s laws, and that must be explained to understand the manifestation of other hereditary patterns. These variations are:

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Incomplete dominance: It occurs when there is no dominance of one characteristic over another. In this case, two alleles can generate an intermediate phenotype when a mixture of the dominant genotypes occurs. For example, mixing a red rose and a white rose can create a pink rose.Multiple alleles: Multiple alleles can exist in a gene, however, only two can be present and generate an intermediate phenotype, without one dominating over the other. For example, as occurs in blood groupsCodominance: Two alleles can be expressed at the same time because the dominant genes can also be expressed without mixing.Pleiotropy: There are genes that can affect various characteristics of other genes.Binding to sex: It is associated with the genes that contain the X chromosome of human beings and that generate different inheritance patterns.Epistasis: Alleles of one gene can mask and affect the expression of alleles of another gene.Complementary genes: It means that there are recessive alleles of different genes that can express the same phenotype.Polygenic inheritance: These are the genes that affect the characteristics of phenotypes such as height, skin color, among others.

Conclusion: the genetics revolution

Techniques have evolved and today genetics has incredible applications for the study of hereditary diseases and possible cures. So too, The control of genetic material has allowed us to make great advances in the field of cloning and assisted reproduction.

Although the famous 3 to 1 ratio was later much discussed, The discoveries of Mendel’s laws opened a path. These discoveries are more remarkable when taking into account the little technology at their disposal. Maybe that’s why his ideas were not immediately accepted.