The 3 Laws of Mendel and the Experiments of the Peas

The 3 laws of Mendel they are the most important statements of biological inheritance. Gregorio Mendel, a monk and Austrian naturalist, is considered the father of Genetics. Through his experiments with plants, Mendel discovered that certain traits were inherited following specific patterns.

Mendel he studied the inheritance experimenting with peas of a plant of the species Pisum sativum that he had in his garden. This plant was an excellent test model because it could be self-pollinated or cross-fertilized, in addition to having several traits that only have two forms.

For example: the feature"color"can be only green or yellow, the feature"texture"can be only smooth or rough, and so with the other 5 features with two forms each.

Gregor Mendel formulated his three Laws in his work published as Experiments on plant hybridization (1865), which he presented at the Brünn Natural History Society, although they were ignored and were not taken into account until the year 1900.

History of Gregor Mendel

Gregor Mendel is considered the father of genetics, due to the contributions he left through his three laws. He was born on July 22, 1822, and it is said that from an early age he was in direct contact with nature, a situation that made him interested in botany.

In 1843 he entered the convent of Brünn and three years later was ordained as a priest. Subsequently, in 1851 he decided to study botany, physics, chemistry and history at the University of Vienna.

After studying, Mendel returned to the monastery and it was there where he conducted the experiments that allowed him to formulate the so-called Mendel's Laws.

Unfortunately, when he presented his work, it went unnoticed and it is said that Mendel abandoned the experiments on inheritance.

However, in the early twentieth century his work began to gain recognition, when several scientists and botanists conducted similar experiments and found their studies.

Mendel's experiments

Mendel studied seven characteristics of the pea plant: color of the seed, shape of the seed, position of the flower, color of the flower, shape of the pod, color of the pod and length of the stem.

There were three main steps for Mendel's experiments:

1-By self-fertilization produced a generation of pure plants (homozygotes). That is, plants with purple flowers always produced seeds that generated purple flowers. He called these plants the generation P (of parents).

2-Then, he crossed pairs of pure plants with different traits and the descendants of these called the second filial generation (F1).

3-Finally, he obtained a third generation of plants (F2) by self-pollining two F1 generation plants, that is, crossing two plants of the F1 generation with the same features.

Results of the experiments

Mendel found some incredible results from his experiments.

F1 generation

Mendel discovered that the F1 generation always produced the same trait, although the two parents had different characteristics. For example, if you crossed a plant with purple flowers with a plant with white flowers, all the descendant plants (F1) had purple flowers.

This is because the purple flower is the trait dominant . Therefore, the white flower is the trait recessive.

These results can be shown in a diagram called the Punnett square. The dominant gene for color is shown with a capital letter and the recessive gene with a lowercase letter. Here the purple color is the dominant gene that is shown with an"M"and the white is the recessive gene that is shown with a"b".

Generation F2

In the F2 generation, Mendel discovered that 75% of the flowers were purple and 25% were white. He found it interesting that although both parents had purple flowers, 25% of the offspring had white flowers.

The appearance of white flowers is due to a gene or recessive trait present in both parents. Here is the Punnett chart showing that 25% of the offspring had two"b"genes that produced the white flowers:

How were Mendel's experiments performed?

Mendel's experiments were carried out with pea plants, a somewhat complex situation since each flower has a male part and a female part, that is, it self-pollinates.

So how could Mendel control the offspring of the plants? How could I cross them?

The answer is simple, to be able to control the offspring of the pea plants, Mendel created a procedure that allowed him to prevent the plants from selfing.

The procedure consisted of cutting the stamens (male organs of the flowers, which contain the pollen sacs, that is, those that produce the pollen) of the flowers of the first plant (called BB) and sprinkle the pollen from the second plant in the pistil (female organ of flowers, which is in the center of it) of the first.

With this action Mendel controlled the process of fertilization, a situation that allowed him to perform each experiment over and over again in order to make sure that the same offspring was always obtained.

This is how he achieved the formulation of what is now known as Mendel's Laws.

Why did Mendel choose the pea plants?

Gregor Mendel chose the pea plants to carry out his genetics experiments, because they were cheaper than any other plant and because the generation time of them is very short and has a large amount of offspring.

The offspring were important, since it was necessary to carry out many experiments in order to formulate their laws.

He also chose them because of the great variety that existed, that is, those of green peas, those of yellow peas, those of round pods, among others.

The variety was important because it was necessary to know what traits could be inherited. Hence, the term of Mendelian inheritance arises.

The 3 laws of Mendel summarized

Mendel's First Law

It is observed that in the parental generation 1 (Father ww, Mother RR), all the descendants have the dominant gene R.

Mendel's first law or the law of uniformity states that upon crossing two pure individuals (homozygotes) all descendants will be equal (uniform) in their traits.

This is due to the dominance of some characters, a simple copy of these is enough to mask the effect of a recessive character. Therefore, both homozygous and heterozygous descendants will present the same phenotype (visible trait).

Mendel's Second Law

Mendel's second law, also called the law of character segregation, states that during the formation of the gametes, the alleles (hereditary factors) are separated (segregated), so that the offspring acquires an allele from each relative.

This genetic principle modified the initial belief that heredity is a uniquely"combination"process in which offspring exhibit intermediate traits between the two parents.

Mendel's Third Law

Mendel's third law is also known as the law of independent separation. During the formation of the gametes, the characters for the different traits are inherited independently from each other.

Currently it is known that this law does not apply to genes on the same chromosome, which would be inherited together. However, the chromosomes do separate independently during the meiosis .

Terms introduced by Mendel

Mendel coined several of the terms that are currently used in the field of genetics, including: dominant, recessive, hybrid.

Dominant

When Mendel used the dominant word in his experiments, he was referring to the character that manifested externally in the individual, whether there was only one of them or if there were two of them.

Recessive

With recessive, Mendel meant that it is a character that is not manifested on the outside of the individual, because a dominant character prevents it. Therefore, for it to prevail it will be necessary for the individual to possess two recessive characters.

Hybrid

Mendel used the word hybrid to refer to the result of a crossing of two organisms of different species or different characteristics.

Similarly, it was he who established the use of the capital letter for the dominant alleles and the lower case for the recessive alleles.

Subsequently, other researchers completed their work and used the rest of the terms that are currently used: gene, allele, phenotype, homozygote, heterozygote.

Mendelian inheritance applied to humans

The traits of human beings can be explained through the Mendelian inheritance, as long as family background is known.

It is necessary to know the family history, since with them you can collect the necessary information about a particular trait.

For this, a genealogical tree is made where each of the traits of the members of the family is described and thus it could be determined who inherited the same.

Example of inheritance in cats

In this example, the color of the fur is indicated by B (brown, dominant) or b (white), while the length of the tail is indicated by S (short, dominant) or s (long).

When the parents are homozygous for each trait (SSbb and ssBB), their children in the F1 generation are heterozygous in both alleles and only show the dominant phenotypes (SsbB).

If the offspring mate with each other, in generation F2 all combinations of fur color and tail length are produced: 9 are brown / short (purple boxes), 3 are white / short (pink boxes), 3 are brown / long (blue boxes) and 1 is white / long (green box).

4 Examples of Mendelian traits

- Albinism : it is a hereditary trait that consists in the alteration of the production of melanin (pigment that humans possess and is responsible for the color of the skin, hair and eyes), so that in many cases there is an absence total of it. This trait is recessive.

- Lobes of the free ear : it is a dominant feature.

- Lobes of the ear joined : it is a recessive trait.

- Widow's hair or beak : This feature refers to the way in which the hairline ends on the forehead. In this case it would end with a peak in the center. Those who present this feature have a letter form"w"upside down. It is a dominant feature.

Factors that alter Mendelian segregation

Heredity linked to sex

The inheritance linked to sex refers to that which is related to the pair of sex chromosomes, that is, those that determine the sex of the individual.

Humans have X chromosomes and Y chromosomes. Women have XX chromosomes, while men have XY.

Some examples of inheritance linked to sex are:

- colour blindness : it is a genetic alteration that makes the colors can not be distinguished. Usually you can not distinguish between red and green, but that will depend on the degree of color blindness that the person presents.

Color blindness is transmitted by the recessive allele linked to the X chromosome, therefore if a man inherits an X chromosome that presents this recessive allele, it will be color blind.

While for women to present this genetic alteration, it is necessary that they have the two altered X chromosomes. That is why the number of women with color blindness is lower than that of men.

- Hemophilia : it is a hereditary disease that, like color blindness, is linked to the X chromosome. Hemophilia is a disease that causes people's blood to not coagulate correctly.

For this reason, if a person suffering from hemophilia is cut, his bleeding will last much longer than that of another person who does not have it. This happens because you do not have enough protein in your blood to control bleeding.

- Duchenne muscular dystrophy : it is a hereditary recessive disease that is linked to the X chromosome. It is a neuromuscular disease, it is characterized by the presence of significant muscle weakness, which develops in a generalized and progressive way.

- Hypertrichosis : it is a hereditary disease that is present in Y chromosome, for which it is only transmitted from a father to a male child. This type of inheritance is called holándrica.

Hypertrichosis is the growth in excess of hair, so that who suffers, has parts of his body that are excessively hairy. This disease is also called the werewolf syndrome, since many of those who suffer from it are almost completely covered with hairs.

References

1. Brooker, R. (2012). Concepts of Genetics (1st ed.). The McGraw-Hill Companies, Inc.
2. Griffiths, A., Wessler, S., Carroll, S. & Doebley, J. (2015). Introduction to Genetic Analysis (11th ed.). W.H. Freeman
3. Hasan, H. (2005). Mendel and the Laws of Genetics (1st ed.). The Rosen Publishing Group, Inc.
4. Lewis, R. (2015). Human Genetics: Concepts and Applications (11th ed.). McGraw-Hill Education.
5. Snustad, D. & Simmons, M. (2011). Principles of Genetics (6th ed.). John Wiley and Sons.
6. Trefil, J. (2003). The Nature of Science (1st ed.). Houghton Mifflin Harcourt.