MENDEL'S 3 LAWS AND THE PEA EXPERIMENTS - BIOLOGY - 2025

The three Mendelian or Mendelian genetics are the most important statements of heredity. Gregory Mendel, an Austrian monk and naturalist, is considered the father of Genetics. Through his experiments with plants, Mendel discovered that certain traits were inherited in specific patterns.

Mendel studied inheritance by experimenting with peas from a Pisum sativum plant that he had in his garden. This plant was an excellent test model because it could self-pollinate or cross-fertilize them, as well as possessing several traits that only have two forms.

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

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

Gregor Mendel Story

Gregor Mendel is considered the father of genetics, due to the contributions he made 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 led him to become interested in botany.

In 1843 he entered the Brünn convent and three years later he was ordained as a priest. Later, 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 that 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 Mendel is said to have abandoned the experiments on heredity.

However, at the beginning of the 20th century his work began to gain recognition, when several scientists and botanists carried out similar experiments and came across his studies.

Mendel's experiments

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

There were three main steps to Mendel's experiments:

1-Through self-fertilization, it produced a generation of pure plants (homozygous). That is, plants with purple flowers always produced seeds that produced purple flowers. He named these plants the P generation (of parents).

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

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

Experiment results

Mendel found some incredible results from his experiments.

Generation F1

Mendel found that the F1 generation always produced the same trait, even though the two parents had different characteristics. For example, if you crossed a purple flowering plant with a white flowering plant, all the offspring plants (F1) had purple flowers.

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

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

Generation F2

In the F2 generation, Mendel found 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 the white flowers is due to a recessive gene or trait present in both parents. Here is the Punnett square 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 and a female part, that is, it self-fertilizes.

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

The answer is simple, in order to control the offspring of the pea plants, Mendel created a procedure that allowed him to prevent the plants from self-fertilizing.

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

With this action Mendel controlled the fertilization process, a situation that allowed him to perform each experiment over and over again in order to make sure that the same offspring were 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 pea plants for his genetic experiments, because they were cheaper than any other plant and because the generation time of the same is very short and has a large number of offspring.

The offspring was important, since it was necessary to carry out many experiments to be able to formulate its laws.

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

Variety was important because it was necessary to know what traits could be inherited. Hence the term Mendelian inheritance arises.

Mendel's 3 laws summarized

Mendel's First Law

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

Mendel's first law or the law of uniformity states that when two pure individuals (homozygous) intersect, all the descendants will be equal (uniform) in their features.

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 offspring 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 gametes, alleles (hereditary factors) separate (segregate), in such a way that the offspring acquire one allele from each relative.

This genetic principle modified the initial belief that inheritance is a purely “combination” process in which the 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 gametes, the characters for the different traits are inherited independently of each other.

It is currently known that this law does not apply to genes on the same chromosome, which would be inherited together. However, chromosomes do separate independently during 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 itself externally in the individual, whether only one of them was found or two of them were found.

Recessive

By recessive, Mendel meant that it is a character that does not manifest itself outside the individual, because a dominant character prevents it. Therefore, for this to prevail it will be necessary for the individual to have two recessive characters.

Hybrid

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

In the same way, he was the one 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 used today: gene, allele, phenotype, homozygous, heterozygous.

Mendelian inheritance applied to humans

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

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

To do this, a genealogical tree is made where each of the traits of the family members are described and thus it could be determined from whom they were inherited.

Example of inheritance in cats

In this example, coat color is indicated by B (brown, dominant) or b (white), while tail length is indicated by S (short, dominant) or os (long).

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

If the pups mate with each other, all combinations of coat color and tail length occur in the F2 generation: 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 of the alteration of the production of melanin (a pigment that human beings have and is responsible for the color of the skin, hair and eyes), which is why on many occasions it exists the total absence of it. This trait is recessive.

- Free earlobes: it is a dominant trait.

- United earlobes: it is a recessive trait.

- Hair or widow's peak: this feature refers to the way the hairline ends on the forehead. In this case it would end with a peak in the center. Those with this trait have a shape of the letter "w" backwards. It is a dominant trait.

Factors that alter Mendelian segregation

Sex-linked inheritance

Heredity 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 sex-linked inheritance are:

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

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

While for women to present this genetic alteration, it is necessary that they have both altered X chromosomes. For this reason, 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 clot properly.

For this reason, if a person with hemophilia cuts himself, his bleeding will last much longer than that of another person who does not have it. This happens because you don't have enough protein in your blood to control bleeding.

–Duchenne muscular dystrophy: it is a recessive hereditary disease that is linked to the X chromosome. It is a neuromuscular disease, 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 on the Y chromosome, for which it is only transmitted from a father to a male child. This type of inheritance is called hollandic.

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

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.). WH 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.