What are Mendel’s Laws?

Mendel’s laws are the principles that establish how inheritance occurs , that is, the process of transmitting the characteristics of parents to their children.

Mendel’s three laws are:

  • First law: principle of uniformity.
  • Second law: principle of segregation.
  • Third law: principle of independent transmission.

These three laws constitute the basis of genetics and its theories. They were nominated by Austrian naturalist Gregor Mendel between 1865 and 1866.

Mendel’s first law: principle of uniformity

The first law or principle of uniformity of the hybrids of the first filial generation establishes that when two purebred individuals (homozygous) are crossed, the first filial generation (heterozygous) will be the same among them (phenotypes and genotypes) and, in addition , the phenotypic trait of one of the parents (dominant genotype) will stand out.

Pure breeds are composed of alleles (specific version of the gene), which determines their outstanding characteristic.

For example:

If purebred plants are crossed, some of red flowers with the dominant genotype (A) and another of purple flowers with the recessive genotype (a), it will result in the first filial generation being equal, that is (Aa), since the dominant genotype (red flower) will stand out, as illustrated below.

first mendel law

Punnet box of the first law

A (red) A (red)
to (purple) Aa Aa
to (purple) Aa Aa

Mendel’s second law: principle of segregation

The second law or principle of segregation is that from the crossing of two individuals of the first filial generation (Aa) a second filial generation will take place in which the phenotype and genotype of the recessive individual (aa) will reappear, resulting in the following: Aa x Aa = AA, Aa, Aa, aa. That is, the recessive character remained hidden in a ratio of 1 to 4.

For example:

If the flowers of the first filial generation (Aa) are crossed, each containing a dominant genotype (A, red color) and a recessive one (a, purple color), the recessive genotype will have the possibility of appearing in proportion 1 of 4, as noted below:

second mendel law

Punnet box of the second law

A (red) to (purple)
A (red) AA Aa
to (purple) Aa aa

Mendel’s third law: principle of independent transmission

The third law or principle of independent transmission consists in establishing that there are traits that can be inherited independently. However, this only occurs in genes that are found on different chromosomes and that do not intervene with each other, or in genes that are in very distant regions of the chromosome.

Likewise, as in the second law, it is best manifested in the second generation of the subsidiary.

Mendel obtained this information by crossing peas whose characteristics, that is, color and roughness, were found on different chromosomes. It was thus that he observed that there are characters that can be inherited independently.

For example:

The cross of flowers with characteristics AABB and aabb, each letter represents a characteristic, and the one that is uppercase or lowercase exposes its dominance.

The first character represents the color of flowers A (red) and (purple). The second character represents the smooth or rough surface of the stems of flowers B (smooth) and b (rough). This crossing would result in the following:

third mendel law
Punnet box of the third law

A (red ) B (smooth) A (red ) b (rough) a (purple) B (plain) a (purple) b (rough)
A (red ) B (smooth) AABB AABb AaBB Aabb
A (red ) b (rough) AABb AAbb Aabb Aabb
a (purple) B (plain) AaBB Aabb aaBB aaBb
a (purple) b (rough) Aabb Aabb aaBb aabb


Variations of Mendel’s laws

Variations of Mendel’s laws or non-Mendelian inheritance are the terms used to refer to the existence of inheritance patterns that were not taken into account in Mendel’s laws, and that must be explained in order to understand the existence of other hereditary patterns.

  • Incomplete dominance: these are the characteristics that not necessarily one dominates the other. Two alleles can generate an intermediate phenotype when a mixture of the dominant genotypes is produced. For example, from the mixture of a red rose and a white rose, a pink rose can be generated.
  • Multiple alleles: in one gene there can be multiple alleles, however, only two can be present and generate an intermediate phenotype, without one dominating over the other. For example, as in blood groups
  • Codominance: two alleles can be expressed at the same time because the dominant genes can also be expressed without mixing.
  • Pleitropia: there are genes that can affect various characteristics of other genes.
  • Linkage to sex: it is associated with genes that contain the X chromosome of human beings and that generate different inheritance patterns.
  • Epistasis: alleles of one gene can cover up 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.

Gregor Mendel

Gregor Mendel’s scientific work was only taken into account from 1900, when scientists Hugo Vries, Carl Correns and Erich von Tschermak took their research and experiments into account.

From that moment his scientific work reached such relevance, which is considered as a milestone in studies on biology and genetics.

Mendel’s laws form the basis of genetics and his theories , which is why he has been considered the father of genetics, since his laws manage to expose what the phenotype of the new individual will be like, that is, his physical characteristics and expression of the genotype .

To determine such knowledge, Mendel conducted various experiments with pea plants of different characters, which he crossed and studied the results of the characters that stood out. Hence, it has determined the existence of dominant characters and recessive characters, that is, genotypes.

In this way, Mendel determined three laws that set out how the offspring and transmission of characters between living beings are carried out.

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