Mendelian Laws

What are Mendel’s Laws?

Mendel’s laws are principles that establish how genetic inheritance occursthat is, the process of transmission of physical and biological characteristics from parents to children.

The characteristics or traits that are inherited are determined by two versions of a gene, called alleles. When the alleles are the same, the individual is homozygous; when the alleles are different, the individual is heterozygous.

Mendel’s three laws are:

  • Principle of uniformity.
  • Segregation principle.
  • Principle of independent transmission.

These principles form the basis of genetics and its theories. They were postulated by the Austrian naturalist Gregor Mendel between 1865 and 1866.

Mendel’s first law: principle of uniformity

The first law or principle of uniformity It establishes that when two homozygous individuals are crossed for a different characteristic, the children (first filial generation) will be heterozygous for that characteristic.

This means that the phenotype (the observable characteristics) and the genotype (the genes that determine the characteristic) of the first generation of children will be identical.

For example:

A plant with two identical alleles is crossed. AA (homozygous dominant) for red flowers with a plant with two alleles ah (homozygous recessive) for purple flowers.

The result will be daughter plants with two different alleles oh (heterozygous) with red flowers, which is the dominant character, as illustrated below:

mendel's first law

The Punnett square is a way of making crosses between individuals. The alleles of one parent are placed in the first row and the alleles of the other parent are placed in the first column. The cells are filled with the combination of the first row and the first column. Thus, the Punnett square of the first generation philia will be:

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to (red) to (red)
to (purple) oh oh
to (purple) oh oh

When crossing two individuals of this first filial generation (heterozygous for a trait), in the second generation the recessive phenotype and genotype will reappear in the proportion 1 to 3, that is, 1 recessive and 3 dominant.

For example:

If the plants of the first filial generation of red color (heterozygous ah)among the children will appear:

  • one with the recessive genotype aa and purple flowers,
  • two heterozygous sons oh with red flowers and
  • a homozygous son AA with red flowers.

mendel's second law

Punnett square of the second filial generation

to (red) to (purple)
to (red) AA oh
to (purple) oh aa

Mendel’s second law: principle of segregation

The second law or principle of segregation It consists in that each version of a gene (allele) for a given characteristic is separated or secreted in the sexual cells of the individual. In this way, the alleles have the same possibility of being inherited by the children.

For example:

A plant homozygous for red flowers will have the genotype AA. The sex cells of this plant will have only one allele A. In contrast, in a heterozygous plant ohhalf of their sex cells will have the allele A and the other half, the allele a.

Mendel’s third law: principle of independent transmission

The third law or principle of independent transmission states that different traits can be inherited independently. Mendel obtained this information by studying the inheritance of two characteristics of peas: color and texture.

For example:

We are going to consider two characteristics of a plant, the color of the flowers (red or purple) and the texture of the stem (smooth or rough). The alleles for flower color are A (dominant red) and a (purple recessive). The alleles for stem texture are B. (smooth dominant) and b (rough recessive).

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If we cross a plant with red flowers and a smooth stem (genotype AABB) with a plant with purple flowers and a wrinkled stem (genotype aabb), the first filial generation will be 100% heterozygous AaBb. In this case, the dominant phenotype will be red flowers and smooth stems, as expected by Mendel’s first law.

If we cross two individuals of the first filial generation (AaBb x AaBb), 16 different combinations are obtained, of which:

  • 9 red flowers and smooth stems
  • 3 red flowers and wrinkled stems
  • 3 purple flowers and smooth stems
  • 1 purple flower and wrinkled stem

mendel's third law

Punnet square AaBb x AaBb

to (red)
B (smooth)
to (red)
b (rough)
to (purple)
B (smooth)
to (purple)
b (rough)
to (red)
B (smooth)
AABB
(red and smooth)
AA Bb
(red and smooth)
aa bb
(red and smooth)
aa bb
(red and smooth)
to (red)
b (rough)
AA Bb
(red and smooth)
AA bb
(red and rough)
aa bb
(red and smooth)
oh bb
(red and rough)
to (purple)
B (smooth)
aa bb
(red and smooth)
aa bb
(red and smooth)
aa bb
(purple and plain)
aa bb
(purple and plain)
to (purple)
b (rough)
aa bb
(red and smooth)
oh bb
(red and rough)
aa bb
(purple and plain)
aa bb
(purple and rough)

See also:

  • gene
  • Genetics.

Variations of Mendel’s Laws

There are some inheritance patterns that do not follow Mendel’s laws. These are known as variations of Mendel’s laws or non-Mendelian inheritance. They correspond to alternative mechanisms to the transmission of hereditary patterns:

  • incomplete dominance: characteristics that one does not necessarily dominate the other. Two alleles can generate an intermediate phenotype when a mixture of the genotypes occurs. For example, from the mixture of a red rose and a white rose a pink rose can be generated.
  • 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 groups.
  • Codominance: two alleles can be expressed at the same time because the dominant genes can also be expressed without mixing. For example, a flower that shows red color and white color at the same time, without mixing.
  • Pleiotropy: when a gene that can affect the expression of other genes. For example, the gene for a genetic defect can affect the development of the individual.
  • Link to sex: It is associated with genes present on the sex chromosomes. For example: white eyes in fruit flies appear in males, while red eyes appear twice as much in females.
  • Epistasis: alleles of one gene can mask and affect the expression of alleles of another gene. For example, some genes in irritable bowel disease.
  • Complementary genes: it refers to the fact that there are recessive alleles of different genes that can express the same phenotype. For example, resistance genes against fungi in wheat.
  • Polygenic Inheritance: These are characteristics determined by several genes, such as height, skin color, among others.
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Gregor Mendel

Gregor Mendel’s research was considered relevant from 1900, when scientists Hugo Vries, Carl Correns and Erich von Tschermak took into account his experiments and results. As a result of this he is considered a milestone in studies on biology and genetics.

Mendel is considered the father of genetics thanks to his laws. These form the basis of genetics and its theoriessince they show what the phenotype of the new individual will be like, that is, its physical characteristics and expression of the genotype.

References

Marks, J. (2008). The construction of Mendel’s laws. Evolutionary Anthropology: Issues, News, and Reviews, 17(6), 250-253.

Bravo A., M. et al. (2007) Biology II. Editorial Santillana. Santiago de Chile.