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Mendel’s Laws of Inheritance

 

Mendel’s Laws of Inheritance

  • Gregor Johann Mendel (1822–1884) is known as “Father of Genetics”.

  • In 1865, Mendel presented the results of his experiments with nearly 30,000 pea plants.

  • He called genes as “factors”, which are passed from parents to offspring’s.

  • Genes, that code for a pair of opposite traits are called “alleles”. He conducted artificial pollination / cross-pollination experiments using several true-breeding varieties having contrasting traits.

Mendel’s Experimental Plant

  • Mendel selected garden pea as his experimental material.

  • Phenotype: Visible expression of genetic constitution e.g., Tall/dwarf

  • Genotype: Genetic constitution of individual e.g., TT, Tt, tt.

Mendel’s Observations

Monohybrid Cross: Cross involving study of inheritance of one character, e.g., height of plant.
Dihybrid Cross: Cross between plants differing in two traits/cross involving study of inheritance of 2 genes or characters.
Homozygous: The individual carrying similar alleles for a trait e.g., TT or tt.
Heterozygous: Individual carrying different alleles for a trait e.g., Tt

Mendel’s Laws of Inheritance

1. LAW OF DOMINANCE

This law states that when two alternative form of a trait or character (genes or alleles) are present in an organism, only one factor expresses itself in F1 progeny and is called dominant while the other that remains masked is called recessive.

Monohybrid Cross

2. LAW OF INDEPENDENT ASSORTMENT

According to this law the two factor of each character assort or separate out independent of the factors of other characters at the time of gamete formation and get randomly rearranged in the offspring producing both parental and new combinations of characters.

3. LAW OF SEGREGATION

This law states that the factors or alleles of the pair segregate from each other during gamete formation, such that a gamete receives only one of the two factors. They do not show any blending.

Incomplete Dominance

  • When neither of the two alleles is dominant and the phenotype of the heterozygote does not resemble any of the parents. The heterozygote expresses intermediate or a mixture of two parents’ traits

  • Example: The flower colour inheritance of snapdragon. On crossing true breeding red (RR) and white flower (rr), we get all pink colour flowers in the F1 generation, which on self-pollination give red: pink: white flowers in the ratio 1:2:1 in the F2 generation.

Co-dominance

The alleles which are able to express themselves independently, even when present together are called co-dominant alleles and this biological phenomenon is called co-dominance.

Test Cross

It is a method devised by Mendel to determine the genotype of an organism. In this cross, the organism with dominant phenotype (but unknown genotype) is crossed with the recessive individual.

Pleiotropy

  • It is the phenomenon in which a single gene exhibits multiple phenotype expressions.

  • The pleiotropic gene affects the metabolic pathways, resulting in different phenotypes.

  • Example: a single gene mutation in the gene coding for the enzyme phenylalanine hydroxylase results in the disease known as phenylketonuria, which is characterised by mental retardation, reduced hair and skin pigmentation.

Polygenic Inheritance

  • It is a type of inheritance, in which a trait is controlled by three or more genes. Some traits are called polygenic traits.

  • The phenotype reflects contribution of each allele and is also influenced by the environment.

  • Example: eye colour, skin pigmentation, height, hair colour, etc.
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