Principles of Inheritance and Variation

Inheritance: The process by which characters are passed on from parent to progeny is called inheritance. Inheritance is the basis of heredity.

Variation: The degree by which progeny differ from their parents is called variation.

Mendel's Laws of Inheritance

True Breeding Line: A breeding line which shows stable trait inheritance and expression for several generations; after having undergone continuous self-pollination; is called a true breeding line. Mendel used true breeding line for his experiments.

Inheritance of One Gene

Monohybrid Cross

When only one pair of contrasting traits is studied in the cross, it is called monohybrid cross.

For monohybrid cross, let us take example of monohybrid cross for stem height, i.e. between tall plants and dwarf plants. During this experiment, Mendel made following observations:

• All plants in the F1 generation were tall plants.
• Some of the plants in the F2 generation were dwarf plants. The ratio of tall plants to dwarf plants was 3:1.
• The tall and dwarf traits did not show blending. In other words, the plants were either tall or dwarf, and there was no plant of medium height.

Conclusion by Mendel

Mendel proposed that something was passed on from parent to progeny, and it was responsible for appearance of a particular trait in progeny. Mendel called them ‘factors’. After the discovery of genes, we know that these ‘factors’ are in fact genes. Therefore, genes are the units of inheritance.

Based on his observations on monohybrid cross, Mendel proposed two laws, which are as follows:

Law of Dominance

Characters are controlled by discreet units called factors. Factors occur in pairs. In a dissimilar pair of factors, one factor dominates (dominant factor) over the other (recessive factor).

Law of Segregation

Members of a pair of alleles segregate during gamete formation. Due to segregation of alleles, a gamete receives either of the allele.

Alleles: Genes which code for a pair of contrasting character are called alleles. They are slightly different forms of the same gene.

Punnett Square: It was developed by a British geneticist, Reginald C. Punnett. It is a graphical representation to calculate the probability of all possible genotypes of offspring in a genetic cross. In a Punnett square, the possible gametes are written on two sides, usually in the top row and left columns. All possible combinations are represented in boxes; after the top row and left column.

Following is the Punnett square showing monohybrid cross between pure tall and pure dwarf plants.

Following Punnett Square shows combination of genotype in F2 generation.

The above Punnette square shows that ratio of pure tall, heterozygous tall and pure dwarf plants was 1: 2: 1. Moreover, ratio of tall plants to dwarf plants was 3:1.

Test Cross: When an organism showing a dominant phenotype (and whose genotype is to be determined) is crossed with the recessive parent (instead of self-crossing), the cross is called Test Cross. The progenies of a test cross can be easily analysed to predict the genotype of the test organism.

Following is an example of test cross when genotype needs to be known. Let us assume that all flowers are of violet colour and violet colour is the dominant trait. Let us cross them with homozygous recessive plants producing white flowers. Two situations may emerge after the test cross.

Situation 1: If plants with dominant trait are homozygous (WW) with violet flowers, then a cross with homozygous (ww) plants with white flowers will produce following result.

In this case, all plants in F2 generation will produce violet flowers because violet colour is the dominant trait. This will prove that genotype of F1 generation was homozygous (WW).

Situation 2: If plants with dominant trait are heterozygous (Ww) with violet flowers, then a cross with homozygous (ww) plants with white flowers will produce following result.

In this case, half of the plants in F2 generation will produce violet flowers (Ww) and remaining half will produce white flowers (ww). This will prove that genotype of F1 generation was heterozygous (Ww).

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