Principles of Inheritance
Mendel conducted experiments on Pisum sativum, the garden peas from 1857 to 1865. The results of which, he published under the title ‘Experiments In Plant Hybridization’ and presented before Brunn society for the sturdy of natural sciences in 1865.
Mendel chose his monastery garden and considered seven out of thirty different traits of pea plants for his experimental studies. There were several reasons for Mendel to have chosen pea plants over others. They were,
- Easily available and easy to grow
- Short life span of one year
- Bisexual flowers
- Naturally self-pollinating
- Availability of contrasting set of characters
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Mendel choose 7 different traits of pea plant for his experiments. Given below is the table which includes those traits.
Pure lines – Pure breeding lines are the organisms which give birth to same kind of organisms, generation after generation. So, all pure breeding tall plants only produced tall plants and dwarf only produced dwarf plants.
The cross is represented using the phenotype, which is the external appearance of the organism, and genotype which is the genetic composition of the trait in consideration.
Each trait, has an alternative version of its gene that is located at a specific location, on a specific chromosome and is called allele. For example, the trait height, has two alleles T, the gene for tallness and t, the gene for dwarfness. Here, the dominant gene is represented in capital letter and the recessive in small letter.
The pure breeding tall plant will have the genotype TT, is called homozygousfor that trait. When a pair of genes in a plant are identical like [TT] or [tt], the plant is considered to be homozygous for that gene pair.
When a pair of contrasting genes are present in plant for a trait like [Tt], it is considered as heterozygous for that gene pair. Such a plant is called hybrid as two parents who are genetically different gave rise to an heterozygous individual.
Dominant gene is the gene which expresses itself in the heterozygous or hybrid condition, i.e. in a condition of Tt, the gene T expresses itself, masking the effect of the gene t. Thus, the plant which is heterozygous for height with genes Tt, will be tall due to effect of gene T. So, T is the dominant gene and hence is represented in capital letter.
Gene for dwarfness, t is called recessive as it remains hidden or unexpressed in presence of dominant gene T. To understand better, consider the cross between pure breeding tall and pure breeding dwarf.
It is represented as follows.
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First filial generation were all tall plants with genotype Tt, thus considered as heterozygous for stem length. Also, because the tallness was dominant over the dwarfness in heterozygous condition, tallness is considered as dominant trait and dwarfness, the recessive trait.
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Phenotypic Ratio – 3 [Tall]:1 [Dwarf]
Genotypic Ratio – 1 [TT]: 2 [Tt]: 1 [tt]
F2 generation or the second generation cross had hybrids from F1 as parents. Hence, they produced two types of gametes, T and t. The cross between these two hybrid parents produced three tall and one dwarf plant, which is represented as phenotypic ratio and two hybrids and pure breeding genotypes which is represented by genotypic ratio.
The second generation cross is represented using a Punnett square. Punnett square is a diagram used to predict an outcome of a particular cross or breeding experiment and is named after Reginald C.Punnet, devised the approach.
Mendel’s First Law of Inheritance: Law of Segregation
Based on the results of the monohybrid cross, proposed that,
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When a pair of contrasting characters are brought together in a hybrid, they separate or segregate during gamete formation.
This is what we see in F2 generation of monohybrid cross. The F2 generation showed the presence of three different gene combinations, TT, Tt and tt. This was possible only due to segregation of genes during gamete formation. A gamete, is hence pure for a particular trait or gene.
Because of this, the law is also famous as Law of Purity of Gametes.