Revision: Classical Genetics Botany HSC Science Class 12 Tamil Nadu Board of Secondary Education

Heredity (heirship or inheritance) is the transmission of genetically based characters from parents to their offsprings.

Mendel's first experiments were with the varieties of garden pea that differed in only one visible character. These are known as monohybrid experiments.

A cross between parents differing in two heritable traits is called a dihybrid cross. e.g., a cross of a pure, tall, round seeded plant with a dwarf, wrinkled-seeded plant.

A test cross is defined as a genetic cross between an individual showing a dominant phenotype with unknown genotype and a homozygous recessive individual to determine the genotype of the dominant individual.

A back cross is defined as a genetic cross between an F₁ hybrid and either of its parental forms (dominant or recessive) to study inheritance of traits.

Incomplete dominance is the inheritance pattern in which neither allele of a gene is completely dominant over the other, so the heterozygous individual shows an intermediate phenotype between the two parental traits.

Co-dominance is the pattern of inheritance in which both alleles of a gene express themselves equally and simultaneously in the heterozygous condition, so both parental traits appear side by side in the phenotype.

The inheritance of a trait that is controlled by two or more genes, where each gene contributes additively to the phenotype and the trait shows continuous variation rather than distinct categories, is called polygenic inheritance.

The law of dominance states that, out of a pair of allelomorphic characters one is dominant and the other recessive.

1. In a pair of contrasting traits, only one trait is expressed—this is the dominant trait.
2. The trait that remains unexpressed is called recessive.
3. The recessive trait can express itself only when both alleles are recessive (homozygous recessive).

Law of segregation states that, when a pair of allelomorphs are brought together in the hybrid (F₁), they remain together in the hybrid without blending but separate complete and pure during gamete formation. 

1. Each pair of alleles separates during gamete formation, with one going into each gamete.
2. No blending occurs; alleles remain pure and distinct.
3. Gametes fuse randomly during fertilisation to form a zygote.

• Gregor Johann Mendel (1822–1884), an Austrian monk, is known as the Father of Genetics for his pioneering work on heredity.
• He studied science and mathematics at the University of Vienna, which helped him apply a quantitative approach to biological problems.
• Mendel conducted systematic hybridization experiments on garden pea (Pisum sativum) from 1856 to 1863.
• From these experiments, he formulated the fundamental Laws of Inheritance, explaining how traits are transmitted across generations.
• Although his work was ignored during his lifetime, it was rediscovered in 1900, leading to widespread recognition and the foundation of modern genetics.

• Back cross is the cross between the F₁ hybrid and either of its parents (dominant or recessive).
• A test cross is a special type of backcross where the F₁ hybrid is crossed with a homozygous recessive parent.
• Backcross is used to obtain desirable traits and may produce all dominant offspring when crossed with a dominant parent.
• A test cross is used to determine the genotype (homozygous or heterozygous) of an organism showing a dominant trait.
• In a test cross, a 1:1 ratio of dominant and recessive traits indicates a heterozygous condition.
• If all offspring show dominant traits in a test cross, the parent is homozygous dominant.
• Test cross is simple, reliable, and widely used in plant breeding and crop improvement.