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.

The trait expressed in a heterozygous condition is called dominant.

An organism having unlike alleles for a character, such as Tt, is called heterozygous.

The trait that remains unexpressed in a heterozygous condition but appears in homozygous form is called recessive.

Alternative forms of the same gene controlling a pair of contrasting traits are called alleles.

An organism having identical alleles for a character, such as TT or tt, is called homozygous.

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

Mendelism refers to the principles of inheritance proposed by Gregor Mendel based on his experiments with pea plants.​ These principles explain that traits are inherited in a predictable manner through discrete hereditary units.

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.

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.

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.
• When two or more genes control the traits having distinct alternate forms, it is called polygenic inheritance.

The law of dominance states that when a pair of alleles or allelomorphs are combined in an F₁ hybrid, only one of them expresses itself, hiding the expression of the other. A monohybrid cross was used to investigate the simultaneous inheritance of a single pair of Mendelian components. A monohybrid cross is one that considers only different versions of a single character. The feature that occurred in the F₁ generation was referred to as dominant, whereas the trait that did not appear in the F₁ population was known as recessive.

Thus, when a pair of alleles come together in an F₁ hybrid, only one of them expresses itself, hiding the expression of the other entirely. In the above example, in the Tt - F₁ hybrid (tall), only ‘T’ expresses itself as dominant, while ‘t’ remains hidden as recessive. This instance illustrates and indicates the law of dominance.

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).

Or

When two homozygous individuals with one or more sets of contrasting characters are crossed, the alleles (characters) that appear in F₁ are dominant and those which do not appear in F₁ are 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.

or

When hybrid (F₁) forms gametes, the alleles segregate from each other and enter in different gametes.

• Genetics is the study of inheritance and variation in living organisms.
• Inheritance means passing traits from parents to offspring.
• Variation refers to differences between offspring and their parents.
• Early humans knew that sexual reproduction causes variation (around 8000–1000 B.C.).
• Humans used selective breeding to develop desirable traits (e.g., Sahiwal cows).

• Gregor Mendel is known as the Father of Genetics; he worked on pea plants (1856–1863).
• He used true-breeding pea plants and studied inheritance using cross-pollination experiments.
• Mendel selected 7 pairs of contrasting traits (e.g., tall/dwarf, round/wrinkled, yellow/green).
• He introduced the concepts of dominant and recessive traits.
• His experiments had a large sample size and statistical analysis, making the results reliable.
• Mendel’s work formed the basic laws of inheritance, explaining how traits pass from parents to offspring.
• His findings were confirmed by repeated experiments across generations.

• 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.

• Incomplete Dominance - Exception to law of dominance; neither allele is completely dominant; F₁ hybrid shows an intermediate expression of both characters.
• Example - Red (RR) × White (rr) in Mirabilis jalapa → F₁ offspring are Pink (Rr); neither red nor white dominates completely.
• F₂ Generation - Selfing of F₁ (Rr × Rr) gives:
  Genotypic ratio - 1RR : 2Rr : 1rr
  Phenotypic ratio - 1 Red : 2 Pink : 1 White
• Both phenotypic and genotypic ratios are 1:2:1 (unlike Mendel's 3:1 phenotypic ratio), which is the key difference from complete dominance.

• Co-dominance - Both alleles of an allelomorphic pair express themselves equally in F₁ hybrids; neither allele is dominant or recessive over the other.
• Example - Red cattle (RR) × White cattle (WW) → F₁ hybrids are Roan (RW); roan coat has a mixture of red and white hair - both traits expressed equally.
• F₂ Generation - Selfing of F₁ (RW × RW) gives:
  Genotypic ratio - 1RR : 2RW : 1WW
  Phenotypic ratio - 1 Red : 2 Roan : 1 White
• In co-dominance, genotypic and phenotypic ratios are identical (1:2:1); key difference from incomplete dominance is that both alleles are fully expressed, not partially.

• Pleiotropy - A single gene controls two or more different non-related traits; such a gene is called a pleiotropic gene; e.g. sickle-cell anaemia gene (Hb^S).
• Example - Normal gene Hb^A is dominant; heterozygous carriers (Hbᴬ/Hbˢ) show mild anaemia with sickle-shaped RBCs under low O₂; homozygous recessive (Hb^S/Hb^A) die of total anaemia.
• Ratio - Cross between two carriers gives 1 Normal: 2 Carriers: 1 Sickle-cell anaemic; since anaemics die, the surviving ratio becomes 2:1 (carriers: normal) instead of the usual 3:1.
• The gene for sickle-cell anaemia is lethal in a homozygous condition but produces sickle-cell trait (mild anaemia) in a heterozygous condition - two different expressions from a single gene.

1. Polygenic traits are controlled by two or more independent genes, with each contributing allele adding an equal, additive increment to the final phenotype.
2. Instead of falling into distinct categories, these traits form a continuous spectrum that characteristically results in a bell-shaped, normal distribution curve within a population.
3. This type of inheritance lacks simple dominant-recessive relationships and gene masking (epistasis); alleles are simply contributing or non-contributing, and the genes assort independently.
4. The final physical expression of these traits is not strictly dictated by genetics, as it is heavily modified by environmental conditions, such as nutrition, affecting a person's height.
5. Prominent examples include human skin colour, human height (involving roughly 400 genes), human eye colour, and wheat kernel colour.