🧬Genetics: Key Terms, Variation, Heredity, and Gene

Why Genetics Matters in Agriculture

When a plant breeder crosses a high-yielding but disease-susceptible wheat variety with a low-yielding but rust-resistant one, genetics explains how the offspring inherit traits from both parents. The entire foundation of crop improvement — from Mendel’s pea experiments to modern marker-assisted selection — rests on understanding heredity (how traits are passed on) and variation (why offspring differ from their parents). Without genetic variation, there would be nothing for breeders to select from.

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What Is Genetics?

• The word “Genetics” derives from the Greek root “gene”, meaning to grow into or to become.
• The term was coined by W. Bateson. ^{UPPSC 2021}
• Bateson also coined the terms homozygous and heterozygous.
• Genetics = the study of heredity and variation.
  • Heredity = traits transmitted from generation to generation (e.g., grain colour, plant height).
  • Variation = differences among individuals of the same species.

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Variation

Hereditary (Genetic) Variation

• Variations in inherited traits that are transmitted across generations.
• Caused by sexual reproduction (independent assortment + crossing over) and mutation (new alleles).
• Examples: stripe patterns in zebra, neck length differences in giraffes.
• Identical twins share the same DNA, so they show no hereditary variation between them.

Environmental (Phenotypic) Variation

• Entirely due to environment; temporary and not inherited.
• Affects only the phenotype (observable appearance), not the genotype (genetic constitution).
• Examples: darker skin from sun exposure (not inherited); a tall plant becoming dwarf under water/nitrogen stress.

Agricultural example: Two rice plants with the same genotype may produce very different yields if one is grown in fertile irrigated soil and the other in poor rainfed conditions. The genetic potential is the same, but the environment modifies expression.

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Continuous vs. Discontinuous Variation

Agricultural significance: Most economically important traits in crops (yield, drought tolerance, grain quality) show continuous variation — making their improvement more complex than simple qualitative traits.

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Causes of Genetic Variation

Importance of Variation

• Variation causes evolution and is the basis of heredity.
• Enables adaptation to environmental changes — essential for species survival.
• In agriculture, genetic variation is the raw material for all breeding programmes.

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Heredity: Genotype and Phenotype

• Johannsen (1909) formulated the genotype-phenotype concept and coined the terms gene, genotype, and phenotype.

Key formula: Phenotype = f(Genotype + Environment)

Exam tip: Johannsen coined three terms — gene, genotype, phenotype. He also renamed Mendel’s “factors” as “genes.”

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Gene — The Hereditary Unit

• A gene is the smallest functional unit of inheritance — chemically, a segment of DNA that controls the synthesis of a polypeptide (enzyme/protein).
• Genes are linearly arranged on chromosomes — the chromosome is the bearer; the gene is the passenger.
• Mendel called genes “factors”; Johannsen renamed them.
• Genes exist in pairs (one from each parent) with two forms: dominant and recessive.

Key Gene-Related Terms

• Barbara McClintock (1983, Nobel Prize) discovered transposable elements (jumping genes) in maize — genes that can move around the genome.

Chromosome Types

• Holandric genes are located on the Y-chromosome — inherited exclusively from father to son.
• Centromere (Kinetomere) = the “driver” of the chromosome — attachment point for spindle fibres during cell division.
• Chromosomal Theory of Inheritance: Genes are located on chromosomes — postulated by Sutton and Boveri.

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History of Hereditary Thought

Weismann’s Germplasm Theory

• The body has two parts: somatoplasm (body) and germplasm (reproductive cells).
• Changes affecting somatoplasm but not reaching germplasm are not heritable.
• Somatoplasm dies with the individual, but germplasm is immortal — it passes from generation to generation.
• This principle is crucial: only changes in reproductive cells (germline) can be inherited.

Agricultural application: When a farmer exposes seeds to gamma rays for mutation breeding, the mutations must occur in the germplasm (reproductive cells) to be inherited. Mutations in somatic tissue alone will not pass to the next generation.

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Physical Basis of Heredity

• Mendel used the term “Marmal” for the genetic factor; Johannsen’s term “gene” is now universally used.
• Genes are located on chromosomes in a linear order.
• Each species has a fixed chromosome number (e.g., rice 2n = 24, wheat 2n = 42).
• Body cells are diploid (2n) — two sets of homologous chromosomes (one from each parent).
• Gametes are haploid (n) — produced through meiosis.
• Homologous chromosomes carry genes for the same traits at the same loci, though alleles may differ.

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Summary Cheat Sheet