👨‍⚖️Mendel's Laws of Inheritance

Why Mendel’s Laws Matter in Agriculture

Every time a plant breeder crosses a disease-resistant variety with a high-yielding one and predicts the outcome in F2, they are applying Mendel’s laws. The 3:1 ratio in a monohybrid cross tells the breeder how many plants to grow to find the desired homozygous recombinant. The test cross is used routinely to verify whether a promising plant is true-breeding (homozygous) before releasing it as a variety. Mendel’s principles remain the bedrock of all breeding programmes — from rice and wheat to cotton and pulses.

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Gregor Mendel — Father of Genetics

• Mendel is known as the Father of Genetics. ^{UPPSC 2021}
• He formulated the Theory of Inheritance through experiments on garden pea in the 1860s.

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Why Garden Pea (Pisum sativum)?

Mendel chose the garden pea because it offered several advantages:

Mendel selected seven pairs of contrasting characters for his study.

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Key to Mendel’s Success

Previous workers studied organisms as a whole complex of characters and failed to find patterns. Mendel succeeded because he:

1. Studied one character at a time — simplified analysis and revealed clear ratios.
2. Selected seven contrasting pairs with clearly distinguishable forms.
3. Chose characters that were independent (on different chromosomes, no linkage).
4. Kept meticulous records with large sample sizes for statistical significance.
5. Used appropriate symbols and terminology.

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Mendel’s Observations (Monohybrid Cross)

• Crossed tall (TT) x dwarf (tt) pea plants.
• F1 generation: All plants were tall → tall is dominant over dwarf.
• F2 generation (F1 selfed): Tall : Dwarf = 3 : 1.
• The same 3:1 ratio appeared in all seven contrasting traits.
• Reciprocal crosses gave identical results (pollen or egg source did not matter).

Agricultural insight: The 3:1 ratio means that in an F2 population of 1000 plants, about 250 will be homozygous for the recessive trait — the breeder can directly identify and select them by phenotype.

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The Three Laws of Mendel

Mendel’s work was rediscovered in 1900 independently by Hugo de Vries (Dutch), Carl Correns (German), and Erich von Tschermak (Austrian).

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1. Law of Segregation (Purity of Gametes)

The most fundamental law of genetics:

In a heterozygous individual, the dominant and recessive alleles remain together without mixing. During gamete formation, the two alleles separate (segregate) so that each gamete receives only one allele.

Four principles underlying segregation:

1. Each hereditary character is determined by a gene (Mendel’s “factor”).
2. Genes occur in pairs (two alleles — one from each parent).
3. During gamete formation, alleles separate so each gamete gets only one allele (occurs during meiosis).
4. At fertilisation, the diploid number is restored (one allele from each parent).

Key point: The word “pure” in “purity of gametes” means alleles do not contaminate each other — T and t remain distinct in a Tt plant.

Homozygous, Heterozygous, and Hemizygous

• Number of gamete types = 2ⁿ (n = number of heterozygous gene pairs).

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2. Law of Dominance (Uniformity of F1)

In a cross between two contrasting varieties, only one trait (the dominant trait) appears in F1. The other trait (recessive) is masked in F1 but reappears in F2.

• All F1 individuals are uniform (same heterozygous genotype) → also called Law of Uniformity of First Filial Generation.
• F1: All tall (Tt). F2: Tall : Dwarf = 3 : 1.

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3. Law of Independent Assortment

Alleles for different characters segregate independently of each other during gamete formation, provided the genes are on different chromosomes (not linked).

• Mendel crossed yellow round (YYRR) x green wrinkled (yyrr) pea.
• F1: All yellow round (YyRr).
• F2: Four phenotypic classes in the ratio 9 : 3 : 3 : 1.

The Punnett Square (checkerboard) is used to work out all possible gamete combinations and predict offspring ratios.

Agricultural application: Independent assortment is why breeders can combine desirable traits from different parents. For example, crossing a high-yielding but susceptible parent with a low-yielding but resistant parent can produce F2 plants that are both high-yielding AND resistant.

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Cross Types and Ratios

Exam formula: For n heterozygous gene pairs: gamete types = 2ⁿ, F2 combinations = 4ⁿ.

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Back Cross and Test Cross

How the Test Cross Works

Agricultural use: The test cross is standard practice in seed production to verify the genetic purity of parental lines before using them in hybrid seed programmes.

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Deviations from Mendel’s Laws

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Why Mendel’s Work Was Not Recognised Until 1900

1. Darwin’s theory of continuous/blending variation dominated scientific thought.
2. Mendel used mathematical calculations — unfamiliar to biologists of the time.
3. Chromosomal behaviour and meiosis were not yet understood.
4. Published in an obscure local journal (Proceedings of the Natural History Society of Brunn).
5. Failed to replicate results with Hieracium (hawkweed) — which reproduces apomictically.

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Summary Cheat Sheet