Principles of Inheritance and Variation - Complete NEET Guide with Diagrams & Practice Questions

Introduction

Welcome to the foundational chapter of Genetics! "Principles of Inheritance and Variation" unravels the mysteries of heredity—why we resemble our parents, yet are also unique. This unit, which includes Molecular Biology and Evolution, is one of the most high-weightage sections in the NEET Biology syllabus, with this chapter alone contributing 3-5 questions on average.

Mastering the concepts laid down by Gregor Mendel, understanding the exceptions to his laws, and learning to solve problems on genetic crosses and pedigree analysis are non-negotiable for a top score. This guide will walk you through every critical concept, from monohybrid crosses to chromosomal disorders, with clear explanations, diagrams, and practice questions to ensure you are fully prepared for the exam.

Key Concepts

Before diving in, let's clarify some basic terms:

Gene: A unit of inheritance; a segment of DNA that codes for a specific trait.
Allele: Slightly different forms of the same gene (e.g., T and t for height).
Genotype: The genetic makeup of an individual (e.g., TT, Tt, tt).
Phenotype: The observable physical characteristic (e.g., tall, dwarf).
Homozygous: Having two identical alleles for a trait (e.g., TT, tt).
Heterozygous: Having two different alleles for a trait (e.g., Tt).

1. Mendel's Laws of Inheritance

Gregor Mendel (1822-1884), the "Father of Genetics," conducted hybridization experiments on garden peas (Pisum sativum) for seven years. He selected 7 pairs of contrasting traits for his study.

Why Pea Plants? Short life cycle, easy to cultivate, large number of offspring, and clear contrasting traits.

Inheritance of One Gene (Monohybrid Cross)

A cross involving the study of a single character (e.g., stem height).

Mendel's Cross: He crossed a true-breeding tall plant (TT) with a true-breeding dwarf plant (tt).
- F₁ Generation: All plants were tall (Tt).
- F₂ Generation (from selfing F₁): Plants were tall and dwarf in a ratio of 3:1.
Phenotypic Ratio (F₂): 3 (Tall) : 1 (Dwarf)
Genotypic Ratio (F₂): 1 (TT) : 2 (Tt) : 1 (tt)

Figure: Diagrammatic representation of a monohybrid cross, showing P, F₁ and F₂ generations with their respective genotypes and phenotypes.

From these observations, Mendel proposed two laws:

Law of Dominance: In a dissimilar pair of factors (alleles), one member of the pair dominates (dominant) the other (recessive).
Law of Segregation: During gamete formation, the two alleles of a pair segregate from each other such that a gamete receives only one of the two factors. This is a universal law with no exceptions.

Test Cross: A cross between an individual with a dominant phenotype (of unknown genotype, e.g., T_) and its homozygous recessive parent (tt). It is used to determine the genotype of the unknown dominant individual.
- If all offspring are dominant, the parent was homozygous dominant (TT).
- If the offspring are in a 1:1 ratio (dominant:recessive), the parent was heterozygous (Tt).

Inheritance of Two Genes (Dihybrid Cross)

A cross involving the study of two characters simultaneously (e.g., seed shape and seed color).

Mendel's Cross: Round Yellow (RRYY) x Wrinkled Green (rryy).
- F₁ Generation: All were Round and Yellow (RrYy).
- F₂ Generation (from selfing F₁): Produced four phenotypes in a specific ratio.
Phenotypic Ratio (F₂): 9 (Round Yellow) : 3 (Round Green) : 3 (Wrinkled Yellow) : 1 (Wrinkled Green).

This led to Mendel's third law:

Law of Independent Assortment: When two pairs of traits are combined in a hybrid, the segregation of one pair of characters is independent of the other pair of characters. This law is applicable only for genes located on different chromosomes or far apart on the same chromosome.

Figure: Punnett square for a dihybrid cross, showing the 9:3:3:1 phenotypic ratio in the F₂ generation.

2. Deviations from Mendelism

Post-Mendelian discoveries revealed more complex inheritance patterns.

Incomplete Dominance

The F₁ hybrid shows a phenotype that is intermediate between the two parental phenotypes.

Example: Flower color in Snapdragon (Antirrhinum sp.).
- Red (RR) x White (rr) → Pink (Rr) in F₁.
- Selfing F₁ (Rr x Rr) gives F₂ with a phenotypic and genotypic ratio of 1 (Red) : 2 (Pink) : 1 (White).

Co-dominance & Multiple Alleles

Co-dominance: The F₁ hybrid resembles both parents, as both alleles express themselves fully and independently.
- Example: ABO blood group in humans. The gene I has three alleles: Iᴬ, Iᴮ, and i.
  - Iᴬ and Iᴮ are dominant over i.
  - When Iᴬ and Iᴮ are present together (genotype IᴬIᴮ), both express themselves, resulting in AB blood type.
Multiple Alleles: When a gene exists in more than two allelic forms. ABO blood group is a classic example.

Pleiotropy

When a single gene exhibits multiple phenotypic effects.

Example: Phenylketonuria in humans. A single gene mutation leads to mental retardation, and a reduction in hair and skin pigmentation.
Example: Starch synthesis in pea seeds. The gene controls both seed shape (round/wrinkled) and starch grain size.

Polygenic Inheritance

Traits that are controlled by three or more genes. The phenotype reflects the additive contribution of each allele.

Example: Human skin color and height. These traits show a continuous variation (a gradient) rather than distinct phenotypes.

3. Chromosomal Theory of Inheritance

Proposed by Walter Sutton and Theodore Boveri in 1902. They noted that the behavior of chromosomes was parallel to the behavior of Mendel's 'factors' (genes).

Key Parallels:
- Both occur in pairs in diploid organisms.
- Both segregate during gamete formation, such that a gamete receives only one of each pair.
- Independent pairs segregate independently of each other.
Conclusion: Genes are located on chromosomes, and it is the segregation and independent assortment of chromosomes that leads to Mendelian ratios.

4. Linkage and Recombination

Experimental Verification: Done by Thomas Hunt Morgan on fruit flies (Drosophila melanogaster).
Linkage: The physical association of genes located on the same chromosome. These genes tend to be inherited together and do not follow the Law of Independent Assortment.
Recombination: The generation of non-parental gene combinations due to crossing over during meiosis.
Morgan's Findings:
- The closer the genes are on a chromosome, the stronger the linkage and the lower the recombination frequency.
- The farther apart the genes are, the weaker the linkage and the higher the recombination frequency.
Gene Mapping: Alfred Sturtevant used recombination frequency to measure the distance between genes and 'map' their positions on the chromosome. 1 map unit = 1% recombination frequency.

5. Sex Determination

XY Type (Humans, Drosophila): Female is homogametic (XX), Male is heterogametic (XY).
XO Type (Grasshopper): Female has two X chromosomes (XX), Male has only one (XO).
ZW Type (Birds): Female is heterogametic (ZW), Male is homogametic (ZZ).
Haplodiploidy (Honey Bee): Females are diploid (32 chromosomes), developing from fertilized eggs. Males (drones) are haploid (16 chromosomes), developing from unfertilized eggs (parthenogenesis).

6. Genetic Disorders

Pedigree Analysis

A chart that represents the inheritance of a specific trait through several generations of a family.

Purpose: To determine the mode of inheritance (dominant/recessive, autosomal/sex-linked) of a trait.
Key Symbols: Square for male, circle for female, shaded symbol for affected individual, horizontal line for mating.

Figure: Standard symbols used in pedigree analysis and representative pedigrees for autosomal dominant and recessive traits.

Mendelian Disorders

Caused by alteration or mutation in a single gene.

Disorder	Type	Key Features
Colour Blindness	X-linked Recessive	Inability to distinguish between red and green. More common in males.
Haemophilia	X-linked Recessive	Defective blood clotting, leading to non-stop bleeding.
Sickle-cell Anaemia	Autosomal Recessive	Substitution of Glu by Val at 6th position of β-globin chain. RBCs become sickle-shaped.
Phenylketonuria (PKU)	Autosomal Recessive	Inborn error of metabolism. Lack of an enzyme to convert phenylalanine to tyrosine.
Thalassemia	Autosomal Recessive	Quantitative problem: reduced synthesis of globin chains (α or β).
Myotonic Dystrophy	Autosomal Dominant	Progressive muscle weakness and wasting.

Chromosomal Disorders

Caused by the absence, excess, or abnormal arrangement of one or more chromosomes.

Aneuploidy: Gain or loss of a chromosome.
- Down's Syndrome: Trisomy of chromosome 21. Short stature, small round head, furrowed tongue.
- Klinefelter's Syndrome: 47, XXY. Male with overall masculine development but also feminine features (gynaecomastia). Sterile.
- Turner's Syndrome: 45, X0. Females are sterile as ovaries are rudimentary. Lack of other secondary sexual characters.
Polyploidy: Presence of more than two sets of chromosomes. Common in plants.

Important Formulas & Equations

Number of Gamete Types: = 2ⁿ (where n = number of heterozygous gene pairs).
Phenotypic Ratios:
- Monohybrid Cross: 3:1
- Dihybrid Cross: 9:3:3:1
- Incomplete Dominance: 1:2:1
- Monohybrid Test Cross: 1:1
Recombination Frequency (%): = (Number of Recombinant Progeny / Total Number of Progeny) × 100

Memory Techniques (Mnemonics)

Mendel's 7 Traits: Use the acronym "F-SPAC-Ce-Po" (Flower-color, Seed-color, Pod-color, Axial/Terminal, Constricted/Inflated, Color-seed, Pod-shape) and visualize the dominant traits.
Chromosomal Disorders:
- Down Syndrome -> Disomy+1 on 21st.
- Klinefelter's -> Boy has a "Knick-knack" extra X (XXY).
- Turner's -> "Turned-off" X chromosome (XO).
Recessive vs. Dominant Disorders: Remember that most enzyme-defect related disorders are recessive (e.g., PKU, Sickle-cell, Thalassemia).

Previous Year Questions (NEET)

Q1. In a cross between a male and female, both heterozygous for sickle-cell anaemia gene, what percentage of the progeny will be diseased? (NEET 2021) a) 50% b) 75% c) 25% d) 100%

Explanation: Sickle-cell anaemia is an autosomal recessive disorder. A cross between two heterozygotes (carriers, HbᴬHbˢ x HbᴬHbˢ) results in a genotypic ratio of 1 (HbᴬHbᴬ) : 2 (HbᴬHbˢ) : 1 (HbˢHbˢ). Only the homozygous recessive (HbˢHbˢ) individuals are diseased, which constitute 1/4 or 25% of the progeny. Answer: (c) 25%

Q2. The term 'linkage' was coined by: (NEET-I 2016) a) T. Boveri b) G. Mendel c) W. Sutton d) T.H. Morgan

Explanation: Thomas Hunt Morgan conducted extensive work on Drosophila and discovered the phenomenon of physical association of genes on a chromosome, which he termed 'linkage'. Answer: (d) T.H. Morgan

Q3. A tall true-breeding garden pea plant is crossed with a dwarf true-breeding garden pea plant. When the F₁ plants were selfed, the resulting genotypes were in the ratio of: (NEET-I 2016) a) 3 : 1 :: Tall : Dwarf b) 1 : 2 : 1 :: Tall homozygous : Tall heterozygous : Dwarf c) 1 : 2 : 1 :: TT : Tt : tt d) All the offspring were tall.

Explanation: This question asks for the genotypic ratio in the F₂ generation of a monohybrid cross. The cross Tt x Tt produces genotypes TT, Tt, and tt in the ratio of 1:2:1 respectively. Answer: (c) 1 : 2 : 1 :: TT : Tt : tt

Q4. A colour-blind man marries a woman with normal sight who has no history of colour blindness in her family. What is the probability of their grandson being colour-blind? (NEET 2015) a) 0.5 b) 1 c) Nil d) 0.25

Explanation: Man (XᶜY) x Woman (XX). All daughters will be carriers (XᶜX) and all sons will be normal (XY). If a carrier daughter (XᶜX) marries a normal man (XY), their sons have a 50% chance of being colour-blind (XᶜY). Therefore, the grandson (via the daughter) has a 0.5 probability. Answer: (a) 0.5

Key Takeaways for Quick Revision

Mendel's Laws (Dominance, Segregation, Independent Assortment) form the basis of genetics.
Law of Segregation is universal; the Law of Independent Assortment is not applicable to linked genes.
Incomplete Dominance (blending phenotype, e.g., pink flowers) and Co-dominance (both express, e.g., AB blood type) are key deviations.
Linkage is the tendency of genes on the same chromosome to be inherited together. Recombination frequency is a measure of the distance between them.
Pedigree analysis is a crucial tool for tracing the inheritance of human genetic disorders.
Clearly differentiate between Mendelian (single-gene) and Chromosomal (aneuploidy/polyploidy) disorders and know key examples of each.

Principle Of Inheritance And Variation