Principles of Inheritance and VariationNEET Botany · Class 12 · NCERT Chapter 2

Introduction to Genetics

Genetics is the branch of biology that studies the inheritance of traits from parents to offspring. It explains why offspring resemble their parents and why they are also slightly different. The principles of inheritance and variation underlie modern medicine, agriculture, evolutionary biology, and biotechnology.

Mendel and His Garden Pea Experiments

Gregor Johann Mendel (1822-1884) was an Austrian monk and the founder of modern genetics. He worked with garden pea (Pisum sativum) in his monastery garden between 1856 and 1863, performing thousands of crosses and recording data carefully. His work, published in 1866, was largely ignored until 1900 when three botanists (de Vries, Correns, Tschermak) independently rediscovered his laws.

Why Garden Pea?

• True-breeding lines: homozygous lines that always produced the same trait, generation after generation.
• Sharp contrasting traits: seven characters with two clear-cut alternatives each.
• Controllable pollination: normally self-pollinating (cleistogamous), but cross-pollination could be done by emasculation + manual pollen transfer.
• Short life cycle: a few months per generation, allowing many generations in a few years.
• Large progeny: hundreds of seeds per cross, allowing reliable statistical analysis.

The 7 Contrasting Characters Mendel Studied

• Seed shape: round vs wrinkled
• Seed colour: yellow vs green
• Flower colour: violet/purple vs white
• Pod shape: full/inflated vs constricted
• Pod colour: green vs yellow
• Flower position: axial vs terminal
• Plant height: tall vs dwarf

Monohybrid Cross (3:1 ratio)

A monohybrid cross involves a SINGLE character (one gene). Mendel crossed true-breeding tall pea plants (TT) with true-breeding dwarf plants (tt).

Cross results

• Parental cross: TT × tt → all F1 are Tt (heterozygous, ALL TALL).
• F1 self: Tt × Tt → F2 shows TWO different phenotypes!
• F2 phenotypic ratio: 3 TALL : 1 DWARF = 3:1.
• F2 genotypic ratio: 1 TT : 2 Tt : 1 tt = 1:2:1.

Test Cross

Looking at a tall pea plant, you cannot tell whether it is homozygous (TT) or heterozygous (Tt) - both look the same. To find out, Mendel devised the TEST CROSS: cross the unknown with a homozygous recessive (tt).

• If unknown is TT: TT × tt → all offspring are Tt → ALL TALL.
• If unknown is Tt: Tt × tt → 1 Tt : 1 tt → 1 TALL : 1 DWARF (1:1 ratio).

Dihybrid Cross (9:3:3:1 ratio)

A dihybrid cross involves TWO characters (two genes) at the same time. Mendel\'s classic dihybrid cross used round-yellow seeds (RRYY) crossed with wrinkled-green seeds (rryy).

• Parental cross: RRYY × rryy → F1 = all RrYy (round yellow).
• F1 self: RrYy × RrYy → F2 shows FOUR different phenotypes.
• F2 phenotypic ratio: 9 round-yellow : 3 round-green : 3 wrinkled-yellow : 1 wrinkled-green = 9:3:3:1.
• The TWO new phenotypes (round-green and wrinkled-yellow) are RECOMBINANTS not present in the parents.

Why 9:3:3:1?

The 9:3:3:1 ratio is the PRODUCT of TWO independent monohybrid 3:1 ratios:

This product rule WORKS only if the two genes assort INDEPENDENTLY. Mendel\'s observation of the 9:3:3:1 ratio is the basis of his 3rd law (independent assortment). Note: this only holds for genes on DIFFERENT chromosomes. Linked genes give different ratios.

Mendel\'s Laws of Inheritance

Law 1: Law of Dominance

When two contrasting alleles for a gene are present in a heterozygote, ONLY the dominant allele is expressed in the phenotype. The recessive allele is masked.

Law 2: Law of Segregation (Law of Purity of Gametes)

During gamete formation, the two alleles of a gene SEPARATE so that each gamete gets only ONE allele. When fertilisation occurs, the alleles combine again. This is why F2 shows the recessive trait reappearing in 1/4 of offspring.

Law 3: Law of Independent Assortment

During gamete formation, the alleles of one gene assort INDEPENDENTLY of the alleles of another gene. This produces all possible gamete combinations in equal proportions, giving the 9:3:3:1 dihybrid F2 ratio.

Incomplete Dominance

When neither allele is fully dominant, the heterozygote shows an INTERMEDIATE / blended phenotype.

• Example: Mirabilis jalapa (4 o\'clock plant). RR (red) × rr (white) → F1 = all Rr (PINK, intermediate). F2 ratio = 1 red : 2 pink : 1 white = 1:2:1.
• Phenotypic ratio EQUALS genotypic ratio (each genotype has a unique phenotype).
• Other examples: snapdragon (Antirrhinum), 4 o\'clock plant.

Codominance and Multiple Alleles (ABO Blood Group)

In CODOMINANCE, BOTH alleles are equally expressed in the heterozygote. The two parental phenotypes are visible side by side, not blended (different from incomplete dominance).

The ABO Blood Group System

• Three alleles: IA, IB, i. (Multiple alleles - more than 2 alleles in the population.)
• IA and IB are CODOMINANT to each other.
• Both IA and IB are DOMINANT over i.
• Four blood groups: A (IA IA or IA i), B (IB IB or IB i), AB (IA IB - codominance), O (ii).
• The ABO gene is on chromosome 9 and encodes a glycosyltransferase that adds sugar groups to the H antigen.

Other Examples

• MN blood group (M, N alleles - both codominant).
• Roan coat in cattle (red x white = roan, mix of red and white hairs visible).

Pleiotropy

PLEIOTROPY is when a SINGLE GENE affects MULTIPLE traits. The opposite of polygenic inheritance.

• Phenylketonuria (PKU): mutations in PAH gene (autosomal recessive). Prevents conversion of phenylalanine to tyrosine. Causes mental retardation, lighter pigmentation, musty body odour, eczema. ONE gene, MANY symptoms.
• Sickle-cell anaemia: mutation in beta-globin gene. Affects RBC shape (sickled), causes anaemia, joint pain, organ damage, increased malaria resistance.
• Marfan syndrome: mutation in fibrillin gene. Affects connective tissue: tall stature, long limbs, eye problems, heart defects.

Polygenic Inheritance

POLYGENIC INHERITANCE is when a SINGLE TRAIT is controlled by MULTIPLE GENES (the opposite of pleiotropy). The traits show CONTINUOUS variation rather than distinct categories.

• Human skin colour: controlled by ~4-6 genes. Each contributes additively. Result: a wide spectrum from very dark to very light.
• Human height: influenced by hundreds of genes plus environment.
• Eye colour intensity: graded; multiple genes contribute.
• Wheat kernel colour: classic example - 3 genes, additive contribution.

Sex Determination

Different organisms use different chromosomal systems to determine sex. The HETEROGAMETIC sex (the one producing 2 types of gametes) determines offspring sex.

• XY (humans, Drosophila): male XY (heterogametic), female XX. SRY gene on Y is the male-determining gene.
• XO (grasshoppers): male XO (only ONE X, no Y, heterogametic), female XX.
• ZW (birds, butterflies): female ZW (heterogametic), male ZZ. OPPOSITE of XY.
• ZO (some moths): female ZO (heterogametic), male ZZ.

Sex-Linked Inheritance

Genes located on the SEX CHROMOSOMES (X or Y) show sex-linked inheritance. Most sex-linked traits in humans are X-LINKED (the X chromosome is large and has many genes; the Y is small).

Why X-Linked Recessive Traits Affect Males More

• Males have only ONE X (XY). They are HEMIZYGOUS for X-linked genes - a single recessive allele expresses immediately.
• Females have TWO X (XX). They need TWO recessive alleles to show the disease (homozygous recessive). Heterozygous females are CARRIERS.
• This is why haemophilia and colour blindness are far more common in males.

Examples of X-Linked Recessive Traits

• Haemophilia: blood clotting disorder. Famous Queen Victoria pedigree.
• Colour blindness: red-green confusion. Affects ~8% of males, ~0.5% of females.
• Duchenne muscular dystrophy (DMD): progressive muscle wasting.

Linkage and Recombination

T.H. MORGAN, working with the fruit fly (Drosophila melanogaster), discovered that genes on the SAME chromosome are inherited TOGETHER (linked) - violating Mendel\'s 3rd law of independent assortment.

• Linkage: physical proximity of genes on the same chromosome causes them to be inherited together.
• Crossing over: during meiosis I, homologous chromosomes exchange segments. This separates linked genes and creates RECOMBINANT gametes.
• Recombination frequency (RF): the percentage of recombinant offspring. Maximum = 50% (effectively unlinked).
• Map distance: 1 centimorgan (cM) = 1% recombination frequency. Used to map gene positions on chromosomes.
• Sturtevant (Morgan\'s student) created the first genetic map.
• Morgan won the Nobel Prize in 1933 for this work.

Chromosomal Disorders

Errors in chromosome number cause significant disorders. Most are caused by NON-DISJUNCTION (failure of chromosomes to separate properly during meiosis).

Aneuploidy vs Polyploidy

• Aneuploidy: abnormal number of one specific chromosome (extra = trisomy, missing = monosomy).
• Polyploidy: extra full chromosome SETS (e.g., 4n, 6n). Common in plants; lethal in most animals.

Major Aneuploidies

• Down\'s syndrome: trisomy 21 (47, +21). Autosomal. Short stature, intellectual disability, heart defects. Risk increases with maternal age.
• Klinefelter\'s syndrome: 47, XXY. Sex chromosomal. Tall, sterile males with some female features.
• Turner\'s syndrome: 45, XO. Sex chromosomal. Short, sterile females with no secondary sexual characteristics.
• Edward\'s syndrome: trisomy 18.
• Patau syndrome: trisomy 13.

Pedigree Analysis

A pedigree is a family tree showing the inheritance of a trait. Standard symbols are used: square = male, circle = female, filled = affected, half-filled = carrier (for recessive traits). Pedigrees help determine the inheritance pattern of a trait.

Common Inheritance Patterns

• Autosomal dominant: appears in every generation; both sexes affected equally; vertical pattern. Examples: Marfan, Huntington\'s.
• Autosomal recessive: may skip generations; carriers do not show; both sexes affected equally; horizontal pattern (siblings affected, parents not). Examples: PKU, sickle cell, albinism.
• X-linked recessive: mostly males affected; skips through carrier mothers; father-to-son transmission impossible. Examples: haemophilia, colour blindness.
• X-linked dominant: father-to-son impossible; affected father gives to ALL daughters.

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