01
Mendelism and the three laws
Foundation for every numerical in the chapter — never skipped in a NEET paper.
Mendel's experimental design
- Gregor Johann Mendel, 1856–1863, garden pea (Pisum sativum) — 14 years, roughly 28,000 plants.
- Why pea: true-breeding varieties available; sharply contrasting traits; bisexual flowers permitting self-pollination; easy artificial cross-pollination by emasculation and bagging; short life cycle; large seed yield.
- Mendel's innovations: use of mathematics and statistics in biology, very large sample sizes, one or two characters studied at a time, records maintained across generations.
| Character | Dominant | Recessive |
|---|---|---|
| Stem height | Tall | Dwarf |
| Flower colour | Violet | White |
| Flower position | Axial | Terminal |
| Pod shape | Inflated | Constricted |
| Pod colour | Green | Yellow |
| Seed shape | Round | Wrinkled |
| Seed colour | Yellow | Green |
Trap
Pod colour → green is dominant, but seed colour → yellow is dominant. These two are routinely swapped in the options.
Terminology asked at definition level
- Gene, allele, locus, factor; homozygous / heterozygous / hemizygous; genotype vs phenotype; dominant vs recessive.
- Monohybrid vs dihybrid; P, F1, F2 generations; true-breeding line.
- Test cross (F1 × homozygous recessive) vs back cross (F1 × either parent) vs reciprocal cross.
- Punnett square — devised by R. C. Punnett.
Monohybrid cross
- TT × tt → F1 all Tt (tall) → F2 3:1 phenotypic, 1:2:1 genotypic.
- Test cross giving 1:1 → parent was heterozygous; all dominant offspring → parent was homozygous.
- Law of Dominance — characters are controlled by discrete factors occurring in pairs; one member is dominant. Explains F1 uniformity and the F2 3:1 ratio.
- Law of Segregation — alleles do not blend; both are recovered in F2; a gamete carries only one allele of a pair (law of purity of gametes). This law has no exceptions.
Dihybrid cross
- RRYY × rryy → F1 RrYy → F2 9:3:3:1 phenotypic; genotypic 1:2:1:2:4:2:1:2:1 (9 genotypes, 4 phenotypes).
- Law of Independent Assortment — segregation of one pair is independent of another; holds only for genes on different chromosomes, or very far apart on the same one.
- Test cross of a dihybrid → 1:1:1:1.
| Quantity | Formula |
|---|---|
| Types of gametes | 2ⁿ |
| Number of genotypes in F2 | 3ⁿ |
| Number of phenotypes in F2 (complete dominance) | 2ⁿ |
| Squares in the Punnett square | 4ⁿ |
| Phenotypic ratio in F2 | expansion of (3 : 1)ⁿ |
| Fraction of homozygotes in F2 | (1/4)ⁿ |
History and nomenclature
- Published 1866 in the Proceedings of the Natural History Society of Brünn; ignored for 34 years.
- Why it was neglected: unfamiliar mathematical approach; no physical evidence for "factors"; stable discrete factors contradicted the prevailing idea of blending inheritance.
- Rediscovered in 1900, independently, by de Vries, Correns and von Tschermak.
- Term "genetics" coined by Bateson; "gene" by Johannsen. Mendel is the father of genetics.
02
Deviations from Mendelism
The densest scoring block. Examples must be memorised with the exact organism.
Incomplete dominance
- F1 is intermediate; phenotypic ratio equals genotypic ratio = 1:2:1.
- Examples: Antirrhinum majus (snapdragon / dog flower) — red × white gives pink; Mirabilis jalapa (four o'clock plant); root shape in radish; feather colour in Andalusian fowl.
- Cause: incomplete expression of the enzyme encoded by the dominant allele — a gene-dosage effect.
Co-dominance
- Both alleles express fully and independently in the heterozygote.
- Examples: AB blood group in the ABO system; roan coat in shorthorn cattle (red and white patches, not an intermediate); MN blood group; HbA + HbS in sickle-cell trait at the molecular level.
Multiple alleles
- More than two alleles of a gene exist in the population, but any individual carries only two.
- ABO blood group — alleles IA, IB, i. IA and IB are co-dominant; both are dominant over i. The gene product is a sugar-transferring enzyme on the RBC surface.
- 6 genotypes → 4 phenotypes: A (IAIA, IAi), B (IBIB, IBi), AB (IAIB), O (ii).
- Practise parent–child possibility and paternity-exclusion problems.
- Coat colour in rabbits: C > cch > ch > c (full > chinchilla > himalayan > albino).
Pleiotropy
- One gene produces multiple phenotypic effects, usually through a single metabolic pathway.
- Examples: phenylketonuria (mental retardation together with reduced hair and skin pigmentation); sickle cell anaemia; starch grain size and seed shape in pea; vestigial wing in Drosophila.
Polygenic (quantitative) inheritance
- A trait governed by three or more genes acting additively; the phenotype is also influenced by the environment; variation is continuous.
- Examples: human skin colour (genes A, B, C), human height, kernel colour in wheat (Nilsson-Ehle).
- Number of phenotypic classes = 2n + 1 where n is the number of gene pairs; F2 distribution follows the binomial expansion.
- Darkest AABBCC, lightest aabbcc; intermediate combinations are the commonest.
Beyond the NCERT text but asked in NEET
Lethal genes — modified 2:1 ratio (yellow mice, aurea in Antirrhinum). Epistasis and complementary or supplementary genes — modified dihybrid ratios 13:3, 12:3:1, 9:7, 9:3:4, 15:1. Bombay phenotype (hh masks ABO expression). Rh factor and erythroblastosis foetalis.
Chromosomal theory of inheritance
- Sutton and Boveri, 1902 — united Mendelian factors with the behaviour of chromosomes; Sutton framed the synthesis.
- Parallelism between the segregation of alleles and the segregation of homologous chromosomes during meiosis.
03
Linkage and recombination
Morgan's numbers are asked verbatim. Learn the two recombination percentages.
- T. H. Morgan, working on Drosophila melanogaster. Why it is an ideal genetic model: life cycle of about two weeks, very many progeny per mating, sexes easily distinguished, many hereditary variations visible under a low-power microscope, grows on simple synthetic medium, only four pairs of chromosomes.
- Linkage — physical association of genes on the same chromosome, so non-parental combinations appear far less often than expected.
- Morgan's dihybrid test crosses: y (yellow body) and w (white eye) are tightly linked, about 1.3% recombination; w (white eye) and m (miniature wing) are loosely linked, 37.2% recombination.
- Recombination frequency is proportional to the distance between genes: 1% recombination = 1 map unit = 1 centiMorgan.
- Alfred Sturtevant — first genetic maps, constructed from recombination frequencies.
- Coupling (cis) versus repulsion (trans) arrangement of linked alleles.
- Comparisons to master: linkage vs independent assortment, and test-cross output for linked vs unlinked genes (linked → parental types in large excess; unlinked → 1:1:1:1).
04
Sex determination
One table carries almost every question here — plus the honeybee peculiarities.
| System | Examples | Female | Male | Heterogametic sex |
|---|---|---|---|---|
| XX–XO | Grasshopper, roundworm, most insects | XX | XO | Male |
| XX–XY | Human, Drosophila, most mammals | XX | XY | Male |
| ZZ–ZW | Birds, some reptiles and fishes, gypsy moth | ZW | ZZ | Female |
| Haplodiploidy | Honeybee | Diploid, 32 | Haploid, 16 — from an unfertilised egg | — |
- Male heterogamety versus female heterogamety; autosomes versus allosomes (sex chromosomes).
- Henking, 1891 — described the X body, later named the X chromosome.
- In humans the father determines the sex of the child: half the sperm carry 22+X and half carry 22+Y, giving a 50:50 probability. The SRY gene on the Y initiates male development.
- Honeybee specifics: parthenogenesis; drones are haploid and produce sperm by mitosis; a drone has no father but does have a grandfather; queen versus worker is decided by nutrition (royal jelly), not genotype.
- Environmental sex determination by incubation temperature in some reptiles is occasionally asked.
05
Mutation
Classification questions dominate; keep the four structural aberrations separate from numerical change.
- A sudden heritable change in DNA or chromosome structure or number; the source of variation and therefore of evolution.
- Gene or point mutation — change in a single base pair, as in sickle cell anaemia.
- Frameshift mutation — insertion or deletion of one or two bases, which shifts the reading frame.
- Structural chromosomal aberrations: deletion, duplication, inversion, translocation — commonly seen in cancer cells.
- Numerical change: aneuploidy (gain or loss of a single chromosome — Down, Turner, Klinefelter) versus polyploidy (gain of a whole set, frequent in plants). Both arise from failure of segregation or of cytokinesis.
- Mutagens: physical — UV, X-rays, gamma rays; and chemical mutagens.
- Hugo de Vries — saltation, large single-step discontinuous variation, from work on Oenothera lamarckiana.
06
Pedigree analysis
Skill-based, not memory-based. Practise until the pattern is identified in under thirty seconds.
- Symbols: square = male, circle = female, shaded = affected, horizontal line = mating, double horizontal line = consanguineous mating, diagonal stroke = deceased, diamond = sex unspecified, arrow = proband.
- Purpose: genetic counselling and tracing a trait across several generations.
| Pattern | How to recognise it |
|---|---|
| Autosomal dominant | Appears in every generation; an affected child always has at least one affected parent; both sexes affected equally. |
| Autosomal recessive | Skips generations; parents are unaffected carriers; frequency rises with consanguineous marriage; both sexes equally affected. |
| X-linked recessive | Far more males affected; criss-cross inheritance — affected male to carrier daughters to affected grandsons; an affected female implies an affected father. |
| X-linked dominant | An affected male passes it to all his daughters and to none of his sons. |
| Y-linked (holandric) | Only males affected; transmitted from father to all sons. |
| Mitochondrial | Purely maternal transmission; all children of an affected mother are affected. |
07
Mendelian genetic disorders
The highest-yield factual block in the chapter. Learn chromosome number, enzyme defect and inheritance mode together.
| Disorder | Inheritance | Molecular or enzyme defect | Key facts |
|---|---|---|---|
| Haemophilia | X-linked recessive | Absent clotting-factor protein in the cascade (factor VIII or IX) | The "royal disease"; Queen Victoria's pedigree; non-stop bleeding from a simple cut; the homozygous female is extremely rare and usually lethal; criss-cross inheritance. |
| Colour blindness (red–green) | X-linked recessive | Defective red or green cone genes | About 8% of males and 0.4% of females; sons of a carrier mother are at risk; never transmitted father to son. |
| Sickle cell anaemia | Autosomal recessive; co-dominant at the protein level | Glutamate → valine at position 6 of the β-globin chain; codon GAG → GUG; gene on chromosome 11 | HbS. HbSHbS affected, HbAHbS is the carrier or sickle-cell trait; sickling occurs at low oxygen tension; heterozygote advantage against malaria; a qualitative defect. |
| Thalassaemia | Autosomal recessive | Reduced synthesis of globin chains — a quantitative defect | α-thalassaemia: HBA1 and HBA2 on chromosome 16. β-thalassaemia: HBB on chromosome 11. Anaemia results from an unbalanced globin-chain ratio. |
| Phenylketonuria | Autosomal recessive | Missing phenylalanine hydroxylase (phenylalanine → tyrosine) | Phenylalanine accumulates and is converted to phenylpyruvic acid, damaging the brain; mental retardation; excreted in urine; managed by dietary restriction. |
| Cystic fibrosis | Autosomal recessive | CFTR chloride channel | Thick mucus in airways and ducts — supplementary example. |
| Albinism · Alkaptonuria | Autosomal recessive | Tyrosinase · homogentisate oxidase | Supplementary examples of enzyme-deficiency disorders. |
| Myotonic dystrophy · Huntington's | Autosomal dominant | Trinucleotide repeat expansion | Supplementary examples of dominant disorders. |
Most-asked comparison
Sickle cell anaemia versus thalassaemia: a structural, qualitative defect in the globin chain against reduced, quantitative synthesis of an otherwise normal chain.
08
Chromosomal disorders
Karyotype notation and the described features are the whole question here.
| Disorder | Karyotype | Cause | Phenotype |
|---|---|---|---|
| Down syndrome | Trisomy 21 · 47 | Non-disjunction; risk rises with maternal age | First described by Langdon Down, 1866. Short stature, small round head, furrowed protruding tongue, partially open mouth, palm crease, broad flat hands with short fingers, retarded physical, psychomotor and mental development. |
| Klinefelter syndrome | 47, XXY | An additional X in a male | Overall masculine build with some feminine development, including gynaecomastia; sterile. |
| Turner syndrome | 45, X0 — monosomy of X | Loss of one X in a female | Sterile, rudimentary ovaries, absent secondary sexual characters, short stature. |
| Edward syndrome | Trisomy 18 | Non-disjunction | Supplementary example. |
| Patau syndrome | Trisomy 13 | Non-disjunction | Supplementary example. |
- Underlying mechanism in every case: failure of segregation (non-disjunction) during meiosis, producing a gamete with n+1 or n−1 chromosomes.
- Distinguish trisomy from monosomy, and euploidy from aneuploidy. Only Down, Klinefelter and Turner are named in the NCERT text.
★
Priority revision list
If time is short, this is the order to revise in.
- The genetic and chromosomal disorders tables — chromosome numbers, enzyme defects, inheritance modes.
- Pedigree pattern identification rules.
- ABO blood group problems, combining multiple alleles with co-dominance.
- Deviations from Mendelism, with the exact example organisms.
- The sex determination table, including the honeybee peculiarities.
- Dihybrid and test-cross ratios plus the gamete and genotype formulae.
- Morgan's linkage percentages (1.3% and 37.2%) and the definition of one centiMorgan.
- Mendel's seven characters — which allele is dominant in each pair.
- Which laws have exceptions (Dominance, Independent Assortment) and which does not (Segregation).
!
Common NEET traps
Each of these has appeared as a distractor in a past paper.
- The Law of Segregation has no exception; Dominance and Independent Assortment do.
- Incomplete dominance is not co-dominance. Roan cattle is co-dominance; pink snapdragon is incomplete dominance.
- Sickle cell anaemia is autosomal recessive at the level of the organism but shows co-dominance at the level of the protein.
- A point mutation is not a frameshift mutation.
- A honeybee drone has no father but does have a grandfather.
- In colour blindness and haemophilia the mother is the carrier; a father can never pass an X-linked recessive allele to his son.
- Down syndrome is not caused by an inherited "Down gene" — it is non-disjunction.
- Thalassaemia is quantitative; sickle cell anaemia is qualitative.
- The number of polygenic phenotypic classes is 2n + 1, not 2ⁿ.