I. INTRODUCTION — GENETICS, INHERITANCE & VARIATION
A KEY DEFINITIONS
| FEATURE | DETAIL |
|---|---|
| Genetics | Branch of biology dealing with inheritance as well as variation of characters from parents to offspring ⭐⭐ |
| Inheritance | Process by which characters are passed on from parent to progeny; basis of heredity ⭐⭐ |
| Variation | Degree by which progeny differ from their parents ⭐⭐ |
| Early human knowledge | Humans knew from 8000–1000 B.C. that one cause of variation was hidden in sexual reproduction ⭐ |
| Exploitation of variation | Through artificial selection & domestication from wild populations → organisms with desirable characters selected ⭐ |
| Example | Sahiwal cows in Punjab — developed from ancestral wild cows through selective breeding ⭐ |
II. MENDEL'S LAWS OF INHERITANCE — EXPERIMENTAL SETUP
A GREGOR MENDEL — THE EXPERIMENT
| FEATURE | DETAIL |
|---|---|
| Organism used | Garden pea (Pisum sativum) ⭐⭐⭐ |
| Duration | 7 years (1856–1863) ⭐⭐⭐ |
| Type of experiments | Hybridisation (artificial pollination / cross pollination) experiments ⭐⭐ |
| First-time application | Statistical analysis & mathematical logic applied to problems in biology ⭐⭐ |
| Sampling size | Large — gave greater credibility to data ⭐ |
| True-breeding lines selected | 14 true-breeding pea plant varieties (as 7 pairs) ⭐⭐⭐ |
| True-breeding definition | A line that, having undergone continuous self-pollination, shows stable trait inheritance and expression for several generations ⭐⭐ |
NEET 2017, 2020: Mendel conducted hybridisation experiments on garden peas for 7 years (1856–1863) and selected 14 true-breeding pea plant varieties.
NEET 2016, 2020: True breeding = nearly homozygous, produces offspring of its own kind.
B SEVEN PAIRS OF CONTRASTING TRAITS
| CHARACTER | DOMINANT TRAIT | RECESSIVE TRAIT |
|---|---|---|
| Stem height | Tall | Dwarf ⭐⭐ |
| Flower colour | Violet | White ⭐⭐ |
| Flower position | Axial | Terminal ⭐⭐ |
| Pod shape | Inflated (Full) | Constricted ⭐⭐ |
| Pod colour | Green | Yellow ⭐⭐ |
| Seed shape | Round | Wrinkled ⭐⭐ |
| Seed colour | Yellow | Green ⭐⭐ |
NEET 2015, 2017: Seven pairs of contrasting traits — TRAP: Mendel did NOT study Pod Length, Seed Size, or Trichomes (Glandular/Non-glandular)!
RE-NEET 2026 (Assertion–Reason): F₁ of Tall × Dwarf are ALL tall and none dwarf BECAUSE stem height is a contrasting trait with tall dominant and dwarf recessive → both statements correct AND the Reason IS the correct explanation.
III. INHERITANCE OF ONE GENE — MONOHYBRID CROSS
A MONOHYBRID CROSS RESULTS
| FEATURE | DETAIL |
|---|---|
| Definition | Study of inheritance of one character at a time ⭐⭐ |
| Cross | Example: TT (Tall) × tt (Dwarf) ⭐⭐ |
| F₁ generation | All tall — resembled only ONE parent (dominant) ⭐⭐ |
| F₁ genotype | Tt (heterozygous) ⭐⭐ |
| F₂ generation (F₁ selfed) | Both traits expressed — 3 Tall : 1 Dwarf ⭐⭐⭐ |
| F₂ Phenotypic ratio | 3 : 1 ⭐⭐⭐ |
| F₂ Genotypic ratio | 1 TT : 2 Tt : 1 tt = 1 : 2 : 1 ⭐⭐⭐ |
| Blending | No blending at either F₁ or F₂ stage — offspring were either tall or dwarf, none in-between ⭐⭐ |
RE-NEET 2026: In F₁ of a monohybrid cross, only the dominant parental character appears — no dwarf, no blending. Dominance of one allele over the other is the CAUSE, not a coincidence.
B KEY TERMINOLOGY & C. PUNNETT SQUARE
| TERM | DETAIL |
|---|---|
| Factors (Mendel's term) | Stably passed from parent to offspring through gametes; now called Genes ⭐⭐⭐ |
| Genes | Units of inheritance — contain information to express a particular trait ⭐⭐⭐ |
| Alleles | Genes which code for a pair of contrasting traits; slightly different forms of the same gene ⭐⭐⭐ |
| Homozygous | Allelic pair identical — e.g., TT or tt ⭐⭐ |
| Heterozygous | Allelic pair dissimilar — e.g., Tt ⭐⭐ |
| Genotype | Genetic constitution — e.g., TT, Tt, tt ⭐⭐ |
| Phenotype | Observable character — e.g., Tall, Dwarf ⭐⭐ |
| Symbol convention | Use same alphabet — T for tall, t for dwarf (NOT T and d — confusing) ⭐ |
| FEATURE | DETAIL |
|---|---|
| Punnett Square developed by | Reginald C. Punnett — a British geneticist ⭐⭐⭐ |
| Definition | Graphical representation to calculate the probability of all possible genotypes of offspring in a genetic cross ⭐⭐ |
| Method | Possible gametes written on two sides (top row + left column); all combinations in boxes ⭐ |
NEET 2024, 2022, 2015: Alleles = different molecular forms of a gene / slightly different forms of the same gene.
NEET 2018, 2021: Punnett Square — developed by a British geneticist.
D LAW OF DOMINANCE (FIRST LAW)
| FEATURE | DETAIL |
|---|---|
| Rule (i) | Characters are controlled by discrete units called factors ⭐⭐⭐ |
| Rule (ii) | Factors occur in pairs ⭐⭐⭐ |
| Rule (iii) | In a dissimilar pair of factors, one member dominates (dominant) & the other is recessive ⭐⭐⭐ |
| Explains | Expression of only one parental character in F₁ monohybrid cross ⭐⭐ |
| Also explains | Expression of both characters in F₂ & the 3:1 proportion ⭐⭐ |
NEET 2024, 2018, 2016: Law of Dominance — factors occur in pairs; in dissimilar pair, one dominates. Exceptions to Law of Dominance = 1. Incomplete Dominance, 2. Co-dominance ⭐⭐⭐.
RE-NEET 2026: Law of Dominance is the explanation for "all F₁ tall, none dwarf" — asked in Assertion–Reason format. The Reason statement must be accepted as the correct explanation.
E LAW OF SEGREGATION (SECOND LAW) & F. TEST CROSS
| FEATURE | DETAIL |
|---|---|
| Basis | Alleles do NOT show blending — both characters recovered as such in F₂ ⭐⭐⭐ |
| Statement | The factors or alleles of a pair segregate from each other such that a gamete receives only one of the two factors ⭐⭐⭐ |
| Homozygous parent | Produces all similar gametes ⭐ |
| Heterozygous parent | Produces two kinds of gametes — each having one allele in equal proportion ⭐⭐ |
| Also called | Law of Purity of Gametes ⭐⭐ |
Test Cross ⭐⭐⭐
| FEATURE | DETAIL |
|---|---|
| Purpose | To determine the genotype of an F₁ or F₂ individual showing dominant phenotype ⭐⭐⭐ |
| Procedure | Organism with dominant phenotype (genotype unknown) crossed with homozygous recessive parent ⭐⭐⭐ |
| Monohybrid test cross ratio | 1 : 1 (if parent is heterozygous) ⭐⭐⭐ |
| Dihybrid test cross ratio | 1 : 1 : 1 : 1 (if parent is dihybrid heterozygous) ⭐⭐ |
NEET 2019, 2016, 2012, 2018: Law of Segregation — alleles segregate during gamete formation; gamete receives only ONE factor from each pair.
NEET 2012, 2016, 2023, 2024: Test cross = F₁ individual crossed with homozygous recessive parent to find genotype. Heterozygous Violet × Homozygous White → out of 40 offspring: 20 Violet, 20 White (1:1 ratio).
NEET 2024 TRAP: If a Tall/Round plant is selfed and produces Wrinkled seeds but NO Dwarf plants → parent genotype must be TTRr (homozygous dominant for height, heterozygous for seed shape).
IV. EXCEPTIONS TO LAW OF DOMINANCE
A INCOMPLETE DOMINANCE
| FEATURE | DETAIL |
|---|---|
| Definition | F₁ phenotype does NOT resemble either parent — it is in between the two ⭐⭐⭐ |
| Classic example | Flower colour in dog flower (Snapdragon / Antirrhinum sp.) ⭐⭐⭐ |
| Cross | RR (Red) × rr (White) → F₁ = Rr (Pink) ⭐⭐⭐ |
| F₂ result | 1 RR (Red) : 2 Rr (Pink) : 1 rr (White) ⭐⭐ |
| F₂ Phenotypic ratio | Changed from 3:1 to 1 : 2 : 1 ⭐⭐⭐ |
| Key feature | Phenotypic ratio = Genotypic ratio = 1 : 2 : 1 ⭐⭐⭐ |
| Explanation | R is NOT completely dominant over r → can distinguish Rr (Pink) from RR (Red) and rr (White) ⭐⭐ |
Explanation of Dominance Concept ⭐⭐
• Gene function: Contains information → produces enzyme → transforms substrate.
• Modified allele could produce: (i) Normal/less efficient enzyme, (ii) Non-functional enzyme, (iii) No enzyme at all.
• If non-functional/no enzyme: Phenotype depends on unmodified (functioning) allele → this is the dominant allele.
• Recessive trait: Due to non-functional enzyme or no enzyme produced.
RE-NEET 2024, NEET 2024, 2019, 2016, 2015, 2018: Incomplete dominance in Snapdragon/Antirrhinum sp. / dog flower; Phenotypic ratio changes from 3:1 to 1:2:1.
NEET 2026 (Match-the-list): Incomplete dominance = Inheritance of flower colour in Antirrhinum sp. (Snapdragon). Do NOT pair it with ABO blood group or skin colour.
B CO-DOMINANCE
| FEATURE | DETAIL |
|---|---|
| Definition | F₁ phenotype resembles BOTH parents ⭐⭐⭐ |
| F₁ result | Both alleles express their own type simultaneously ⭐⭐⭐ |
| Phenotypic & Genotypic ratio | 1 : 2 : 1 ⭐⭐ |
| Classic example | ABO blood grouping — Iᴬ Iᴮ (AB blood group) → both A and B sugars expressed ⭐⭐⭐ |
| Controlled by | Gene I ⭐⭐ |
| Number of alleles | Three — Iᴬ, Iᴮ, and i ⭐⭐⭐ |
| Iᴬ and Iᴮ | Produce slightly different forms of sugar on RBC surface ⭐⭐ |
| Allele i | Does NOT produce any sugar ⭐⭐ |
| Iᴬ and Iᴮ over i | Completely dominant over i ⭐⭐ |
| Iᴬ and Iᴮ together | Both express own sugar = Co-dominance ⭐⭐⭐ |
| Total Phenotypes & Genotypes | 4 Phenotypes (A, B, AB, O), 6 Genotypes, 3 Alleles ⭐⭐⭐ |
NEET 2026 (Match-the-list): Co-dominance = ABO blood groups (Iᴬ Iᴮ). Trap option pairs co-dominance with Antirrhinum — wrong.
C ABO BLOOD GROUPING
Genetic Basis of Blood Groups ⭐⭐⭐
| ALLELE P1 | ALLELE P2 | GENOTYPE | BLOOD TYPE |
|---|---|---|---|
| Iᴬ | Iᴬ | Iᴬ Iᴬ | A |
| Iᴬ | Iᴮ | Iᴬ Iᴮ | AB |
| Iᴬ | i | Iᴬ i | A |
| Iᴮ | Iᴬ | Iᴬ Iᴮ | AB |
| Iᴮ | Iᴮ | Iᴮ Iᴮ | B |
| Iᴮ | i | Iᴮ i | B |
| i | i | ii | O |
NEET 2020, 2018, 2015, 2017, 2024, RE-NEET 2024: ABO blood group controlled by gene I with three alleles (multiple alleles); AB blood group = co-dominance; Phenotypes = 4, Genotypes = 6, Alleles = 3.
NEET 2024, 2017: If child is O⁺ (ii) → BOTH parents must carry 'i' allele. If Father is B⁺ and Mother is A⁺ and Child is O⁺ → Father = Iᴮ i, Mother = Iᴬ i. Parent Iᴬ Iᴮ × Iᴬ i → produces 4 Genotypes and 3 Phenotypes!
NEET 2026: Mother heterozygous A (Iᴬ i) × Father heterozygous B (Iᴮ i) → children = AB : A : B : O = 1:1:1:1 → probability of O blood group = 25%.
RE-NEET 2026: For a child with blood group 'O' (ii), the IMPOSSIBLE parental combination is Father Iᴬ Iᴮ × Mother Iᴬ i — an Iᴬ Iᴮ parent carries NO 'i' allele. All other combinations (Iᴬi × Iᴮi, Iᴬi × Iᴬi, Iᴮi × Iᴮi) are possible.
An AB parent (Iᴬ Iᴮ) can NEVER produce an O child, and an O parent (ii) can never produce an AB child.
D MULTIPLE ALLELES
| FEATURE | DETAIL |
|---|---|
| Definition | More than two alleles governing the same character ⭐⭐⭐ |
| Best example | ABO blood grouping — three alleles (Iᴬ, Iᴮ, i) ⭐⭐⭐ |
| In an individual | Only two alleles can be present ⭐⭐ |
| Multiple alleles found | Only when population studies are made ⭐⭐⭐ |
V. PLEIOTROPY
A PLEIOTROPY — KEY CONCEPTS
| FEATURE | DETAIL |
|---|---|
| Definition | A single gene product may produce more than one effect (multiple phenotypic expression) ⭐⭐⭐ |
| Such gene called | Pleiotropic gene ⭐⭐ |
| Mechanism | Effect of gene on metabolic pathways contributing towards different phenotypes ⭐⭐ |
| Examples | Starch synthesis in pea seeds ⭐⭐⭐ • Phenylketonuria (PKU) in humans ⭐⭐⭐ |
B STARCH SYNTHESIS IN PEA SEEDS — PLEIOTROPY EXAMPLE
| FEATURE / GENOTYPE | DETAIL |
|---|---|
| Gene | One gene with two alleles: B and b ⭐⭐ |
| BB homozygotes | Efficient starch synthesis → large starch grains → Round seeds ⭐⭐ |
| bb homozygotes | Less efficient → smaller starch grains → Wrinkled seeds ⭐⭐ |
| Bb heterozygotes | Round seeds (B dominant for seed shape) BUT intermediate starch grains (incomplete dominance for grain size) ⭐⭐⭐ |
| Key insight | Same gene shows dominance for one phenotype (seed shape) and incomplete dominance for another phenotype (starch grain size) ⭐⭐⭐ |
NEET 2016, 2023, 2024: Pleiotropy — single gene affecting multiple phenotypic expression (starch synthesis in pea seeds / Phenylketonuria).
NEET 2018 TRAP: Starch grain size in pea seeds = example of Pleiotropy/Incomplete dominance → NOT multiple alleles!
NEET 2026 (Match-the-list): Pleiotropy = Phenylketonuria disease in humans (single gene → mental retardation + reduced hair & skin pigmentation). Starch synthesis in peas is the other NCERT example.
VI. INHERITANCE OF TWO GENES — DIHYBRID CROSS
A DIHYBRID CROSS BASICS
| FEATURE | DETAIL |
|---|---|
| Definition | Study of inheritance of two characters at a time ⭐⭐ |
| Cross | RRYY (Round, Yellow) × rryy (Wrinkled, Green) ⭐⭐ |
| F₁ genotype & phenotype | RrYy (all Round, Yellow) — resembled one parent ⭐⭐ |
| F₂ Phenotypic ratio | 9 : 3 : 3 : 1 ⭐⭐⭐ |
| F₂ Genotypic ratio | 1:2:2:4:1:2:1:2:1 ⭐⭐ |
| F₂ Phenotypes | 9 Round Yellow : 3 Wrinkled Yellow : 3 Round Green : 1 Wrinkled Green ⭐⭐ |
| Derivation of 9:3:3:1 | (3 Round : 1 Wrinkled) × (3 Yellow : 1 Green) ⭐⭐ |
| F₁ gamete types | 4 types — RY, Ry, rY, ry (each 25% i.e., 1/4th) ⭐⭐ |
| Punnett Square size | 16 squares (4×4) ⭐⭐ |
NEET 2022: In a dihybrid cross (Yellow/Constricted × Green/Inflated), F₂ Yellow/Inflated = 9/16 = 56.25%.
NEET 2025: RRYY (Round Yellow) × rryy (Wrinkled Green), genes R and Y assorting independently → F₂ phenotypic ratio = 9 : 3 : 3 : 1. Distractors offered: 9:7, 1:2:1, 3:1.
B LAW OF INDEPENDENT ASSORTMENT (THIRD LAW) & C. IMPORTANT FORMULAS
| FEATURE | DETAIL |
|---|---|
| Statement | "When two pairs of traits are combined in a hybrid, segregation of one pair of characters is independent of the other pair of characters" ⭐⭐⭐ |
| Basis | 50% R and 50% r segregation is independent from 50% Y and 50% y segregation ⭐⭐ |
| Types of gametes / Phenotypes | 2ⁿ (n = number of heterozygous pairs) ⭐⭐⭐ |
| Types of genotypes | 3ⁿ ⭐⭐⭐ |
| N (HETEROZYGOUS PAIRS) | GAMETE TYPES | PHENOTYPES | GENOTYPES |
|---|---|---|---|
| 1 (Monohybrid) | 2 | 2 | 3 |
| 2 (Dihybrid) | 4 | 4 | 9 |
| 3 (Trihybrid) | 8 | 8 | 27 |
NEET 2025: The phrase "genes follow independent assortment" in the stem is the signal for the standard 9:3:3:1 — NOT a modified/epistatic ratio like 9:7.
VII. CHROMOSOMAL THEORY OF INHERITANCE
A WHY MENDEL'S WORK REMAINED UNRECOGNIZED (1865–1900)
| REASON | DETAIL |
|---|---|
| 1. Communication | Not easy in those days — work could not be widely publicised ⭐ |
| 2. Concept not accepted | Genes as stable & discrete units that don't blend → not accepted by contemporaries ⭐⭐ |
| 3. Mathematics in biology | Totally new and unacceptable to biologists of his time ⭐⭐ |
| 4. No physical proof | Could not provide physical evidence for existence of factors or what they were made of ⭐ |
| Published in & Unrecognized till | Published in 1865; Remained unrecognized till 1900 ⭐⭐ |
B REDISCOVERY (1900)
| FEATURE | DETAIL |
|---|---|
| Year | 1900 ⭐⭐⭐ |
| Three scientists | de Vries, Correns & von Tschermak ⭐⭐⭐ |
| Achievement | Independently rediscovered Mendel's results on inheritance of characters ⭐⭐ |
| Additionally | Advancements in microscopy → discovery of chromosomes (colored bodies — visualised by staining) ⭐ |
| By 1902 | Chromosome movement during meiosis worked out ⭐ |
C SUTTON & BOVERI — CHROMOSOMAL THEORY
| FEATURE | DETAIL |
|---|---|
| Scientists | Walter Sutton and Theodore Boveri ⭐⭐⭐ |
| Year | 1902 ⭐⭐ |
| Observation | Behaviour of chromosomes was parallel to the behaviour of genes ⭐⭐⭐ |
| Used | Chromosomal movement to explain Mendel's laws ⭐⭐ |
| Theory | Chromosomal Theory of Inheritance ⭐⭐⭐ |
Parallelism between Chromosomes and Genes ⭐⭐
| CHROMOSOMES | GENES |
|---|---|
| Occur in pairs | Occur in pairs |
| Segregate at gamete formation → only one to each gamete | Segregate at gamete formation → only one to each gamete |
| Independent pairs segregate independently of each other | One pair segregates independently of another pair |
| Two alleles on homologous sites on homologous chromosomes | — |
NEET 2020, 2022: Sutton and Boveri — chromosomal theory of inheritance.
D T.H. MORGAN & DROSOPHILA
| FEATURE | DETAIL |
|---|---|
| Scientist | Thomas Hunt Morgan and his colleagues ⭐⭐⭐ |
| Achievement | Experimental verification of chromosomal theory of inheritance ⭐⭐⭐ |
| Organism used | Drosophila melanogaster (tiny fruit fly) ⭐⭐⭐ |
Why Drosophila? ⭐⭐⭐
| REASON | DETAIL |
|---|---|
| 1 | Grown on simple synthetic medium in laboratory ⭐⭐ |
| 2 | Life cycle complete in about two weeks ⭐⭐⭐ |
| 3 | Single mating → large number of progeny ⭐ |
| 4 | Clear sex differentiation — male & female easily distinguishable ⭐⭐ |
| 5 | Many types of hereditary variations visible with low power microscopes ⭐⭐ |
NEET 2020: Morgan — experimental verification using Drosophila; life cycle ~2 weeks; simple synthetic medium; many hereditary variations.
VIII. LINKAGE AND RECOMBINATION
A LINKAGE AND RECOMBINATION — KEY CONCEPTS
| FEATURE | DETAIL |
|---|---|
| Discovered by | T.H. Morgan (during dihybrid crosses in Drosophila with sex-linked genes) ⭐⭐⭐ |
| Observation | Two genes on same chromosome → F₂ ratio deviated significantly from 9:3:3:1 ⭐⭐ |
| Parental combinations | Much higher than non-parental type ⭐⭐⭐ |
| Term "Linkage" | Coined by Morgan → physical association of two or more genes on the same chromosome ⭐⭐⭐ |
| Term "Recombination" | Generation of non-parental gene combinations ⭐⭐⭐ |
| Tightly linked genes | Show very low recombination (e.g., white & yellow = 1.3% recombination) ⭐⭐ |
| Loosely linked genes | Show higher recombination (e.g., white & miniature wing = 37.2% recombination) ⭐⭐ |
B ALFRED STURTEVANT — GENETIC MAPPING
| FEATURE | DETAIL |
|---|---|
| Scientist | Alfred Sturtevant (student of Morgan) ⭐⭐⭐ |
| Contribution | Used frequency of recombination between gene pairs as a measure of distance between genes ⭐⭐⭐ |
| Mapped | Positions of genes on chromosomes → genetic maps ⭐⭐ |
| 1 Map Unit (centimorgan) | = 1% recombination frequency ⭐⭐⭐ |
| 50% recombination | Means genes show independent assortment (located on different chromosomes or very far apart on same chromosome) ⭐⭐⭐ |
| Application | Genetic maps used as starting point in genome sequencing (e.g., Human Genome Sequencing Project) ⭐ |
NEET 2015, 2016, 2018, 2019, 2023: Linkage coined by Morgan; if parental types > recombinant types → genes are linked. Alfred Sturtevant mapped gene positions using recombination frequency; 1 Map Unit = 1% recombination. 50% recombination = independent assortment. Translocation = movement of a gene from one linkage group to another!
IX. POLYGENIC INHERITANCE
A POLYGENIC INHERITANCE — KEY CONCEPTS
| FEATURE | DETAIL |
|---|---|
| Definition | Traits controlled by three or more genes ⭐⭐⭐ |
| Nature of traits | Not distinct — spread across a gradient (continuous variation) ⭐⭐ |
| Effect of alleles | Additive — phenotype reflects contribution of each allele ⭐⭐⭐ |
| Also considers | Influence of environment ⭐⭐ |
| Examples | Human skin colour, Human height ⭐⭐⭐ |
Skin Colour Model ⭐⭐
| GENOTYPE | SKIN COLOUR |
|---|---|
| AABBCC (all dominant) | Darkest ⭐⭐ |
| aabbcc (all recessive) | Lightest ⭐⭐ |
| 3 dominant + 3 recessive alleles | Intermediate ⭐ |
NEET 2016, 2018, 2024, RE-NEET 2024: Polygenic inheritance = trait controlled by 3 or more genes; additive effect; examples = Human skin colour & Human height.
NEET 2025: Pattern of inheritance of polygenic traits = Non-Mendelian inheritance pattern (multiple genes, additive effect, continuous variation + environmental influence). NOT Mendelian, NOT autosomal dominant, NOT X-linked recessive.
NEET 2026 (Match-the-list): Polygenic inheritance = Human skin colour.
X. SEX DETERMINATION
A HISTORICAL BACKGROUND
| FEATURE | DETAIL |
|---|---|
| Henking (1891) | Traced a specific nuclear structure during spermatogenesis in insects ⭐⭐ |
| Observation | 50% sperm received this structure, 50% did not ⭐ |
| Named | X body — but could not explain significance ⭐⭐ |
| Later conclusion | X body = X-chromosome ⭐⭐ |
B TYPES OF SEX DETERMINATION
1. XO Type ⭐⭐⭐
| FEATURE | DETAIL |
|---|---|
| Found in | Most insects — especially Grasshopper ⭐⭐⭐ |
| Female karyotype | XX (possess pair of X-chromosomes + autosomes) ⭐⭐ |
| Male karyotype | XO (only ONE X-chromosome + autosomes) ⭐⭐ |
| Gametes | All eggs have X; sperms → 50% have X, 50% have NO sex chromosome ⭐⭐ |
| Sex of offspring | Egg + X-sperm = Female; Egg + no-X-sperm = Male ⭐⭐ |
| Type of heterogamety | Male heterogamety (male determines sex) ⭐⭐⭐ |
NEET 2022, 2018, 2019, 2023: Grasshopper = XO type of sex determination, male heterogametic (male has only one X chromosome).
NEET 2026: In a grasshopper population, members with 23 chromosomes = MALES (XO) and members with 24 chromosomes = FEMALES (XX) — respectively. Sequence of the options is the trap.
2. XY Type ⭐⭐⭐
| FEATURE | DETAIL |
|---|---|
| Found in | Insects (Drosophila) and Mammals including Humans ⭐⭐⭐ |
| Female karyotype | XX ⭐⭐ |
| Male karyotype | XY (Y-chromosome is distinctly smaller than X) ⭐⭐⭐ |
| Type of heterogamety | Male heterogamety ⭐⭐⭐ |
3. ZW Type ⭐⭐⭐
| FEATURE | DETAIL |
|---|---|
| Found in | Birds (Fowl) ⭐⭐⭐ |
| Male karyotype | ZZ (pair of similar sex chromosomes) ⭐⭐ |
| Female karyotype | ZW (pair of dissimilar sex chromosomes) ⭐⭐ |
| Type of heterogamety | Female heterogamety (female determines sex of progeny) ⭐⭐⭐ |
NEET 2019: Birds = ZW type, female heterogamety → female determines sex of baby.
C SEX DETERMINATION IN HUMANS & D. HONEY BEE
| FEATURE | DETAIL |
|---|---|
| Type | XY type ⭐⭐ |
| Total chromosomes | 46 (23 pairs) — 22 pairs autosomes + 1 pair sex chromosomes ⭐⭐ |
| Female vs Male | Female = XX (1 type ovum); Male = XY (2 types sperm: 50% X, 50% Y) ⭐⭐⭐ |
| Who determines sex? | Genetic makeup of the SPERM determines sex of child ⭐⭐⭐ |
| Each pregnancy | Always 50% probability of either male or female child ⭐⭐ |
Sex Determination in Honey Bee (Haplodiploid System) ⭐⭐⭐
| FEATURE | DETAIL |
|---|---|
| System | Haplodiploid sex-determination system ⭐⭐⭐ |
| Basis | Based on number of chromosome sets an individual receives ⭐⭐ |
| Fertilised egg (Sperm + Egg) | Develops as Female (Queen or Worker) = Diploid (32 chromosomes) ⭐⭐⭐ |
| Unfertilised egg | Develops as Male (Drone) = Haploid (16 chromosomes) ⭐⭐⭐ |
| Males develop by | Parthenogenesis ⭐⭐⭐ |
| Males produce sperm by | Mitosis (NOT meiosis — they are already haploid) ⭐⭐⭐ |
Special Features of Haplodiploid System ⭐⭐
- Males do not have father ⭐⭐
- Males cannot have sons ⭐⭐
- Males have a grandfather ⭐⭐
- Males can have grandsons ⭐⭐
NEET 2026 (Multi-statement): TRUE → fertilised egg develops as female (queen/worker); unfertilised egg develops as male by parthenogenesis; male has HALF the chromosome number of female; honeybees have haplodiploid system. FALSE → "Males produce sperm by meiosis" — drones produce sperm by MITOSIS.
NEET 2026: "In which animal do haploid cells divide mitotically to produce gametes?" → Male honeybees (drones). Not grasshopper, earthworm or frog males.
XI. MUTATION
A MUTATION — KEY CONCEPTS
| FEATURE | DETAIL |
|---|---|
| Definition | Alteration of DNA sequences → changes in genotype and phenotype of organism ⭐⭐⭐ |
| Leads to | Variation in DNA (along with recombination) ⭐⭐ |
| Chromosomal mutations | Loss (deletions) or Gain (insertion/duplication) of a segment of DNA → alteration in chromosomes ⭐⭐ |
| Chromosomal aberrations common in | Cancer cells ⭐⭐⭐ |
B TYPES OF GENE MUTATIONS
| TYPE | DETAIL |
|---|---|
| Point mutation | Change in a single base pair of DNA ⭐⭐⭐ |
| Point mutation example | Sickle cell anaemia ⭐⭐⭐ |
| Frame-shift mutation | Deletions or insertions of base pairs of DNA ⭐⭐⭐ |
| Mutagens | Chemical and physical factors that induce mutations ⭐⭐ |
| Mutagen example | UV radiations ⭐⭐ |
NEET 2026: The sixth mutant codon of the β-globin gene causing polymerisation of Hb under low O₂ and sickling of RBC = GUG (normal GAG) — classic point mutation / single base substitution.
XII. GENETIC DISORDERS
A PEDIGREE ANALYSIS
| FEATURE | DETAIL |
|---|---|
| Definition | Study of family history about inheritance of a particular trait across several generations ⭐⭐⭐ |
| Why needed | Control crosses not possible in humans → pedigree provides an alternative ⭐⭐ |
| Use | Strong tool to trace inheritance of a specific trait, abnormality or disease ⭐⭐⭐ |
Symbols Used in Pedigree Analysis ⭐⭐⭐
| SYMBOL | MEANING |
|---|---|
| □ | Male ⭐ |
| ○ | Female ⭐ |
| ◇ | Sex unspecified ⭐ |
| ■ / ● (Filled) | Affected individuals ⭐⭐ |
| □—○ (Single horizontal line) | Mating ⭐ |
| □══○ (Double horizontal line) | Consanguineous mating (mating between relatives) ⭐⭐⭐ |
NEET 2023, 2022: Pedigree symbols — know all standard symbols. Double horizontal line between parents = Consanguineous mating.
NEET 2025: Pedigree-based numerical → probability of an F₃ child being disease-free AND a carrier (heterozygous) = 1/4. Read the question carefully: "no disease + carrier" ≠ "affected"; carrier = one mutant allele only.
Pedigree questions now come as probability calculations, not just symbol identification — first deduce the mode of inheritance (X-linked recessive here), then compute.
B CLASSIFICATION OF GENETIC DISORDERS
| CATEGORY | CAUSED BY | EXAMPLES |
|---|---|---|
| Mendelian Disorders | Alteration/mutation in single gene ⭐⭐⭐ | Colour blindness, Haemophilia, Sickle-cell anaemia, PKU, Thalassemia, Cystic fibrosis ⭐⭐ |
| Chromosomal Disorders | Absence/excess/abnormal arrangement of one or more chromosomes ⭐⭐⭐ | Down's syndrome, Klinefelter's syndrome, Turner's syndrome ⭐⭐ |
C MENDELIAN DISORDERS — DETAILED
1. Colour Blindness ⭐⭐⭐
| FEATURE | DETAIL |
|---|---|
| Type | Sex-linked (X-linked) recessive disorder ⭐⭐⭐ |
| Defect | In either red or green cone of eye ⭐⭐ |
| Result | Failure to discriminate between red and green colour ⭐⭐ |
| In males vs females | About 8% males affected vs 0.4% females ⭐⭐⭐ |
| Why more in males | Males have only ONE X chromosome; females have two ⭐⭐ |
| Son of carrier woman | 50% chance of being colour blind ⭐⭐ |
| Daughter colour blind | Only if mother is carrier AND father is colour blind ⭐⭐⭐ |
NEET 2022, 2016, 2014, 2012: Colour blindness = sex-linked recessive; defect in red/green cone. 8% males, 0.4% females. Colorblind Man × Normal Homo Woman → 0% sons colorblind. Normal Man × Carrier Woman → 50% sons colorblind. Colorblind Woman × Man (whose mother was Colorblind) → 100% progeny colorblind!
2. Haemophilia ⭐⭐⭐
| FEATURE | DETAIL |
|---|---|
| Type | Sex-linked (X-linked) recessive disease ⭐⭐⭐ |
| Transmission | From unaffected carrier female to some male progeny ⭐⭐⭐ |
| Defect | Single protein in the cascade of proteins involved in blood clotting is affected ⭐⭐⭐ |
| Symptom | Simple cut → non-stop bleeding ⭐⭐ |
| Female haemophilic | Extremely rare — mother must be at least carrier AND father must be haemophilic (unviable in later life) ⭐⭐⭐ |
| Famous carrier | Queen Victoria — pedigree shows many haemophilic descendants ⭐⭐⭐ |
NEET 2022, 2020, 2016, 2013: Haemophilia = X-linked recessive; carrier female to male progeny; blood clotting protein affected.
3. Sickle-cell Anaemia ⭐⭐⭐
| FEATURE | DETAIL |
|---|---|
| Type | Autosome-linked recessive trait ⭐⭐⭐ |
| Chromosome | Chromosome 11 (β-globin gene) ⭐⭐ |
| Controlled by | Single pair of alleles: Hbᴬ and Hbˢ (Hbᴬ Hbᴬ normal; Hbᴬ Hbˢ carrier; Hbˢ Hbˢ diseased) ⭐⭐⭐ |
| Molecular defect | Substitution of Glutamic acid (Glu) by Valine (Val) at 6th position of β-globin chain ⭐⭐⭐ |
| Codon change | GAG → GUG (single base substitution at 6th codon = point mutation) ⭐⭐⭐ |
| RBC shape change | Biconcave disc → Elongated sickle-shaped (under low oxygen tension due to polymerisation) ⭐⭐⭐ |
| Nature of defect | Qualitative problem — incorrectly functioning globin ⭐⭐⭐ |
NEET 2022, 2021, 2020, 2016: Sickle-cell anaemia = autosomal recessive; Glu → Val; GAG → GUG; point mutation. Qualitative defect of Hb (vs Thalassemia = Quantitative!).
NEET 2026: Disorder caused by substitution of Glutamic acid (Glu) → Valine (Val) at the 6th position of the β-globin chain = Sickle-cell anaemia. Distractors: Thalassemia, PKU, Haemophilia.
NEET 2026: Mutant 6th codon = GUG (from GAG) → Hb polymerises under low oxygen tension → biconcave disc changes to sickle shape.
4. Phenylketonuria (PKU) ⭐⭐
| FEATURE | DETAIL |
|---|---|
| Type | Autosomal recessive trait; inborn error of metabolism ⭐⭐⭐ |
| Enzyme lacking | Phenylalanine hydroxylase (converts phenylalanine → tyrosine) ⭐⭐⭐ |
| Consequence | Phenylalanine accumulates → converted to phenylpyruvic acid and derivatives ⭐⭐ |
| Symptoms | Mental retardation (accumulation in brain), hair loss & reduction in skin pigmentation ⭐⭐⭐ |
| Also example of | Pleiotropy (single gene → multiple phenotypic effects) ⭐⭐ |
NEET 2016: PKU = autosomal recessive; inborn error of metabolism; pleiotropic.
5. Thalassemia ⭐⭐⭐
| FEATURE | DETAIL |
|---|---|
| Type | Autosome-linked recessive blood disease ⭐⭐⭐ |
| Defect | Mutation or deletion → reduced rate of synthesis of one of the globin chains (α or β) ⭐⭐⭐ |
| Nature of defect | Quantitative problem — synthesising too few globin molecules ⭐⭐⭐ |
| TYPE | CHAIN AFFECTED | CHROMOSOME | GENE(S) |
|---|---|---|---|
| α-Thalassemia | α-globin chain ⭐⭐ | Chromosome 16 ⭐⭐⭐ | HBA1 & HBA2 (two closely linked genes) ⭐⭐⭐ |
| β-Thalassemia | β-globin chain ⭐⭐ | Chromosome 11 ⭐⭐⭐ | HBB (single gene) ⭐⭐⭐ |
NEET 2024, 2022, 2020, 2017, 2013: Thalassemia = autosomal recessive; Quantitative defect (too few globin molecules) vs Sickle-cell (Qualitative). α-Thalassemia = Chr 16 (HBA1, HBA2); β-Thalassemia = Chr 11 (HBB).
6. Cystic Fibrosis & 7. Myotonic Dystrophy ⭐⭐
| DISORDER | DETAIL |
|---|---|
| Cystic Fibrosis | Mendelian disorder listed in NCERT among common Mendelian disorders ⭐ |
| Myotonic Dystrophy | Autosomal dominant trait ⭐⭐⭐ (mentioned in NCERT pedigree figure as representative example of autosomal dominant trait) |
NEET 2022: Myotonic Dystrophy = Autosomal dominant trait.
D CHROMOSOMAL DISORDERS — DETAILED
| CONCEPT | DETAIL |
|---|---|
| Aneuploidy | Gain or loss of chromosome(s) due to failure of segregation of chromatids during cell division ⭐⭐⭐ |
| Polyploidy | Increase in whole set of chromosomes due to failure of cytokinesis after telophase; common in plants ⭐⭐⭐ |
| Trisomy vs Monosomy | Trisomy = additional copy (2n+1); Monosomy = lacking one chromosome (2n−1) ⭐⭐ |
Three Chromosomal Disorders ⭐⭐⭐
| DISORDER | KARYOTYPE | CAUSE | KEY FEATURES |
|---|---|---|---|
| 1. Down's Syndrome | 47 (Trisomy 21) ⭐⭐⭐ | Additional copy of chromosome 21 (Langdon Down, 1866) ⭐⭐ | Short stature, small round head, furrowed tongue, palm crease, mental retardation ⭐⭐⭐ |
| 2. Klinefelter's Syndrome | 47, XXY ⭐⭐⭐ | Additional copy of X-chromosome in male ⭐⭐⭐ | Overall masculine development + Gynaecomastia (breast development), Sterile ⭐⭐⭐ |
| 3. Turner's Syndrome | 45, XO (monosomy) ⭐⭐⭐ | Absence of one X chromosome in female ⭐⭐⭐ | Sterile female, rudimentary ovaries, lack secondary sexual characters ⭐⭐⭐ |
RE-NEET 2024, NEET 2024, 2023, 2019, 2016, 2013: Aneuploidy = failure of segregation. Polyploidy = failure of cytokinesis (plants). Down's = Trisomy 21. Klinefelter's = 47,XXY (Gynaecomastia, sterile). Turner's = 45,XO (sterile female, rudimentary ovaries).
RE-NEET 2026 (Statement-based): BOTH statements are INCORRECT when swapped → Down's syndrome = presence of an ADDITIONAL copy of chromosome 21 (Trisomy 21, 47); Turner's syndrome = ABSENCE of one X chromosome (45, XO). NTA deliberately interchanged the two causes.
XIII. RAPID REVISION — 10 MASTER COMPARISON TABLES
TABLE 1: Mendel's Three Laws ⭐⭐⭐
| LAW | STATEMENT | CROSS | KEY RATIO |
|---|---|---|---|
| Law of Dominance | Factors occur in pairs; in dissimilar pair, one dominates | Monohybrid | Explains 3:1 in F₂ |
| Law of Segregation | Alleles of a pair segregate; gamete receives only one factor | Monohybrid | 1:2:1 (genotypic) |
| Law of Independent Assortment | Segregation of one pair is independent of the other pair | Dihybrid | 9:3:3:1 |
TABLE 2: Dominance vs Incomplete Dominance vs Co-dominance ⭐⭐⭐
| FEATURE | COMPLETE DOMINANCE | INCOMPLETE DOMINANCE | CO-DOMINANCE |
|---|---|---|---|
| F₁ resembles | One parent (dominant) | Neither parent (intermediate) | Both parents |
| F₂ Phenotypic ratio | 3 : 1 | 1 : 2 : 1 | 1 : 2 : 1 |
| F₂ Genotypic ratio | 1 : 2 : 1 | 1 : 2 : 1 | 1 : 2 : 1 |
| Phenotypic = Genotypic? | NO | YES | YES |
| Example | Tall/Dwarf pea | Snapdragon flower colour | ABO blood group (Iᴬ Iᴮ) |
TABLE 3: Monohybrid vs Dihybrid Cross ⭐⭐⭐
| FEATURE | MONOHYBRID | DIHYBRID |
|---|---|---|
| Characters studied | 1 | 2 |
| F₂ Phenotypic ratio | 3 : 1 | 9 : 3 : 3 : 1 |
| F₂ Genotypic ratio | 1 : 2 : 1 | 1:2:2:4:1:2:1:2:1 |
| F₁ gamete types | 2 | 4 |
| Punnett Square size | 4 (2×2) | 16 (4×4) |
| Test cross ratio | 1 : 1 | 1 : 1 : 1 : 1 |
TABLE 4: Sex Determination Types ⭐⭐⭐
| TYPE | ORGANISM | MALE | FEMALE | WHO DETERMINES? |
|---|---|---|---|---|
| XO | Grasshopper (insects) | XO ⭐⭐ | XX | Male (heterogametic) |
| XY | Humans, Drosophila | XY ⭐⭐ | XX | Male (heterogametic) |
| ZW | Birds | ZZ ⭐⭐ | ZW | Female (heterogametic) |
| Haplodiploid | Honey bee | Haploid (16) ⭐⭐ | Diploid (32) | No. of chromosome sets |
TABLE 5: Mendelian Disorders — Master Table ⭐⭐⭐
| DISORDER | INHERITANCE | CHR/GENE | KEY FEATURE |
|---|---|---|---|
| Colour Blindness | X-linked recessive ⭐⭐ | X chromosome | 8% males, 0.4% females |
| Haemophilia | X-linked recessive ⭐⭐ | X chromosome | Blood clotting; Queen Victoria |
| Sickle-cell Anaemia | Autosomal recessive ⭐⭐ | Chr 11 (β-globin) | Glu→Val; GAG→GUG; Qualitative |
| PKU | Autosomal recessive ⭐ | Autosomal | Phenylalanine hydroxylase lacking |
| Thalassemia | Autosomal recessive ⭐⭐ | α: Chr 16; β: Chr 11 | Quantitative defect |
| Cystic Fibrosis | Mendelian ⭐ | — | Listed in NCERT |
| Myotonic Dystrophy | Autosomal dominant ⭐⭐ | — | Only autosomal dominant in list |
TABLE 6: Chromosomal Disorders — Master Table ⭐⭐⭐
| DISORDER | KARYOTYPE | CAUSE | KEY FEATURES |
|---|---|---|---|
| Down's Syndrome | 47 (Trisomy 21) ⭐⭐⭐ | Extra copy of Chr 21 | Short, round head, furrowed tongue, palm crease, mental retardation |
| Klinefelter's Syndrome | 47, XXY ⭐⭐⭐ | Extra X in male | Masculine + Gynaecomastia, Sterile |
| Turner's Syndrome | 45, XO ⭐⭐⭐ | Missing X in female | Sterile, rudimentary ovaries, no secondary sexual characters |
TABLE 7: Sickle-cell Anaemia vs Thalassemia ⭐⭐⭐
| FEATURE | SICKLE-CELL ANAEMIA | THALASSEMIA |
|---|---|---|
| Inheritance | Autosomal recessive | Autosomal recessive |
| Type of defect | Qualitative (incorrectly functioning globin) ⭐⭐⭐ | Quantitative (too few globin molecules) ⭐⭐⭐ |
| Molecular basis | Point mutation (Glu→Val) | Mutation or deletion |
| Chain affected | β-globin only | α-globin OR β-globin |
| Chromosome | 11 | α: 16; β: 11 |
| Gene(s) | β-globin gene | α: HBA1, HBA2; β: HBB |
TABLE 8: Mutation Types ⭐⭐⭐
| TYPE | CAUSE | EXAMPLE |
|---|---|---|
| Point mutation | Change in single base pair | Sickle-cell anaemia (GAG→GUG) |
| Frame-shift mutation | Deletion or insertion of base pairs | — |
| Aneuploidy | Failure of segregation of chromatids | Down's (Trisomy 21), Turner's (Monosomy X) |
| Polyploidy | Failure of cytokinesis | Common in plants |
TABLE 9: Key Scientists & Contributions ⭐⭐⭐
| SCIENTIST | CONTRIBUTION | YEAR |
|---|---|---|
| Gregor Mendel | Hybridisation experiments on peas; Laws of Inheritance | 1856–1863 |
| de Vries, Correns, von Tschermak | Independently rediscovered Mendel's results | 1900 |
| Henking | Discovered X-body in insect spermatogenesis | 1891 |
| Walter Sutton & Theodore Boveri | Chromosomal Theory of Inheritance | 1902 |
| T.H. Morgan | Experimental verification; Linkage; Drosophila work | — |
| Alfred Sturtevant | Genetic mapping using recombination frequency | — |
| Reginald C. Punnett | Developed Punnett Square | — |
| Langdon Down | First described Down's syndrome | 1866 |
TABLE 10: ABO Blood Group — Complete ⭐⭐⭐
| GENOTYPE | BLOOD TYPE | ALLELES PRESENT | SUGAR ON RBC |
|---|---|---|---|
| Iᴬ Iᴬ | A | Iᴬ, Iᴬ | A-type sugar |
| Iᴬ i | A | Iᴬ, i | A-type sugar |
| Iᴮ Iᴮ | B | Iᴮ, Iᴮ | B-type sugar |
| Iᴮ i | B | Iᴮ, i | B-type sugar |
| Iᴬ Iᴮ | AB | Iᴬ, Iᴮ | Both A & B sugars (Co-dominance) |
| ii | O | i, i | No sugar |
XIV. COMMON EXAM TRAPS — QUICK REFERENCE
CONSOLIDATED PYQ Q&A TABLE ⭐⭐⭐
| TRAP / QUESTION | CORRECT ANSWER |
|---|---|
| Mendel's experiments lasted how many years? | 7 years (1856–1863) ⭐ |
| How many true-breeding varieties selected? | 14 (7 pairs) ⭐ |
| Did Mendel study Pod Length / Seed Size / Trichomes? | NO — these are TRAPS ⭐⭐⭐ |
| What is a true-breeding line? | Undergone continuous self-pollination → stable trait inheritance ⭐ |
| Mendel published when? | 1865 ⭐ |
| Unrecognized till? | 1900 ⭐ |
| Rediscovered by? | de Vries, Correns, von Tschermak (independently, in 1900) ⭐ |
| Reasons for non-recognition? | Poor communication; discrete factors not accepted; mathematics in biology new; no physical proof ⭐ |
| Genes = ? | Units of inheritance; contain information for trait expression ⭐ |
| Alleles = ? | Slightly different forms of the same gene ⭐⭐ |
| Punnett Square — developed by? | Reginald C. Punnett (British geneticist) ⭐ |
| Law of Dominance — what are the three rules? | (i) Discrete factors, (ii) Occur in pairs, (iii) In dissimilar pair one dominates ⭐⭐ |
| Exceptions to Law of Dominance? | Incomplete Dominance & Co-dominance ⭐⭐ |
| Incomplete dominance — example? | Snapdragon (Antirrhinum sp. / dog flower) — Red × White = Pink ⭐⭐ |
| Incomplete dominance — F₂ ratio? | Phenotypic = Genotypic = 1:2:1 ⭐⭐⭐ |
| Co-dominance — F₁ resembles? | Both parents ⭐⭐ |
| Co-dominance — example? | ABO blood group (Iᴬ Iᴮ = AB) ⭐⭐ |
| ABO — how many alleles / phenotypes / genotypes? | 3 alleles, 4 phenotypes, 6 genotypes ⭐⭐⭐ |
| Multiple alleles found in? | Population studies (individual has only 2) ⭐⭐ |
| Pleiotropy = ? | Single gene → multiple phenotypic effects ⭐⭐ |
| Pleiotropy examples? | Starch synthesis in peas, PKU ⭐⭐ |
| Is starch grain size in peas = multiple alleles? | NO — it is Pleiotropy/Incomplete dominance ⭐⭐⭐ |
| Test cross = ? | Dominant phenotype × Homozygous recessive parent ⭐⭐ |
| Test cross monohybrid ratio? | 1 : 1 ⭐⭐ |
| Test cross dihybrid ratio? | 1 : 1 : 1 : 1 ⭐⭐ |
| Dihybrid F₂ ratio? | 9 : 3 : 3 : 1 ⭐⭐ |
| Formula: gamete/phenotype types? | 2ⁿ (n = no. of heterozygous pairs) ⭐⭐ |
| Formula: genotype types? | 3ⁿ ⭐⭐ |
| Law of Independent Assortment? | Segregation of one pair is independent of another pair ⭐⭐ |
| Chromosomal Theory — proposed by? | Walter Sutton & Theodore Boveri ⭐⭐ |
| Experimentally verified by? | T.H. Morgan (using Drosophila) ⭐⭐ |
| Why Drosophila? | Simple medium; 2-week life cycle; large progeny; easy sex differentiation; many hereditary variations ⭐⭐ |
| Linkage — coined by? | T.H. Morgan ⭐⭐ |
| Linkage = ? | Physical association of genes on same chromosome ⭐⭐ |
| Recombination = ? | Generation of non-parental gene combinations ⭐⭐ |
| If parental types > recombinant types? | Genes are linked ⭐⭐⭐ |
| Tightly linked = ? | Very low recombination ⭐ |
| Loosely linked = ? | Higher recombination ⭐ |
| Genetic mapping — by? | Alfred Sturtevant ⭐⭐ |
| 1 Map Unit (centimorgan) = ? | 1% recombination frequency ⭐⭐⭐ |
| 50% recombination = ? | Independent assortment (genes on different chromosomes or very far apart) ⭐⭐⭐ |
| Translocation = ? | Movement of gene from one linkage group to another ⭐⭐ |
| Polygenic inheritance = ? | Trait controlled by 3 or more genes; additive effect ⭐⭐ |
| Polygenic examples? | Human skin colour, Human height ⭐⭐ |
| AABBCC = ? | Darkest skin colour ⭐ |
| aabbcc = ? | Lightest skin colour ⭐ |
| Henking (1891) discovered? | X-body in insect spermatogenesis ⭐ |
| Grasshopper — sex determination type? | XO type — male heterogamety ⭐⭐⭐ |
| Humans, Drosophila — type? | XY type — male heterogamety ⭐⭐ |
| Birds — type? | ZW type — female heterogamety ⭐⭐⭐ |
| In ZW — who determines sex? | Female (heterogametic) ⭐⭐ |
| Honey bee sex determination? | Haplodiploid system ⭐⭐ |
| Honey bee — females? | Diploid (32) — Queen, Workers ⭐⭐ |
| Honey bee — males (Drones)? | Haploid (16) — from unfertilised eggs (parthenogenesis) ⭐⭐ |
| Drones produce sperm by? | Mitosis (NOT meiosis) ⭐⭐⭐ |
| Drones have father? | NO ⭐⭐ |
| Drones can have sons? | NO ⭐⭐ |
| Who determines sex of child in humans? | Sperm (male's genetic makeup) ⭐⭐ |
| Each pregnancy — probability? | 50% male or female ⭐ |
| Point mutation example? | Sickle-cell anaemia ⭐⭐ |
| Frame-shift mutation = ? | Deletions/insertions of base pairs ⭐⭐ |
| Chromosomal aberrations common in? | Cancer cells ⭐⭐ |
| Mutagen example? | UV radiations ⭐ |
| Aneuploidy = ? | Failure of segregation of chromatids → gain/loss of chromosomes ⭐⭐⭐ |
| Polyploidy = ? | Failure of cytokinesis → whole set increase; common in plants ⭐⭐ |
| Consanguineous mating symbol? | Double horizontal line in pedigree ⭐⭐⭐ |
| Colour blindness —% males? | 8% ⭐⭐ |
| Colour blindness —% females? | 0.4% ⭐⭐ |
| Colorblind Man × Normal Homo Woman → sons? | 0% colorblind ⭐⭐⭐ |
| Normal Man × Carrier Woman → sons? | 50% colorblind ⭐⭐⭐ |
| Colorblind Woman × CB-carrier Man → progeny? | 100% colorblind ⭐⭐ |
| Haemophilia — carrier? | Queen Victoria ⭐⭐ |
| Female haemophilic possible? | Extremely rare (mother carrier + father haemophilic) ⭐⭐ |
| Sickle-cell — amino acid change? | Glutamic acid (Glu) → Valine (Val) at 6th position ⭐⭐⭐ |
| Sickle-cell — codon change? | GAG → GUG ⭐⭐⭐ |
| Sickle-cell — qualitative or quantitative? | Qualitative ⭐⭐⭐ |
| Thalassemia — qualitative or quantitative? | Quantitative ⭐⭐⭐ |
| α-Thalassemia — chromosome & genes? | Chr 16; HBA1 & HBA2 ⭐⭐⭐ |
| β-Thalassemia — chromosome & gene? | Chr 11; HBB ⭐⭐⭐ |
| PKU — enzyme lacking? | Phenylalanine hydroxylase ⭐⭐ |
| PKU — accumulates what? | Phenylalanine → phenylpyruvic acid ⭐⭐ |
| Down's syndrome — cause? | Trisomy of chromosome 21 ⭐⭐⭐ |
| Down's syndrome — first described by? | Langdon Down (1866) ⭐ |
| Klinefelter's — karyotype? | 47, XXY ⭐⭐⭐ |
| Klinefelter's — key feature? | Masculine + Gynaecomastia; Sterile ⭐⭐⭐ |
| Turner's — karyotype? | 45, XO ⭐⭐⭐ |
| Turner's — key features? | Sterile female; rudimentary ovaries; lack secondary sexual characters ⭐⭐⭐ |
| Myotonic Dystrophy — inheritance? | Autosomal dominant ⭐⭐ |
| If child is O blood group (ii)? | Both parents must carry 'i' allele ⭐⭐ |
| Tall/Round selfed → Wrinkled but NO Dwarf? | Parent genotype = TTRr ⭐⭐⭐ |
| Iᴬ Iᴮ × Iᴬ i → how many phenotypes? | 3 phenotypes, 4 genotypes ⭐⭐ |
| RE-NEET 2026 A–R: F₁ Tall × Dwarf all tall because tall is dominant? | Both statements correct AND Reason IS the correct explanation ⭐⭐⭐ |
| NEET 2026 Match: Incomplete dominance? | Antirrhinum sp. flower colour — NOT ABO, NOT skin colour ⭐⭐⭐ |
| NEET 2026 Match: Co-dominance? | ABO blood groups (Iᴬ Iᴮ) — NOT Antirrhinum ⭐⭐⭐ |
| Iᴬi × Iᴮi → probability of O child? | 25% (1:1:1:1 = AB:A:B:O) ⭐⭐⭐ |
| Impossible parents of an O child? | Father Iᴬ Iᴮ × Mother Iᴬ i (AB parent has no i allele) ⭐⭐⭐ |
| AB parent produce O child? O parent produce AB child? | NEVER and NEVER ⭐⭐⭐ |
| NEET 2026 Match: Pleiotropy? | Phenylketonuria (also starch synthesis in peas) ⭐⭐⭐ |
| NEET 2025: RRYY × rryy independent assortment F₂? | 9:3:3:1 — NOT 9:7 / 1:2:1 / 3:1 ⭐⭐⭐ |
| "Independent assortment" in stem signals? | Standard 9:3:3:1 — NOT epistatic 9:7 ⭐⭐⭐ |
| NEET 2025: Polygenic inheritance pattern? | Non-Mendelian (additive + environment) ⭐⭐⭐ |
| NEET 2026 Match: Polygenic inheritance? | Human skin colour ⭐⭐⭐ |
| Grasshopper 23 vs 24 chromosomes? | 23 = MALES (XO); 24 = FEMALES (XX) ⭐⭐⭐ |
| Haploid cells divide mitotically to make gametes? | Male honeybees (drones) — NOT grasshopper / earthworm / frog ⭐⭐⭐ |
| Honey bee FALSE statement? | "Males produce sperm by meiosis" — they use MITOSIS ⭐⭐⭐ |
| Pedigree F₃ disease-free AND carrier? | Probability = 1/4 (carrier ≠ affected) ⭐⭐⭐ |
| RE-NEET 2026 swapped statements? | Down's = extra Chr 21 (47); Turner's = missing X (45, XO) ⭐⭐⭐ |