Covers NCERT Class 12 Biology, Chapter "Principles of Inheritance and Variation" — Mendelian genetics, chromosomal theory, sex determination, mutation and human genetic disorders. One of the most reliably tested chapters across recent NEET papers.
Monohybrid & dihybrid cross ratios and test-cross applications, incomplete dominance vs co-dominance (flower colour, ABO blood group), multiple alleles and pleiotropy, chromosomal theory of inheritance and linkage, sex-determination mechanisms (XX-XY, XO, ZW), pedigree symbols and pattern recognition, and human genetic disorders — both Mendelian (haemophilia, colour blindness, sickle-cell anaemia, thalassemia, PKU) and chromosomal (Down, Klinefelter, Turner syndrome).
Mendel's Law of Dominance: in a heterozygote, one allele (dominant) expresses over the other (recessive). Law of Segregation: the two alleles of a gene separate during gamete formation, each gamete getting only one. Law of Independent Assortment: alleles of different genes (on different chromosome pairs) segregate independently of each other.
A test cross (heterozygote × homozygous recessive) is used to determine an organism's actual genotype when the phenotype alone can't distinguish homozygous dominant from heterozygous. Monohybrid test cross gives a 1:1 ratio.
Incomplete dominance (e.g. Mirabilis jalapa flower colour) gives an intermediate F1 phenotype and a 1:2:1 phenotypic ratio in F2 — genotypic and phenotypic ratios become identical, unlike simple Mendelian dominance.
Co-dominance: both alleles express fully and simultaneously — classic example is the AB blood group, where both I^A and I^B alleles are expressed together on the same red blood cells.
ABO blood group is controlled by multiple alleles (I^A, I^B, i) at a single locus, with I^A and I^B co-dominant over i, and both dominant to i.
Independent dihybrid cross gives a classic 9:3:3:1 F2 phenotypic ratio; linked genes (located close together on the same chromosome) deviate from this ratio because they don't assort independently — Morgan's Drosophila work established this.
Sex determination: humans and most mammals use the XX (female)–XY (male) system; grasshoppers use XX–XO; birds/some reptiles use ZW–ZZ (female is the heterogametic sex, opposite to mammals); honeybees use haplodiploidy (no sex chromosomes at all).
Mutation is any sudden, heritable change in DNA sequence — point mutations (single base change, e.g. sickle-cell anaemia) versus chromosomal aberrations (changes in chromosome number/structure, e.g. Down syndrome's trisomy 21).
Pedigree analysis uses standard symbols (square = male, circle = female, filled = affected, horizontal line = mating, vertical line = offspring) to trace inheritance patterns across generations in humans, where controlled crosses aren't possible.
Original questions modelled on recurring NEET question types — not verbatim reproductions of any official paper.
Scaled from direct NCERT recall (Q1–Q6) to mixed NEET-level difficulty (Q7–Q15).
Purely a recall device — not a reinterpretation of the source stories.
Think of a family with two possible heirs to a throne, one loud and visible in every portrait (the dominant allele), one quiet and unseen unless both parents happen to carry only that quiet line (the recessive allele, expressed only when homozygous). In each generation, only one heir's trait is painted into the portrait — but the quiet heir's blood never disappears, just waits, exactly as Mendel's Law of Segregation describes: both alleles are passed on and separate cleanly into gametes, even when only one is seen.
Picture a treasurer's ledger with three possible entries at one single account line — I^A, I^B, and i — but every person only carries two of the three at once. I^A and I^B are both "loud" entries that show up equally when they appear together (co-dominance, giving blood group AB); i is the "silent" entry, visible only when no loud entry is present at all (giving blood group O). One locus, three possible entries, but always exactly two per ledger — the essence of multiple allelism.
"3-1 alone, 9-3-3-1 together, 1-1 when tested" — monohybrid F2 is 3:1, dihybrid F2 is 9:3:3:1, and any test cross collapses back to equal ratios (1:1 or 1:1:1:1) because the recessive parent contributes nothing but a "blank" gamete.