Principles of Inheritance
and Variation — 4-day mastery plan

Learn (2.5 h)

• Why pea was the ideal choice: true-breeding lines, contrasting traits, self-pollinating but easy to cross, short life cycle, large progeny.
• The 7 pairs of contrasting traits — memorise the table, both columns.
• Steps in making a cross: emasculation → pollination. Know which parent is female.
• Monohybrid cross: F₁ all tall, F₂ 3:1 phenotypic and 1:2:1 genotypic, no blending.
• Terminology, precisely: factor, gene, allele, homozygous, heterozygous, genotype, phenotype, monohybrid.
• Punnett square mechanics + the binomial form (½T + ½t)².
• Test cross: dominant phenotype of unknown genotype × homozygous recessive. All dominant → homozygous; 1:1 → heterozygous.
• Law of Dominance (3 clauses) and Law of Segregation — know which observation each explains.
• Incomplete dominance: Antirrhinum, RR red × rr white → Rr pink, F₂ 1:2:1. Genotypic ratio unchanged; only the phenotypic ratio changes.
• Explanation of dominance via enzyme logic: modified allele → normal/less efficient enzyme, non-functional enzyme, or no enzyme.
• Co-dominance and multiple alleles: gene I, alleles IA, IB, i; 6 genotypes, 4 phenotypes.
• Dominance is not autonomous — the Bb starch grain example.

Lock

Drill (1.5 h)

• Draw, by hand, 10 monohybrid Punnett squares including 2 incomplete-dominance ones. No shortcuts — the hand-drawing is what makes it automatic under time pressure.
• 8 blood-group problems: given parents' groups, list all possible and all impossible children's groups.
• NCERT exercises 1, 2, 4, 5, 6, 12, 13.

Check (25 min)

Set 1 below, closed book. Target: 10/10. Anything wrong goes into the error log with the NCERT line that answers it.

Learn (2.5 h)

• Dihybrid cross RRYY × rryy → F₁ RrYy → F₂ 9:3:3:1. Fill the 16-cell square yourself at least twice.
• 9:3:3:1 as (3:1)(3:1). Each trait pair still segregates 3:1 on its own.
• Law of Independent Assortment — statement in NCERT's exact words.
• Four gamete types RY, Ry, rY, ry at 25% each; general rule 2ⁿ gamete types for n heterozygous loci.
• Cytological basis: independent alignment of the two chromosome pairs at the metaphase plate (Possibility I / II figure).
• Why Mendel's work was ignored till 1900 — the four reasons — and who rediscovered it.
• Chromosomal theory of inheritance: Sutton and Boveri paralleled chromosome behaviour with gene behaviour; Sutton united it with Mendelian principles and named it.
• Morgan and Drosophila melanogaster: why it suited the work (synthetic medium, ~2-week life cycle, many progeny, sexes easily told apart, visible variations).
• Linkage vs recombination; tight linkage → low recombination. Cross A: y–w. Cross B: w–m.
• Sturtevant used recombination frequency to map genes; maps later used in genome sequencing.
• Polygenic inheritance: additive alleles, environment matters, human skin colour with A/B/C, AABBCC darkest, aabbcc lightest.
• Pleiotropy: one gene, many phenotypes, usually via metabolic pathways — phenylketonuria.

Lock

Drill (1.5 h)

• Two full 4×4 dihybrid squares from scratch; then tabulate all 9 genotypes with their phenotypes and counts (1:2:1:2:4:2:1:2:1).
• Practise the shortcut method: solve dihybrid ratios as a product of two monohybrid ratios. Do 6 problems both ways to prove they match.
• Gamete-count questions: AaBbCcDd, AABbCc, AaBBCcDdEe — how many types, and write them out for the first.
• NCERT exercises 3, 7, 8, 9.

Check (25 min)

Set 2 below, closed book.

Learn (2.5 h)

• Henking's X body (1891) → recognised later as the X chromosome; origin of the terms sex chromosome and autosome.
• XO type (grasshopper): male has one X, female has XX; some sperms carry X, some don't. Male heterogamety.
• XY type (humans, Drosophila): male XY, female XX. Male heterogamety. Equal chromosome number in both sexes.
• ZW type (many birds): female ZW, male ZZ. Female heterogamety — the egg decides the chick's sex.
• Sex determination in humans: 22 pairs autosomes + XY / XX; 50% X-sperm, 50% Y-sperm; the sperm determines the child's sex, and every pregnancy is a fresh 50:50.
• Honey bee haplodiploidy: fertilised egg → female (queen or worker); unfertilised egg → drone by parthenogenesis. Female 32, male 16. Drones make sperm by mitosis. A drone has no father and can have no sons, but has a grandfather and can have grandsons.
• Mutation: alteration of DNA sequence → change in genotype and phenotype; along with recombination it generates variation.
• Deletion/insertion/duplication of segments → chromosomal aberrations (common in cancer cells). Single base pair change → point mutation (sickle-cell anaemia). Insertions/deletions of base pairs → frame-shift.
• Mutagens: chemical and physical; UV radiation is a mutagen.
• Pedigree analysis: purpose, and every symbol in Figure 4.13 — including the diamond for unspecified sex, the double line for consanguineous mating, and the numbered diamond for multiple unaffected offspring.

Lock

Drill (1.5 h)

• Redraw the honey bee sex-determination flow chart (Figure 4.13, p. 71) from memory, with all chromosome numbers.
• Build a comparison table: XO / XY / ZW — who is heterogametic, chromosome numbers, example organisms.
• 10 pedigree problems. Use this decision routine: affected in every generation? → dominant. Skips generations, both sexes equally, consanguinity present? → autosomal recessive. Almost only males affected, through unaffected mothers? → X-linked recessive.
• NCERT exercises 10, 11, 14.

Check (25 min)

Set 3 below, closed book.

Learn (2 h)

• The two categories: Mendelian disorders (single gene) vs chromosomal disorders (number/arrangement of chromosomes).
• Colour blindness — X-linked recessive, red/green cone defect, 8% of males and 0.4% of females; a daughter is affected only if mother is a carrier and father is colour blind.
• Haemophilia — X-linked recessive; a clotting-cascade protein is affected; carrier mother → affected sons; affected females extremely rare; Queen Victoria's pedigree.
• Sickle-cell anaemia — autosome-linked recessive; alleles HbA/HbS; only HbSHbS is diseased; HbAHbS is a carrier showing sickle-cell trait; Glu → Val at the 6th position of the β-globin chain; codon GAG → GUG; polymerisation under low oxygen tension changes the biconcave disc to a sickle shape.
• Phenylketonuria — autosomal recessive; missing enzyme converts phenylalanine → tyrosine; phenylalanine accumulates as phenylpyruvic acid and derivatives; also excreted in urine.
• Thalassemia — autosome-linked recessive; reduced synthesis of a globin chain. α: genes HBA1 and HBA2 on chromosome 16. β: gene HBB on chromosome 11. Thalassemia is a quantitative problem; sickle-cell is a qualitative one.
• Aneuploidy (failure of chromatid segregation) vs polyploidy (failure of cytokinesis; common in plants). Trisomy vs monosomy.
• Down's syndrome — trisomy of 21, total 47; described by Langdon Down, 1866; short stature, small round head, furrowed tongue, partially open mouth, broad palm with characteristic crease, retarded development.
• Klinefelter's — 47, XXY; masculine build plus gynaecomastia; sterile. Turner's — 45, X0; rudimentary ovaries; sterile.

Revise (1 h)

• Read the NCERT Summary (p. 77–78) aloud, twice.
• Recite the whole fact vault below without looking. Anything she hesitates on gets a star and goes on a card.

Mock (1.5 h)

• Set 4 below, then a full 45-question chapter test in one sitting, phone away, strict 50 minutes, with negative marking applied.
• Then the part most students skip: for every wrong or guessed question, write the exact NCERT sentence that settles it. That list is her 8 August revision sheet.