1. Chapter at a glance
- Inheritance is the process by which characters are passed on from parent to progeny; variation is the degree by which progeny differ from their parents.
- Mendel conducted hybridisation experiments on garden pea (true-breeding lines) using seven pairs of contrasting traits and applied statistical analysis to propose laws of inheritance.
- Genes are the units of inheritance; alleles are slightly different forms of the same gene that code for a pair of contrasting traits.
- In a monohybrid cross, the F₁ shows only the dominant trait while the F₂ shows both traits in 3:1 phenotypic ratio and 1:2:1 genotypic ratio.
- In a dihybrid cross, the F₂ shows the phenotypic ratio 9:3:3:1 due to independent segregation of the two pairs of characters.
- Chromosomes and genes occur in pairs; their behaviour during meiosis explains Mendel’s laws (chromosomal theory of inheritance).
- Linkage (physical association of genes on the same chromosome) reduces recombination; sex determination mechanisms include XO, XY, ZW and haplodiploid systems.
- Genetic disorders are classified as Mendelian (single-gene, e.g., haemophilia, sickle-cell anaemia) or chromosomal (aneuploidy/polyploidy, e.g., Down’s, Turner’s, Klinefelter’s syndromes).
2. Definitions and laws (exactly as framed in the text)
- Inheritance: the process by which characters are passed on from parent to progeny; it is the basis of heredity.
- Variation: the degree by which progeny differ from their parents.
- True-breeding line: one that, having undergone continuous self-pollination, shows the stable trait inheritance and expression for several generations.
- Gene: the unit of inheritance; it contains the information required to express a particular trait.
- Alleles: genes which code for a pair of contrasting traits (slightly different forms of the same gene).
- Genotype: the allelic pair of genes (e.g., TT, Tt, tt).
- Phenotype: the descriptive terms for the trait (e.g., tall, dwarf).
- Dominant factor / Recessive factor: in a pair of dissimilar factors, one member dominates (dominant) the other (recessive).
- Law of Dominance: (i) Characters are controlled by discrete units called factors. (ii) Factors occur in pairs. (iii) In a dissimilar pair of factors one member of the pair dominates (dominant) the other (recessive).
- Law of Segregation: the factors or alleles of a pair segregate from each other such that a gamete receives only one of the two factors.
- Law of Independent Assortment: when two pairs of traits are combined in a hybrid, segregation of one pair of characters is independent of the other pair of characters.
- Incomplete dominance: the F₁ has a phenotype intermediate between the two parents (e.g., pink flowers in Snapdragon).
- Co-dominance: both alleles express their own phenotypes equally in the heterozygote (e.g., AB blood group).
- Multiple alleles: more than two alleles governing the same character (e.g., Iᴬ, Iᴮ, i).
- Linkage: physical association of two genes on the same chromosome.
- Recombination: generation of non-parental gene combinations.
- Test cross: crossing an organism showing dominant phenotype with the recessive parent to determine its genotype.
- Mutation: alteration of DNA sequences resulting in changes in genotype and phenotype.
- Pedigree analysis: study of the family history about inheritance of a particular trait over several generations.
3. Important diagrams and activities
- Figure 4.1 – Seven pairs of contrasting traits in pea plant: shows the traits Mendel selected.
- Figure 4.2 – Steps in making a cross in pea: illustrates artificial pollination/cross-pollination technique.
- Figure 4.3 – Diagrammatic representation of monohybrid cross: demonstrates 3:1 phenotypic and 1:2:1 genotypic ratios with no blending.
- Figure 4.4 – Punnett square for monohybrid cross: shows calculation of all possible genotypes and the 3:1 ratio.
- Figure 4.5 – Diagrammatic representation of a test cross: shows how genotype of a dominant-phenotype plant is determined.
- Figure 4.6 – Monohybrid cross in Snapdragon (incomplete dominance): shows 1:2:1 phenotypic ratio instead of 3:1.
- Figure 4.7 – Dihybrid cross (seed colour + shape): demonstrates 9:3:3:1 ratio and independent assortment.
- Figure 4.8 & 4.9 – Meiosis and independent assortment of chromosomes: illustrates chromosomal basis of segregation and independent assortment.
- Figure 4.11 – Linkage and recombination in Drosophila: shows deviation from 9:3:3:1 due to linkage.
- Figure 4.12 – Sex determination (XY in humans/Drosophila; ZW in birds): shows chromosomal differences between sexes.
- Figure 4.13 – Symbols used in human pedigree analysis: standard notation for tracing inheritance.
4. Common misconceptions and exam pitfalls
- Dominance is not always complete; incomplete dominance or co-dominance must be identified from the given ratio (1:2:1 or both parental phenotypes).
- Phenotypic ratio 3:1 does not reveal genotype; a test cross is required to distinguish TT from Tt.
- Law of Independent Assortment applies only to unlinked genes; linkage causes deviation from 9:3:3:1.
- Sex of the child is determined by the sperm (X or Y), not the egg; the statement “mother is responsible” is incorrect.
- Multiple alleles exist in a population but only two alleles are present in any individual.
- Pleiotropy (one gene → multiple phenotypes) and polygenic inheritance (multiple genes → one phenotype) are distinct and must not be confused.
- In pedigree analysis, autosomal recessive shows skipping generations and equal male/female affection; X-linked recessive shows criss-cross pattern and rare female affection.
5. Formula sheet
No mathematical formulas with SI units appear in the chapter. The only mathematical expressions given are the expected Mendelian ratios derived from binomial expansion:
- Monohybrid: (½T + ½t)² = ¼TT + ½Tt + ¼tt → phenotypic ratio 3:1
- Dihybrid: (3 Round : 1 Wrinkled) × (3 Yellow : 1 Green) = 9:3:3:1
These ratios are to be remembered exactly as stated; no other formulae are provided.