Ch 4Principles of Inheritance and Variation — Class 12 Zoology, concept-first.

Genetics is the branch of biology concerned with heredity and variation — how characteristics and features are passed from parents to their offspring across successive generations.

Mendel worked with genes that came in exactly two alternative forms — a dominant allele and a recessive allele, like tall and dwarf in garden pea.

Two people's blood is not chemically interchangeable. Mixing incompatible blood types causes agglutination — visible clumping — of red blood cells (erythrocytes), because the surface of every red cell…

Karl Landsteiner identified two distinct surface antigens on human red blood cells, which he named antigen A and antigen B.

Beyond the A and B antigens, human red cells carry a third, independent surface antigen — the Rh factor, more precisely the 'D' antigen of the Rh blood group system — first found in the blood of the r…

Rh incompatibility becomes medically important specifically in pregnancy, and specifically when the mother is Rh-negative and the father is Rh-positive, because the foetus can then inherit the father'…

Sex determination is simply the mechanism by which a species establishes and fixes the distinction between male and female individuals.

Human and mammalian sex determination is dominated by a single master switch (SRY), but Drosophila proved that sex determination does not have to work that way. C.B.

Because females in an XY system carry two X chromosomes while males carry only one, there is a built-in dosage problem: without some correction, females would produce twice as much of every X-linked g…

A sex-linked trait is any trait whose gene sits on one of the sex chromosomes rather than on an autosome, and its inheritance pattern is called sex-linked inheritance.

Three human conditions are named in this chapter as classic examples of X-linked gene inheritance: red-green colour blindness (daltonism), haemophilia, and Duchenne's muscular dystrophy.

Y-linked, or holandric, genes sit in the non-homologous region of the Y chromosome — the part with no counterpart on X — and because every son inherits his Y chromosome directly and only from his fath…

Karyotyping is the technique of isolating an individual's complete chromosome set from a single cell and arranging the chromosomes into their homologous pairs for inspection; the resulting diagrammati…

A pedigree is a 'family tree' constructed using a standardised set of genetic symbols (Fig. 4.10) to record how a specific phenotypic trait has appeared, generation after generation, within one family…

Mendelian disorders are caused by an alteration or mutation in a single gene, and — unlike chromosomal disorders — they follow the ordinary Mendelian pattern of inheritance from parent to offspring, a…

A normal human somatic cell is diploid (2n), carrying 46 chromosomes arranged in 23 homologous pairs. Chromosomal disorders arise when this number or the internal structure of individual chromosomes i…

Not every inherited character is controlled by a gene sitting on a nuclear chromosome. Certain characters instead depend on genomes found outside the nucleus altogether — in the chloroplast, in the mi…

Limnaea peregra is a freshwater snail whose shell coils spirally, either clockwise (dextral) or anticlockwise (sinistral); dextral coiling is the normal form and sinistral coiling is a mutant characte…

Sonneborn and his co-workers discovered that some strains of the ciliate Paramecium aurelia carry self-replicating cytoplasmic particles called kappa particles, and that these strains — called 'killer…

The chapter closes with three related but distinct ways genetics has been proposed to improve the human condition.

This is the chapter's own end-of-chapter Evaluation set exactly as printed in the Tamil Nadu Class 12 Zoology textbook: 24 multiple-choice questions (numbered 1-24, testing recall and short calculatio…

Sample papers and previous-year board questions for this subject.