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

Every living organism resembles its parents more than it resembles an unrelated member of its own species — children look like their parents, seedlings grown from a particular variety of pea breed tru…

Gregor Mendel selected the garden pea, Pisum sativum, as his experimental organism, and this choice was itself one of the most consequential decisions in the history of biology, because it is precisel…

A monohybrid cross is a genetic cross in which the parents differ in, and the experimenter therefore follows, just a single pair of contrasting characters at a time, keeping every other character cons…

Mendel's Law of Segregation — sometimes also called the Law of Purity of Gametes — is the second of his three great laws, and it states that the two alleles of a gene present together in a heterozygou…

Incomplete dominance is the first, and simplest, of several genuine exceptions to Mendel's Law of Dominance that later geneticists discovered once they extended the study of inheritance beyond Mendel'…

Co-dominance is a second important deviation from simple Mendelian dominance, and it is worth distinguishing carefully from incomplete dominance (§4.5), with which it is frequently and easily confused…

A dihybrid cross is a genetic cross that follows the simultaneous inheritance of two DIFFERENT genes at once, each controlling a separate, independently observable character, rather than the single ch…

Mendel's own three laws described, with remarkable precision, the PATTERN by which hereditary factors are transmitted from parent to offspring, expressed purely as ratios of observable phenotypes — bu…

Polygenic inheritance describes a pattern of inheritance in which a single observable phenotypic character is controlled not by just one gene, as in every example discussed so far in this chapter, but…

Pleiotropy describes the biological phenomenon in which a single gene, through the effects of its single protein product, ends up influencing — or is directly responsible for producing — MULTIPLE, and…

Sex determination is the general term for the genetic mechanism by which the sex of a developing individual — male or female — is fixed at the very moment of fertilisation, and it varies considerably…

Honeybees (Apis mellifera) display an entirely different, and genetically quite unusual, mechanism of sex determination called haplodiploidy, which relies not on any distinct, dedicated PAIR of sex ch…

Mendel's Law of Independent Assortment (§4.7) holds true, without exception, only for genes that are located on entirely DIFFERENT chromosome pairs from one another, or, even for genes located on the…

Sex-linked inheritance refers specifically to the characteristic pattern by which genes located on one of the two SEX chromosomes — in practice, almost always the X chromosome, since the much smaller…

A Mendelian disorder, by definition, is a genetic disorder caused by a mutation or other structural alteration affecting just a SINGLE gene, and whose characteristic pattern of transmission through th…

Pedigree analysis is the essential, and in practice indispensable, method used to study exactly how a particular genetic trait or an inherited disorder has been transmitted through the successive gene…

Chromosomal disorders form a category of genetic disorder that is fundamentally distinct, in its underlying cause, from the single-gene Mendelian disorders discussed in the previous section (§4.15): r…

This chapter traced the complete arc of classical genetics, beginning with Gregor Mendel's own carefully designed pea-breeding experiments and building outward, step by step, all the way to the molecu…

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