Ch 2Classical Genetics — Class 12 Botany, concept-first.

Genetics is the branch of biology that studies how living things pass their traits from one generation to the next.

Genetics is often described as the science of heredity and variation together, and the two ideas are two sides of the same coin.

Mendelism is the name given to the entire body of concepts that Gregor Johann Mendel established through his original research on plant hybridisation in the garden pea.

Gregor Johann Mendel (1822-1884) was born in Heinzendorf, Silesia (present-day Hyncice) and, after studying botany, physics and mathematics at the University of Vienna, entered the monastery of St.

Mendel's theory of inheritance - the particulate theory - proposed that heredity is carried by minute, discrete particles or factors (now called genes), rather than by any kind of fluid 'blending' bet…

Mendel noticed that a single trait, such as stem height, could be expressed in two different ways - tall or dwarf - because the underlying gene can exist in alternative forms for the same trait, calle…

From his monohybrid cross results, Mendel proposed two rules, today called his laws of inheritance. The Law of Dominance states that characters are controlled by discrete factors that occur in pairs,…

A monohybrid cross studies the inheritance of a single character - for example plant height - and therefore tracks the two alleles of one gene through a cross.

Mendel chose two sharply contrasting traits for each character he studied, which made it logical to assume that two distinct hereditary factors were involved.

A test cross is the cross of an individual of unknown genotype with a homozygous recessive individual, and its purpose is to reveal that unknown genotype.

A back cross is a cross between an F1 hybrid and either one of its two parental genotypes, and it comes in two forms - a dominant back cross (F1 crossed back to the dominant parent) and a recessive ba…

A dihybrid cross follows two characters, each controlled by a separate gene with two alleles, through a cross - for example seed shape (round R dominant over wrinkled r) together with cotyledon colour…

The dihybrid test cross applies the same logic as a monohybrid test cross to two genes at once: the F1 dihybrid (round-yellow, RrYy) is crossed with the homozygous double-recessive parent (wrinkled-gr…

A trihybrid cross extends the same reasoning to three gene pairs at once, and demonstrates that Mendel's Law of Segregation and Law of Independent Assortment both hold even when more than two characte…

Beyond simple monohybrid, dihybrid and trihybrid crosses, real inheritance often produces phenotypic ratios that do not match the clean 3:1, 9:3:3:1 or 27:9:9:9:3:3:3:1 patterns - exceptions to strict…

Intragenic (intra-allelic, or intralocus) gene interactions take place between the different alleles of the same gene, at the same chromosomal locus.

Incomplete dominance is an allelic interaction in which one allele is not completely dominant over the other, so the heterozygote shows a phenotype intermediate between the two homozygous parents - ge…

Codominance is a different kind of allelic interaction from incomplete dominance: instead of blending into an intermediate phenotype, both alleles of the heterozygote are expressed simultaneously and…

A lethal allele is one capable of causing the death of the organism that carries it. E. Baur reported the first such case in 1907, in the snapdragon (Antirrhinum sp.), where it behaves as a recessive…

Pleiotropy is the phenomenon in which a single gene affects several apparently distinct traits at the same time, altering more than one part of the organism's phenotype through one shared underlying m…

Intergenic (interlocus, or non-allelic) gene interactions take place between the alleles of different genes sitting at different loci, unlike intragenic interactions, which involve only the alleles of…

Polygenic inheritance is the pattern in which several genes together contribute additively to a single trait, and it is what allows Mendelian genetics to explain continuously varying (quantitative) tr…

DNA is the universal genetic material, and genes located in the nuclear chromosomes follow ordinary Mendelian inheritance.

This chapter traced classical (transmission) genetics from Mendel's original pea experiments through to its more complex extensions.

The Evaluation exercise closing this chapter has 37 items. The first 22 are single-best-answer multiple-choice questions spanning extra-nuclear inheritance, gamete combinatorics in di- and trihybrid g…

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