- The word “gene” was coined by geneticist Wilhelm Johannsen in 1909 to simply describe what parents passed to their offspring. The detailed structure of DNA associated with genes was discovered much later.
- Genes are specific sequences of nucleotides on a chromosome, that encode particular proteins which are expressed in the form of some particular feature of the body.
- GENES are the specific parts (DNA segments) of a chromosome which determine the hereditary characteristics.
Definitions [6]
Define the following term:
Gene
Genes are specific sequences of nucleotides on a chromosome that encode particular proteins which are expressed in the form of some particular feature of the body. They are the units of heredity which are transferred from parents to offspring and are responsible for some specific characteristics of the offspring.
Definition: Genetics
Genetics is the study of heredity i.e. transmission of body features (both similarities and differences) from parents to offspring and the laws relating to such transmission.
Definition: Genes
Definition: Genome
GENOME is the full complement of DNA (including all genes and the intergenic regions) of an organism.
Definition: Monohybrid Experiments
Mendel's first experiments were with the varieties of garden pea that differed in only one visible character. These are known as monohybrid experiments.
Definition: Mutation
Mutation is a sudden change in one or more genes, or in the number or in the structure of chromosomes.
or
Mutation is a phenomenon which results in alteration of DNA sequences and consequently results in changes in the genotype and the phenotype of an organism.
Key Points
Key Points: Monohybrid Cross
| Parameter | Monohybrid Cross |
|---|---|
| Meaning | Cross involving 1 pair of contrasting characters |
| Example | TT × tt (Tall × Dwarf) |
| Phenotypic Ratio | 3:1 (Tall: Dwarf) |
| Genotypic Ratio | 1: 2: 1 |
| Conclusion | Tallness is dominant; dwarfness is recessive |
Key Points: Molecular Structure of DNA
1. DNA structure was first studied by Rosalind Franklin (1953); later explained by Watson and Crick, who proposed the double helix model (Nobel Prize, 1962).
2. DNA is a macromolecule made of two complementary strands twisted into a double helix.
3. Each strand is made up of nucleotides, which include phosphate, sugar (pentose), and a nitrogenous base.
4. There are four nitrogenous bases:
- Adenine (A) pairs with Thymine (T) (2 hydrogen bonds)
- Guanine (G) pairs with Cytosine (C) (3 hydrogen bonds)
5. The two strands form a ladder-like structure, with bases as rungs and sugar-phosphate as the backbone.
Key Points: Deoxyribonucleic Acid (DNA)
- Miescher (1869) isolated a substance from white blood cell nuclei (pus from bandages) and called it nuclein — the first discovery of nucleic acid.
- Nuclein properties — High phosphorus content + acidic nature → renamed nucleic acid.
- Two types of nucleic acids: DNA and RNA.
- Early belief — Scientists thought proteins were the genetic material (large, complex, varied). DNA was wrongly considered simple and unimportant.
- 1928–1952 — Over 25 years, three key experiments proved that DNA (not protein) is the genetic material.
- Role of DNA — It is stable, can replicate accurately, and passes traits to the next generation — making it the true genetic material.
Key Points: Mutations
- A mutation is a sudden heritable change in DNA sequences that leads to changes in the genotype and phenotype of an organism.
- Loss of DNA segment = deletion; gain of DNA segment = insertion/duplication; both cause chromosomal aberrations, commonly seen in cancer cells.
- Frame-shift mutation - caused by loss or gain of a DNA segment; Point mutation - change in a single base pair (e.g., sickle cell anaemia).
- Physical mutagens that cause mutation include UV radiation, X-rays, alpha, beta and gamma rays; Chemical mutagens include mustard gas, phenol and formalin.
- Mutation is an important source of genetic variation in organisms, alongside recombination.
