- 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 an 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: Deoxyribonucleic Acid (DNA)
- DNA was established as the primary genetic material and formally modelled as a double helix by Watson and Crick in 1953.
- The structural blueprint relied heavily on Erwin Chargaff’s chemical base equivalence rules and Rosalind Franklin’s X-ray diffraction data.
- The fundamental building block of DNA is a nucleotide, which comprises a five-carbon deoxyribose sugar, a phosphate group, and a nitrogenous base.
- A nucleotide is distinct from a nucleoside, as a nucleoside contains only the nitrogenous base and pentose sugar without the attached phosphate group.
- The double helix is formed by two antiparallel polynucleotide chains running in opposite directions (5′→3′ and 3′→5′) coiled in a clockwise, right-handed fashion.
- The physical architecture places the hydrophilic sugar-phosphate backbone on the exterior, while the information-carrying nitrogenous bases stack flat on the interior.
- Structural stability is maintained horizontally by complementary base pairing (A = T via two hydrogen bonds; G = C via three hydrogen bonds) and vertically by strong covalent phosphodiester bonds.
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.
