Basics of Genetics

Classical Genetics Mendel's Principles Gregor Johann Mendel (1822-1884), known as the Father of Genetics Conducted a series of experiments using pea plants and showed that traits are passed from parents to offspring in predictable ways. By quantitative data analysis of results, he concluded that each trait in the pea plant is controlled by a pair of factors and that during gamete formation, members of a gene pair separate from each other. Mendel's Experiment Mendel chose the garden pea, Pisum sativum, for his hybridization experiments because it was easy to cultivate and had a short life cycle. Naturally, it is self-fertilizing in nature, but it is easy to cross-breed experimentally. Mendel selected seven visible characters (or traits) in the pea plant. These characters have the following features: Qualitative in nature, i.e., characters (or traits) that are easily classified into distinct phenotypic classes. Each character has two alternative forms. The relationship between two alternative forms shows complete dominance. Genes governing traits are located on either different chromosomes or distantly located on the same chromosome. One gamete governs one trait. Characters Studied by Mendel No. Character Dominant Recessive 1 Stem length Tall Dwarf 2 Flower position Axial Terminal 3 Flower colour Violet White 4 Seed coat colour Grey White 5 Pod shape Inflated Constricted 6 Pod colour Green Yellow 7 Seed form Round Wrinkled Flower colour is positively correlated with Seed coat colours. Seeds with white seed coats were produced by plants that had white flowers and those with grey seed coats came from plants that had violet flowers.   Gene and Allele Gene is a segment of DNA that determines a specific character. Each gene may exist in alternative forms known as alleles, which code for different versions of a particular inherited character.   It can also define alleles as genes occupying corresponding positions on homologous chromosomes and controlling the same characteristic (e.g., the height of a plant) but producing different effects (tall or short).   If alternative forms do not affect phenotype or have very small effects that can be recognized only by special techniques, they are called isoalleles. Genes and Alleles Chromosomes in diploid cells exist in pairs. One member of each pair is derived from the female gamete and one from the male gamete, and thus are identical with respect to their genes and are called homologous chromosomes. There can be many alleles for a particular gene in a population, but for a diploid organism, only two such alleles can exist in any given individual. Alleles for any particular gene present at a specific locus on a chromosome are called the alleles for that gene. Wild-type and Mutant alleles Prevalent alleles in a population are called wild-type alleles. They typically encode proteins that are made in the right amount and function normally. Alleles that are present at less than 1% in a population and have been altered by mutation are called mutant alleles.   Such alleles usually result in a reduction in the amount or function of a wild-type protein and are most often inherited in a recessive fashion.   Alleles of same gene: Homozygous recessive alleles Homozygous dominant alleles Heterozygous alleles Genotype and Phenotype The terms Genotype and Phenotype are used in genetics to distinguish physical appearance from genetic constitution. The genotype is defined as the genetic constitution of an individual for any particular trait. A diploid organism with a genotype consisting of two identical alleles is homozygous for that locus. One that has a genotype consisting of two different alleles is heterozygous for the1 locus.   Eg: Homozygous dominant genotype (TT) Homozygous recessive genotype (tt) Heterozygous genotype (Tt) Phenotype is observable properties of an individual for any particular trait. It depends on its genotype. Genotypic classes with respect to a particular diploid or polyploid level (e.g., monoploid, diploid, or polyploid) and the number of alleles of that gene in a population. If there are 2 alleles (A and a) for a gene, then: Genotype classes will be 2 (AA and aa) in a diploid organism. 3 (AA, Aa, and aa) in a triploid organism. 4 (AAAA, AAAa, AAaa, Aaaa, and aaaa) in a tetraploid organism. Monoploid Genotype classes: A, a Diploid Genotype classes: AA, Aa, aa Triploid Genotype classes: AAA, Aaa, aaa