Principles of Inheritance and Variation

Mendel’s Laws of Inheritance

Mendel used garden peas (Pisum sativum) to perform his hybridization experiment to study genetics for seven years i.e. from 1856 to 1863 which led him to propose the laws of inheritance. He first prepared self-pollination of the pea lines to obtain true breeds of particular characters and then performed cross-pollination/artificial pollination of those pea lines having contrasting characters. He selected seven pairs of contrasting characters i.e. 14-true breeding pea plant varieties. The contrasting pairs of characters are listed below;...

Inheritance of One Gene

Mendel crossed tall and dwarf pea plants collected their seeds, and grew them to produce the first hybrid generation called the first filial progeny or F1 generation. Then he crosses the seeds to obtain the second filial progeny or F2 generation. Similarly, he crossed other contrasting characters containing plants to obtain F1 and F2 generations.   After these crossings, he concluded that;...

Inheritance of Two Genes

Mendel also worked and crossed pea plants that differed into two characters and got a typical phenotypic ratio of  9 : 3 : 3: 1 and genotype ratio  1: 2: 2: 4: 1: 2: 1: 2: 1....

Chromosomal Theory of Inheritance

Mendel’s work was not accepted at that time because due to a lack of communication between the scientific communities, the use of maths in his work was not acceptable to some biologists, lack of physical proof of his work on the genes, etc. reasons. Then after years of his death, his work was rediscovered by three scientists namely Carl Correns, Hugo De Vries, and Von Tschermrk independently. They were able to do so due to the development of a microscope so that cell division and its chromosomes were observable....

Linkage and Recombination

The term linkage and crossing over is introduced by TH Morgan. The physical association of parental genes in a chromosome is called linkage, such genes are called Linked genes and recombination is used to describe non-parental gene combinations. When two genes in a dihybrid cross were situated on the same chromosome, the proportions of parental gene combinations were much higher than in the non-parental type. Linked genes are exceptions to the law of independent assortment (3rd law)....

Pleiotropy and Polygenic Inheritance

Both these phenomena are exceptions to Mendel’s laws of Inheritance. In pleiotropy, a single gene can exhibit multiple phenotypic expressions. It is the effect of a gene on metabolic pathways which contributes to different phenotypes. E.g. White-eye mutation in Drosophila results in changes in body color, and starch grain size in pea seed and seed size where a single gene control both starch grain size and seed shape....

Sex Determination

The chromosome involved in sex determination is called the sex chromosome (Allosome). Whereas the chromosome in an organism other than the sex chromosome is called somatic chromosomes (Autosomes). Henking (1891) studied spermatogenesis in some insects in which he observed that 50% of sperm received a nuclear structure after spermatogenesis, other 50% of sperm did not receive it, he named them as ‘X-body’ is now called X-chromosome. There are various mechanisms of sex determination, these are;...

Mutation

The mutation is a sudden heritable change in DNA sequences resulting in changes in the genotype (and sometimes phenotype) of an organism. It is the alteration in chromosomes that result in abnormalities or aberrations that can be easily observed in cancer cells. The substance that causes mutations are called Mutagens and these are of two types;...

Pedigree Analysis

The analysis of a trait or traits in several generations of a family is called pedigree analysis. It is represented in the form of a hierarchical chart that is called the pedigree tree. It helps to trace the inheritance of a specific trait or abnormality or disease....

Genetic Disorders

The disorders that occur due to defect in the genetic structure of a person is called a genetic disorder. These disorders can get transmitted from one generation to another. There are mainly 2 types of genetic disorders Mendelian disorders and Chromosomal disorders....

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