 |
Heredity and Variation |
| |
All living organisms reproduce. Reproduction results in the formation of offspring of the same kind. A pea plant produces only pea plants each time it reproduces. A rat produces only rats. Humans produce only humans. |
|
Pre-Mendelian Theories on Heredity |
| |
There have been several explanations on the possible mechanism of inheritance of traits from the parent to the offspring, put forth from time to time by different biologists. All these early theories presume that the characteristics of the two parents somehow mix during inheritance. Hence these ideas came to be known as blending theories on inheritance. Following are some of the early ideas on heredity. |
|
Mendelian Genetics |
| |
Mendel (1822-1884) was born in to the family of a poor peasant in Moravia, Austria. He received his school education with utmost difficulty due to poverty in the family. In 1843 he joined a church as a monk where in 1847 he became the abbot (head) of the monastery at Brunn, Austria (now called Bruno in Czechoslavakia). |
|
Mendel's Experiments |
| |
Mendel was primarily a monk in a monastery. In addition to his normal duties of preaching in the church, Mendel evinced a keen interest in the maintenance of the garden in the premises of the church. In the course of his routine rounds in the garden, Mendel was keenly observing the pattern of inheritance of certain characters in some of the plants. He became interested in investigating the mechanism by which the characters are transferred from the parent plants to their offspring. He decided to conduct some experiments in this direction. After careful examination and thinking, Mendel selected the pea plants (Pisum sativum) for his experiments. |
|
Monohybrid Inheritance |
| |
In the initial set of experiments, Mendel concentrated only on the pattern of inheritance of a single pair of contrasting characters. This pattern of inheritance involving only one pair of contrasting characters is known as monohybrid inheritance. |
|
Dihybrid Inheritance |
| |
It is a cross involving two pairs of contrasting characters. For this experiments on dihybrid inheritance. Mendel selected the contrasting characters in the seed coat and cotyledons. He conducted a cross pollination between a pure breeding plant with round seed coat and yellow coloured cotyledons and a pure breeding plant with wrinkled seed coat and green coloured cotyledons. In the F1 generation, all the resulting plants had round seed coat and yellow coloured cotyledons. |
|
Test Cross |
| |
Mendel devised a system of conducting verification for the results obtained by him. It is known as test cross. It is a cross between F1 plant and the recessive parent. A test cross-conducted for the monohybrid inheritance results in the two opposite characters expressing in a ratio of 1:1. |
|
Punnet Square |
| |
The genotypes and phenotypes resulting from various combination of gametes can be easily determined by Punnet squares, devised by Reginald C. Punnet (1875 1967). Hence each of the possible gametes is placed in an individual column or a row, with vertical column representing the female and horizontal row the male parent. The gametes are then arranged in all possible combinations and the resulting genotypes are entered in the boxes along with the phenotypes. |
|
Reasons for Mendel's Success |
| |
Many scientists earlier to the period of Mendel had attempted breeding experiments in both plants and animals. However, most of them failed to arrive at any specific conclusions or principles to explain the pattern of inheritance. In fact, many of these scientists did not even keep precise records of the results obtained by them in their experiments. |
|
Chromosomal Basis of Inheritance |
| |
Heredity depends upon the protoplasmic continuity between the parent and the offspring. It is the gametes that establish the continuity between parent and offspring and hence, the mechanism of inheritance operates across this exceedingly slender protoplasmic bridge. The machinery of inheritance lies mainly in the nucleus or more particularly the chromosomes of the cells which form the gametes. |
|
Chromosome Theory of Inheritance |
| |
In the year 1902, Boveri and Sutton proposed the idea of chromosome theory of inheritance which was subsequently confirmed by Morgan in the year 1933. According to this theory, the Mendelian factors which are now called as genes, are located on the chromosomes and it is the chromosomes which segregate and assort independently during meiosis and recombine during the process of fertilization. |
|
Non-Mendelian Inheritances |
| |
The rediscovery of Mendelism and the subsequent idea of chromosomal basis of inheritance, paved way for several significant discoveries in the field of heredity. Many other patterns of inheritance which cannot be explained on the basis of Mendel's laws alone, were discovered in plant and animals. Such patterns of inheritance are described as non-Mendelian inheritance. |
|
Incomplete Dominance |
| |
It is a type of intragenic (or interallellic) interaction where both the alleles of a given trait express as a blend (mixture) as against a normal Mendelian pattern where one allele is dominant over the other. As a resulting of this blending, an intermediate character is expressed. This situation occurs due to the fact that the dominant gene is not in a position to completely suppress the expression of recessive gene. With the result, the heterozygous offspring will be phenotypically and genotypically different from either of the homozygous parent. Following are the two familiar examples of incomplete dominance. |
|
Multiple Alleles |
| |
The transfer of blood from one person to another is called blood transfusion. In all cases of blood transfusion, it is necessary to match the blood group of the recipient with the blood group of donor. The following table represents the blood group matching. |
|
Codominance |
| |
Co-dominance represents a situation where two allelic genes when present together in an individual, express their traits independently instead of showing a typical dominant recessive relationship. As a result the heterozygous progeny of the F2 generation show a phenotype that is different from both the homozygous parent. |
|
Epistasis |
| |
Epistasis is the best example for gene interaction. It is a pattern of inheritance where a pair of genes situated at one locus, prevent the expression of a pair of genes situated at another locus. Such genes are called inhibiting genes or epistatic genes (Epi = above/over gene, static = standing). It is an intergenic or non allelic form of gene interaction. The basic genes, the expression of which is prevented by the epistatic genes, are called as hypostatic genes. Epistasis reduces the number of phenotypes in the F2 generation of a dihybrid cross. |
|
Pleiotropy |
| |
It is a phenomenon where a given gene has multiple phenotypic effects. As a result, the gene not only influences the trait which it expresses, but also influences many other traits. Such a gene is called pleiotropic gene. |
|
Quantitative Inheritance |
| |
It is a pattern of inheritance where a single phenotypic trait is governed by more than one pair of genes. Hence, there is formation of many possible genotypes as a result of which such traits show a wide range of phenotypes. Such a trait is called polygenic or multifactor trait and the pattern of inheritance is called polygenic or quantitative inheritance. |
|
Summary |
| |
The earlier belief on heredity was that characteristics of the two parents mix and express in the offspring. These ideas came to be known as the blending theory of inheritance. |
