Gene library

А chromosome of an organism broken into small fragments comprising one or few genes with the help of restriction endonuc1eases. Each fragment is joined to а suitable vehicle DNA forming recombinant DNAs of different nature. These are then introduced into а host (bacterial, yeast, plant or animal) cell. The foreign DNA fragment replicates as the host cell divides. This produces а group of cells, each containing а foreign DNA fragment comprising one or а few genes. The daughter cells identical to the parent cell are referred to as a clone of cells. The process is called DNA cloning. Many clones of the cells, each clone containing а different fragment of the same foreign DNA, are thus formed. The various clones representing all the genes of an organism are referred to as gene library of that organism.

Allelic and non-allelic interaction of genes Summary of Mendel’s Hypotheses

The summary of Mendel’s hypotheses given below uses terms prevalent in genetic these days.

1. Each trait of an organism is controlled by a pair of alleles (principle of unit characters).

2. In an organism having two unlike alleles for a given trait, one (the dominant allele) may express and the other (the recessive allele) may remain unexpressed (principle of dominance).

3. During gamete formation, the allele pairs of different traits segregate independently of each other (principle of independent assortment).

4. Each allele is transmitted from generation to generation as a discrete unchanging unit.

5. Each organism unherits one allele for each trait from each parent.

Genetic principles discovered after Mendel.

Since the times of Mendel, many important genetic principles have been discovered. These are sometimes referred to as Non – Mendelian genetics as these can not be explained on the basis of Mendel’s principles alone. However, such cases can be explained with suitable modifications in Mendel’s laws. It may be better to describe these principles as the post – Mendelian genetics. Some of these principles are discussed there.

1. Incomplete Dominance (Blended Inheritance)

In Mendel’s pea experiment, dominance was essentially complete. Consequently, there was practically no difference between homozygous and the heterozygous plants in the expression of a dominant character. For instance, a (TT) tall pea plant was almost similar to a (Tt) tall pea plant. However, this is not true of all characters or organisms. There are characters or genes are not dominant or recessive. In such cases, both the genes of contrasting conditions of a characters express as a blend (mixture). With the result, the hybrid produced by crossing two pure individuals does not resemble either of them, but is midway between them. This is known as incomplete or partial dominance, also blended or intermediate inheritance. It is due to the fact that the dominant character or genes is not in a position to completely suppress the recessive one. With the result, the heterozygote has a different phenotype (as a well as a different genotype) from homozygotes for either allele.

Blended inheritance is an exception to out come of Mendel’s monohybrid crosses. It is fortunate that he did not select organisms showing this condition as it may have complicated his work and may even have failed him in framing his valuable genetic principles.

Examples. Blended inheritance is found in both plants and animals. Good examples are seen in Mirabilis jalaba, the four-o’clock plant. Antirrhinum majus, the snapdragon and Andalusian fowl.

а) Four – O’clock Plant. A cross between a plant pure for red flowers and a plant pure for a white flowers yields hybrid plants with pink flowers in F generation. Neither red nor white is completely dominant, so that both colours appear in the hybrids as a blend which is pink. This is obviously a contradiction of Mendel’s assumption that no blending of characters takes place. When two of the hybrid plants with pink flowers are crossed, the F generation includes plants with red, pink and white flowers in the usual Mendelian ratio of 1:2:1. This cross shows condition as it may have complicated his work and may even have failed him in framing his valuable genetic principles.