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Mendel's theory of genetics
Back in the history, we can recall the crops grown by the process of inbreeding and selection for better yield. At that time farmers were unaware about the actual reason of obtaining a better breed but slowly science and precisely biologist have developed several measures to get the reason behind the improved results.
Earlier many biologists tried to find out about the inheritance and the basis for this factor but they failed to forward the idea on it and collect all the necessary evidences. So, it was Gregor Johan Mendel, who first bought up a clear picture on genetics and he revealed many more of the hidden facts on this subject.
Mendel was born in 1822 in central Europe of Heinzendorf. For several years he studied philosophy, and later 1843 were admitted to Augustinian Monastery of St. Thomas in Brno. For some years he acquired teaching experience, then from 1851 to 1853 he attended University of Vienna, and there he studied Physics and Botany and after that he again went to Brno and taught botany and Physics there. This way for several years he brightens up his career and hence thus, in 1856 he started his first experiments on hybridization. After hat until the end of life he kept researching on different aspects of genetics and now we have basic for every topic on genetics and all this is possible with the foundation set up by Mendel, his hard work and effort.
Mendel, in 1866 published his results on various experiments of genetics. His experiment started on a pea plant (Pissum sativum). The regions for which he chose pea plant for the experiment are:
- It was easy to grow.
- It can be easily hybridize artificially.
- Pea plant can self fertilize and hence cross-breed.
- Pea plant reproduces well and matures in a single season.
From this pea plant grown in his garden, Mendel performed an experiment to disclose different facts about inheritance. From this pea plant he observed several visible traits like the height of the stem, seed shape and color, pod shape and color and pod and flower arrangement. He took pea plant with these contrasting characters and crossed them. At first his crosses were simple that evolved only one pair of contrasting traits, thus called monohybrid cross. Example the cross between true-breeding plants with stems having contrasting character like tall and dwarf, the result of first generation also called (f1) generation had all the tall plants. Now when self cross was performed from the members of f1 generation, he observed that 787 out of 1064 were tall, 277 were dwarf. The ratio thus obtained was 3:1 after the self cross. Likewise, he performed similar crosses considering other traits as well. Thus, he observed that in all the crosses that he performed, f1 and f2 pattern of inheritance were similar independent of the sexes of the plants.
From this experiment Mendel gave 3 major postulates:
1. Unit factors in pairs: genetic factor are controlled by unit factors that exist in pairs in an individual organism. In the crosses between tall and dwarf stems, a specific factor existed for each trait. Thus, a diploid individual receives one from each parent. since, these factors always comes in pair, the possible combinations comes out to be, TT for tallness, tt for dwarf and Tt for hybrid, therefore one of this trait in an individual plant determines plant height and similar is case with other trait as well.
2. Dominance or receiveness: when two unit factors that are unlike each other but are present in an individual for describing a single character, in that case one is dominant that is expressed whereas other becomes recessive. Like the cross between tall and dwarf plant, in F1 generation only tall plants can be seen whereas in the F2 generation tall and dwarf both can be seen when self crossed. In the F1 generation though both unit factor were present during cross, but only the dominant i.e. T was expressed whereas other remained hidden.
3. Segregation: at the time of formation of gametes, the paired unit factor separate randomly, so that each gametes receive one or the other unit factor.
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These are some of the law given by Mendel from his experiment. Here, several terms were used to describe character, crosses and traits, some of them that can be generally encountered in this chapter are:
Phenotype: the outer expression or physical appearance of a trait is called phenotype.
Allele: the unit factor discussed above that is the alternative form of gene is called allele.
Allelic pair: the combination of two alleles that comprise the gene pair.
Genotype: represents the genetic makeup of an individual.
Homozygous: when identical allele makes up the genotype, it is called homozygous.
Heterozygote: when different alleles make up the genotype, it is called heterozygote.
Test cross: cross between the organisms with unknown genotype but known phenotype with homozygous recessive individual is called test cross.
Reciprocal cross: the cross that can be made between two individual independent of sexes is called reciprocal cross.
Beside the above mentioned points, Mendel has given one more postulate:
4. Principle of independent assortment: it states that during gamete formation, segregating pair of unit factors assort independently of each other. i.e. all possible combination of gametes will be formed in equal frequencies. In previous experiment where he obtained 3:1 ratio with single contrasting characters but in case of dihybrid cross where more than one characters are involved the ratio comes out to be 9:3:3:1.
Therefore, Mendel described and explained these postulates through mathematical calculation. And now two approaches regarding such observation are used:
Punnett square: the result of recombination during fertilization i.e. genotypes and phenotypes can be easily picturized using a square known as punnet square. Named after Reginald C. Punnett, and thus devised by biologist in order to determine the probability of offspring having a particular genotype. Hence, is the simple graphical way to represent the cross in effective way.
Forked line method: in this case each contrasting pair of traits is considered separately and then result is combined using forked line method. This method is based on rules of probability and hence is used in case of dihybrid or trihybrid crosses.
Beside the monohybrid cross involving only one contrasting character, slowly Mendel proceeded towards the complex portion where he performed a more difficult crosses involving two and three contrasting character called dihybrid and trihybrid cross respectively.
Therefore, some of the reasons for Mendel success in his experiments of inheritance are:
- Choice of suitable organism.
- His restriction of experiments to one or few contrasting traits.
- Accurate record of all the process in quantitative form.
- His postulates on the experiments.
At the initial stage several reasons led to unrewarding of Mendelian experiment but with passing time when after many more of investigation on genetics by other researcher, his work and his efforts well all accepted and hence treasured because he developed the basis for transmission of hereditary traits.
Beside Mendel work on genetics, he was also able to explain many genetically inherited diseases and now it was his work that has set up the foundation for biologist and scientist to work on various inheritable diseases and get complete information on their transfer and curable measures. Presently, we can hear a lot about sickle cell anemia, cystic fibrosis, Tay - Sachs disease, and many more are all inherited according to the Mendelian fashion. Thus, at least we now have certain methodology either for the prevention or certain measure that can be adapted for treatment of these diseases.
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