Basic Concepts Of Genetics

Have you ever wondered why an elephant always gives birth only to a baby elephant and not some other animal? Or why a mango seed forms only a mango plant and not other plants?

Given that they do, are the offspring identical to their parents? Or do they show differences in some of their characteristics? Have you ever wondered why siblings sometimes look so similar to each other? Or sometimes even so different?

These and several related questions are dealt with, scientifically, in a branch of biology known as Genetics. This subject deals with the inheritance, as well as the variation of character from parents to off springs Inheritance is the process by which character are passed on from parent to progeny; it is the basis of heredity. Variation is the degree by which progeny differ from their parents.

Human knew from early as 8000-1000 B.C. that one of the cause of variation was hidden in sexual reproduction, They exploited the variation that were naturally present in the wild population of plants and animals to selectively breeds and select of organism that possessed desirable characters. For example, through artificial selection and domestication from ancestral wild cows, we have well-known German breed Cows and India Breeds etc.We must, however, recognize that though our ancestors knew about the inheritance of character and variation, they had very little idea about the scientific basis of these phenomena.

MENDEL’S LAWS OF INHERITANCE

7 Contrasting Characters Of Pea

It was during mid nineteenth century that headway was made in the understanding of inheritance.  Gregor Mendel, conduct hybridization experiment on garden peas for seven years (1856-1863) and proposed the law of inheritance in living organism. During Mendel’s investigation into inheritance patterns it was for the first time that statistical analysis and mathematical logic were applied to problems in biology. His experiment has a large sampling size, which gave greater credibility to the data that he collected. Also, l the confirmation of his inferences from experiment on successive generation of his test plants proved that his result pointed to general rule of ideas. Inheritance rather than being unsubstantiated

Mendel conducted such artificial pollination/cross pollination experiment using several true breeding pea lines. A true breeding line is one that, having undergone continuous self pollination, shows the stable trait inheritance and expression for several generations. Mendel selected 14 true breeding pea plant varieties as pair which are similar except for one character with contrasting trait. Some of the contrasting trait selected were smooth or wrinkled seeds, yellow or green seeds, smooth or  inflated pods, green or yellow pods, and tall or dwarf plants.

INHERITANCE OF ONE GENE

Let us take example of one such hybridization experiment carried  out by Mendel where he crossed  tall and dwarf pea plants to study the inheritance of one gene .he collected the seeds to produce as a result of this cross and grew them to generate plants of the first hybrid generation. This generation is also called as first filial progeny or the F1.Mendel observed that all the f1 progeny plant were tall, like one of its parents; none where dwarf. He made similar observation for the other pair of trait-he found that the f1 always resemble either on e of the parents and that the trait of the other parent was not seen in them.

Checkerboard diagram of Di Hybrid Cross

Mendel then self pollination the tall F1 plants and to his surprised found that in the filial second generation some of the off spring were dwarf the character that was not seen in the f1 generation was now expressed. Then progeny of f1 filial plant that were dwarf were ¼ of the F2 plant while ¾ of the F2 plant were tall. The tall and dwarf trait were identical to their parental type did not show any blending, that is all the offspring were either tall or dwarf, none were of in between height.

Similar result were obtained with the other trait that he studied: only one of the parental trait was expressed in the F1 generation while at the F2 stage both the trait were expressed in the proportion 3:1. The contrasting trait did not show any blending at either F1 or F2 stage.



Di Hybrid Cross

Based on these observations, Mendel proposed that something was being stably passed down, unchanged, from parents to offsprings through the gametes, over successive generations. He called these things as “factors”. Now we call them as genes. Genes, therefore, are the units of inheritance. They contain the information that is required to express a particular trait in an organism. Genes which code for a pair of contrasting traits are known as alleles,  i.e: they are slightly different forms of the same gene. If we use alphabetical symbols for each genes, then the capital letter is used for the traits expressed at the F1 stage and the small alphabet for the other trait. For example, in case of the character of height, T is used for the Tall trait and the t for the Dwarf and T and t are alleles of each other, Hence, in plants the pair of alleles for height would be TT,Tt or tt. Mendel also proposed that in a true breeding, tall or dwarf pea variety the allelic pair of genes for height are identical or homozygous, TT and tt respectively. TT and tt are called the genotype of the plant while the driscriptive terms tall and dwarf are the phenotypes. What then would be the phenotype of a plant that had a genotype Tt?

Mono Hybrid Cross

As Mendel found the phenotype of the F1 hetrozygote—Tt exactly like the TT parent in appearance, he proposed that in a pair of dissimilar factors, one Dominant the other(as in the F1) and hence is called the Dominant Factor while the other is recessive. In this case T(for tallness) is dominant over t(for dwarfness), that is recessive. He observed identical behavior for all the other characters/ traits pairs that he studied. It is convenient( and logical) to use the capital and lower case of an alphabetical symbol to remember this concept of Dominance and Recessiveness. (Do not use capital T for tall and d for dwarf because you will find it difficult to remember weather T and d are alleles of the same genes/characters or not). Alleles can be similar as in the case of homozygotes TT and tt or can be dissimilar as in the case of hetrozygote—Tt.  Since the Tt plant is heterozygous for genes controlling one character(height), it is a mono hybrid and the cross between TT and tt  is a monohybrid cross.