Mendelian Genetics and its Extension

Mendelian Genetics and its Extension

Gregor Johann Mendel

Heredity is defined as transmission of shorten characters and qualities from one generation to next generation. In the 18^th Century Carolos Linnaeus and other Biologists have also conducted experiment in plants and become successful in obtaining the hybrids. But the first scientific approach in the study of inheritance was made by Gregor Johann Mendel who has conducted experiment on garden pea from 1857-1865 and finally published his findings in 1866 in the journal Natural history Society of brown. But unfortunately these contributions of Mendel were overloaded by the scientific world of that time.

After 34 years i.e. in 1900 these again rediscovered by three different scientists of Europe and they were Hugo De Vries (Holland), Karl Correns (Germany), E.V. Tschermak (Austria).

The term genetics was introduced by W.Batson (1906).

Mendels Experiment

Mendel studied the inheritance of different characters in garden pea (Pisum Sativum). Mendel selected the garden pea for his experiment due to following result.

i.                    These are easy to cultivate annual plant. So study of several generations is possible in a short period.

ii.                  It has perfect bisexual flower containing both male and female parts and thus, the flower are mainly self pollinating.

iii.                Pea plants show a number of sharp and distinct contrasting characters.

iv.               The hybrids are fertile. So experiment can continue for many generations. Here a large number of offspring help in reaching a correct conclusion.

Selection of traits

Mendel selected 7 pairs of contrasting character and all were related to dominant and recessive.

1.     Length of stem-Tall and Dwarf

2.     Position of flower-Axial and Terminal

3.     Color of flower-Purple and White

4.     Shape of pot-Inflated and Constricted

5.     Color of pot-Green and Yellow

6.     Shape of seed-Round and Wrinkled

7.     Color of seed coat-Yellow and Green

Monohybrid cross

Mendel perform his monohybrid cross experiment by taking only one character from each parent. So a cross between two individuals which differ in a single set of contrasting characters is known as Monohybrid cross. In his experiment Mendel crossed pure varieties tall and dwarf plant and these plants produced their seeds. From these seeds the F1 generation was obtained as per Mendelian experiment all the F1 plants appeared as tall (About 100%).

In his next experiment Mendel allowed these F1 plant to self pollinate and produced the F2 generation. Here the obtained plants and F2 having both tall and dwarf varieties in 3:1 ratio i.e. 75% of the F2 become tall plants and 25% become dwarf plants.

Picture

Here the phenotypical ratio becomes 3:1. But as per the genetical constituents these ratio becomes 1:2:1. Because out the three tall plants only one is pure (Homozygous) but the rest two tall plants are hybrids (Heterozygous).

From the results of this experiment Mendel concluded that something is present in the germ cell which responsible for the transmission for a particular characters. He named this something as element which was later on termed as factor by Karl Correns and now it is term as Gene by Johann Sen.

From this Monohybrid experiment Mendel postulated his first 2 law

(1)Law of Dominance

(2)Law of Segregation

Law of Dominance

According to Mendel when two Homozygous  individual with one or more set of contrasting character are crossed, the characters that only appeared in F1 are said to be Dominant character and the character which do not appeared at all in F1 is called Recessive character. In his Monohybrid experiment the character for Tallness suppresses the character for Dwarfness and thus the Dwarf character is not able to express itself (Though it is present). Thus, Tallness dominates over Dwarfness.

Such dominance or recessiveness of genes can be explained on the basis of enzymatic function of gene. The dominant genes are capable of synthesizing active polypeptide that form functional enzyme where as the recessive gene synthesized incomplete or non functional polypeptides. Therefore the dominant gene produces a specific phenotype even if in heterozygous condition whereas the recessive gene failed to do so.

The phenomenon of dominance is of practical importance as in most cases the harmful recessive characters are suppressed by the normal dominant character in hybrids.

A person hybrid for any characters like Hemophilia or Colorblindness appeared perfectly normally and these harmful recessive genes can exist for several generations without expressing themselves.

Exception

The law of dominance is applicable in most cases but, it is not always found to be true because sometimes. The dominant gene fails to completely suppress the recessive gene. Some examples are-

(1 ) Incomplete Dominance

In case of 4’0 clock plant (Mirabilis Jalapa). Here the pure varieties bear red and white flowers one crossing these two for F1.Plants appeared with pink flower. In that way when these F1 plants were interbreed red, pink, and white flower appeared in F2 in the ratio of 1:2:1 i.e. 1 red, 2 pink and 1 white.

(Picture)

Thus there is a blending of color or character and thus it is known as Blending inheritance or partial inheritance or incomplete dominance.

(2)Co-Dominance (Complete Dominance)

In some cases the dominant character is not able to suppress the recessive character and therefore both the genes express themselves equally in the hybrids i.e. both the character exhibit side by side. The best example of co-dominance comes from cattle i.e. a cow with brown coat is crossed with a bull with white coat. Here the F1 hybrid developed both brown and white color side by side i.e. both the colors appeared in the offspring almost in equal proportion. It proves that both the characters behave like dominant and none of them are recessive but both of them behave actual relation with each other.

The process of co-dominancy is also found in blood groups of human being. In case of ABO blood group system the gene IA and the gene IB are responsible for the antigens A and B respectively. (I= Isohaemoglutinogen).

Law of segregation

This law state that when a pair of contrasting character are brought together in a Heterozygote, the two members of allelic pair remain together without being contaminated and when gametes are form from these hybrids, the two factors separate out from each other. So according to Mendel these alleles have kept their identity and in each generation they are free to separate from each other.

Example- As per Mendel’s Monohybrid experiment the two character i.e. tallness and dwarfness do not totally mix in F1. But they remain together without contaminating each other. Each one keeps its individuality and at the time of gamete formation each gamete (For tallness and dwarfness) recessive only. One gene therefore the gametes are produced in equal proportion i.e. 50% contains the gene T and other 50% contain the gene t.

Here Mendel indicates that these gametes are pure either for tall or for dwarf and therefore this law is otherwise known as Law of purity of gametes.

Dihybrid cross

Mendel also conducted experiment by taking two pairs of contrasting character at the same time. He performed a cross between two pea plants with two different characters i.e. shape of the seed and color of the seed coat. One Homozygous pea plant had round and yellow seed and the other homozygous pea plant had wrinkled and green seed.

(Picture)

By crossing these two for F1 Mendel found only round and yellow seeds in majority amount but appearance of wrinkled green becomes either negligible or nil.

On self pollination the F1 plants produced 4 different types of plant in the ratio of 9:3:3:1 where two of these combinations were similar to original parental variety where as the other two were of new varieties. Here the Genotypical ratio becomes 1:2:2:4:1:2:1:2:1

Thus according to law of dominance here Mendel explained that the round and yellow character are dominant over wrinkled and green. So that almost all the F1 offspring and majority of F2 offspring were become round and yellow so by taking this experiment Mendel postulated that the 3^rd law of inheritance i.e. Law of independent assortment.

Law of independent assortment

This law state that if the inheritance of more than one pair of character is studied simultaneously the factor for each pair of character for assort out independently of the other pair i.e. Two alleles of a particular character is entirely independent of each other. In this way Mendel concluded that a dominant allele of a character not only combines with a dominant allele of another character but also with the recessive allele. Similarly a recessive allele can also combine with a dominant allele of other character to form the gamete. Thus a gene responsible for a certain character does not interfere with another gene of a different character.

Example- in Mendel dihybrid crosses experiment the genes for shape of the seed do not interfere with the genes of color of the seed and vice versa. So during gamete formation they behave independently.

                Mendelian Genetics and its Extension