Mendel’s Law of Heredity and Experiments | Genetics

In this article we will discuss about Mendel’s law of heredity and experiments.

From his experiments on plant crossing Mendel established the basic laws of genetics. He carried his plant crossing experiments with ordinary garden peas (Pisum sp.). Mendel selected pure tall and pure dwarf plants for crossing.

These are contrasting characters. It was found that when two contrasting characters are brought together in a cross, one is dominant (expressed or evident) in the next (F₁) generation, and the other remains recessive (latent or recedes from view).

Likewise, tallness is dominant over dwarfness. The factors responsible for a pair of alternative or contrasted characters, are now termed alleles (allelomorph). A class of offspring as determined by genetic composition is referred to as a genotype and one determined by its appearance is referred to as phenotype.

Monohybrid Cross:

When individuals differing in one pair of contrasting characters are crossed, such an experiment is known as a monohybrid cross.

Description:

Pure tall and pure dwarf plants were crossed and all the seeds obtained produced hybrid tall in the first generation (F₁), the dwarfness remained suppressed. As a result of random crossing in the first generation the plants of the second generation (F₂) were, on-the average, 75% tall and 25% dwarf. Thus, dwarfness which was suppressed in the first generation became expressive, in a quarter of the second generation.

When the F₂ dwarf plants were self-pollinated only dwarf plants resulted (F₃), whereas among the tall F₂ plants 25% produced pure tall and 50% produced tall and dwarf plants in the ratio 3: 1, showing that in 50% plants of F₂ the introduced characters were not completely separated.

Thus of the tall and dwarf characters the tall become dominant in F₁ generation but in the following F₂ generation these two characters were segregated or separated, in an average 3 to 1 ratio.

Experiment with long-winged and vestigeal-winged Vinegar fly. Long wing dominant over vestigeal wing.

Dihybrid Cross:

When individuals differing in two pairs of contrasting characters are crossed, such an experiment is known as dihybrid cross.

1. Mendel crossed a variety of peas with yellow and round seeds with one having green and wrinkled seeds. He knew that yellow colour is dominant over green and that wrinkled seed surface is recessive to round. The F₁ generation hybrid seeds were, as expected, yellow and round. This F₁ hybrid produced four types of gametes and a Punnett checker board shows 16 possible combinations in F₂ generation (Fig. 27).

The F₂ phenotypes include 9 yellow-round (YR) like the dominant parents, and 1 green-wrinkled (gw) like the recessive parents. Two new combinations appeared of which 3 were green-round (gR) and 3 yellow-wrinkled (Yw). The 9:3:3:1 ratio is characteristic of a dihybrid cross.

Considering one pair of contrasting characters there are 9 + 3 = 12 yellow to 3 + 1 = 4 green and 9 + 3 = 12 round to 3 + 1 = 4 wrinkled individuals. Thus, the ratio of 3 dominant to 1 recessive in each case follows Mendel’s first law, as in a monohybrid cross. Thus Mendel found that each pair of character is inherited independently of the other.

2. The dihybrid cross may be illustrated in the case of guinea pig where black coat (B) is dominant over white (b) and rough coat (R) is dominant over smooth (r). The mating of white-rough with black-smooth results black-rough hybrid. The black- rough of F₁ generation of each sex produces four kinds of gametes.

When the members of F₁ generation are inter-crossed sixteen possible combinations appear in the F₂ generation. The F₂ phenotypes include 9 black-rough (BR) like the dominant parents, and 1 white-smooth (br) like recessive parents.

Two new combinations have appeared, of which there are 3 black-smooth (Br) and 3 white-rough (bR). As in dihybrid crosses with pea 9: 3: 3: 1 ratio is obtained (Fig. 28). It is the characteristics of a dihybrid cross.

Mendel formulated the following laws from these experiments:

1. Law of segregation (from monohybrid cross):

Whatever unit characters may be introduced into the zygote at fertilization such factors are not completely blended, but are separated out in the gametes it produces.

2. Law of independent assortment (from dihybrid cross):

When considering two or more pairs of unit characters the inheritance of each pair of unit character is independent of the other.

Linkage and Crossing Over:

Each chromosome carries several or many genes. When two or more genes in the same chromosome are tied and do not leave one another ordinarily, they are said to be .linked. The linked genes always tend to be transmitted together to the offspring’s. If it happens the linkage will be complete, and the two or more characters will occur together.

Linkage is the tendency of two or more genes in the same chromosome to remain together in the process of inheritance. But such is not always the case. A pair of chromosomes exchange segments at the reduction division. This exchange is known as crossing over (Fig. 29).

In Drosophilia, grey body (+) and long wing (+) characters are dominant over black body (b) and vestigeal wing (v). When pure grey-long is crossed to blackvestigeal, the offspring’s produced in F₁ generation are grey-long hybrid (++ /bv).

In the F₁ female genotype,-the chromosomes might come together and separate without crossing over between + and +, giving eggs of two non-crossover classes ++ and b v. The second possibility is that crossing over might take place between + and +, giving eggs of two crossover classes + v and b +.

If a cross is made between F₁ female to a black-vestigeal (double recessive) male, four categories of offspring’s will be produced. Grey-long, black-vestigeal are non- crossover classes and constitute more than 50%, grey-vestigeal, and black-long are crossover classes which are always less than 50%. So tendency of the two genes to remain in original combination is always greater.