Mendel’s Law of Heredity | Genetics

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

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 filial (f₁) generation, and the other re­frains recessive (latent or recedes from view).

Likewise, tallness is dominant over dwarfness. The factors responsible for a pair of alternative or contrasted charac­ters are 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.

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 became 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 vestigial-winged vinegar fly. Long wing dominant over vestigeal wing.

Di-hybrid Cross:

When individuals differing in two pairs of contrasting characters are crossed, such an experiment is known as di-hybrid cross:

1. Mendel crossed a variety of peas producing yellow and round seeds with one producing green and wrinkled seeds. He knew that yellow colour (Y) is dominant over green (g) and that wrinkled seed- surface (w) is recessive to round (R).

The f₁ generation hybrid seeds were, as expected, yellow and round. This fi hybrid produced four types of gametes and a Punnett checker board shows 16 possible combinations in f₂ generation (Fig. 46.1).

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 di-hybrid 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 seed-producing 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 di-hybrid 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 fi generation of each sex produces four kinds of gametes.

When the members of f₁ generation are intercrossed, 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 di-hybrid crosses with pea, 9 : 3:3:1 ratio is obtained (Fig. 46.2). It is the characteristic of a di-hybrid 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 di-hybrid cross):

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