Mendel's Law Inheritance Experiments

Mendel Experiments

Inheritance is the obtaining of genetic traits or factors by the progeny from their parents. Genetics deals with two concepts including the inheritance of traits and variations of traits to the offspring from parents. During the mid-nineteenth century, the mystery behind genetics was cracked by a monk named Gregor Mendel. Reasons for Mendel's success was his method of working as he maintained the statistical record of all the experiments and analyzed them. He selected genetically pure breed line and purity was tested by self-crossing the progeny for several generations. Mendel's laws are still true because they take place in sexually reproducing organisms or parents as they are of pure breeding.


Gregor Johann Mendel Experiment

  • Selection of Material: Garden pea was selected by Mendel for his experimental material.

  • Selection of Traits: 7 pairs of alternating or contrasting characters were selected by  Mendel.


Mendel's Experiments

Monohybrid Cross: Mendel made a cross between two pure plants having contrasting characters for a single plant called monohybrid cross.

Pure tall and dwarf plants were crossed by Mendel. All the plants are tall hybrids that belonged to the F1 generation which were self-pollinated. The plants were both tall and dwarf of the F2 generation in approximate 3:1 ratio phenotypically and 1:2:1 genotypically.

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Mendel's Explanation 

The above results were explained by Mendel after presuming that tallness and dwarfs of the plants were determined by a pair of contrasting factors or genes (determiners). A plant is claimed as tall only if it has determiners for tallness (represented by T) and a plant is a dwarf as it has genes for dwarfness (represented by t). These determiners are received from either parent and it occurs in pairs. Depending on this behavior, the tallness is depicted as a dominant character and dwarfs as recessive (law of dominance). When gametes are formed, the determiners are never contaminated. These units factors segregate so that each gamete gets either of the alternative factors. The two entities separate out when F1 hybrids (Tt) are self-pollinated. Afterward, they unite without depending on each other producing tall and dwarf plants (law of segregation). The Monohybrid test cross-ratio is 1:1.


Dihybrid Cross: Mendel made a cross between two pure plants having a pair of contrasting factors i.e color and shape of seed called a Dihybrid Cross. 

Mendel conducted an experiment to study the segregation and transmission of 2 pairs of contrasting traits at a time. Mendel found that in the F1  generation only round and yellow seeds are produced after crossing between round yellow and wrinkled green seeds.  But in the F2 generation,  4 types of combinations were observed.


Traits

Combinations

Round yellow

9 Parental combinations

Round green

3 Non-parental combinations

Wrinkled green

3 Non-parental combinations

Wrinkled yellow

1 Parental combinations


Thus, the offspring of the F2 generation were produced in the ratio of 9:3:3:1 phenotypically and 1:2:2:4:1: 2:1:2:1 genotypically. This ratio is called the dihybrid ratio.

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Mendel's Explanation: 

Results are explained by Mendel after assuming that wrinkled and green characters are recessive and round and yellow characters are dominant so all the F1 offsprings are round yellow. In F2 generations, since all the 4 characters were assorted out independent of the others. Mendel told that a pair of alternating characters behave without depending on the other pair i.e seed color does not depend on the seed coat. 

Therefore, at the time of gamete formation genes for round and wrinkled characters of the seed coat were assorted out without any dependence of the yellow or green color of the seed. As a result, 4 types of gametes with two old and two new combinations i.e YR, Yr, yR, yr were formed from the F1 hybrid. These 4 types of gametes on random mating produce four types of offspring in the ratio of 9:3:3:1 in the F2 generation ( law of independent assortment). The Dihybrid test cross-ratio is 1:1:1:1.


 Mendel's Law of Inheritance

  • Law of Segregation: This law states that 2 members of of the allelic pair without being contaminated, stay together when a pair of genes are brought together in a hybrid,  and the two separate out from each other when gametes are formed from the hybrid, and only 1 enters each gamete as seen in the monohybrid and dihybrid cross. This is the reason that the law of segregation is also described as the law of purity of gametes.

  • Law of Independent Assortment: 2 or 3 characters are taken during a dihybrid and trihybrid cross. These characters segregate independently of the others in the F2 generation.

FAQ (Frequently Asked Questions)

1. Explain Incomplete Dominance?

After Mendel, many cases were recorded where F1 hybrids produced were not related to the single parents but still exhibited characters blending of both the parents. This is called incomplete dominance.


Example: First is,  4 o'clock plant (Mirabilis jalapa). When parents with red flowers (RR) are crossed with plants having white flowers (rr) the hybrid F1 plants bear pink flowers (Rr). When these F1 plants with pink flowers are self-pollinated, they develop red, pink, and white-flowered plants in the ratio of 1:2:1. Snapdragon or dog flower (Antirrhinum majus) is another example of incomplete dominance.

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2. Explain Codominance?

Codominance: In codominance, both the genes of an allelomorphic pair in F1 hybrid express themselves equally in ratio 1:2:1  both genotypically as well as phenotypically in the F2 generation.

Example:  Codominance can be seen in coat color in cattle, and in Andalusian fowl.

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Codominance in Blood: The human blood group has alleles Ia and Ib which are said to be codominant because both alleles are expressed in the phenotype AB. They never check the expression of each other and solely produce its antigen. In the case of the alleles,  codominance as well as a dominant recessive inheritance for the blood groups is seen in the human beings

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