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Mendel’s Law of Inheritance: Experiments and Insights

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What Are Mendel’s Key Experiments and Their Biological Significance?

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.

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FAQs on Mendel’s Law of Inheritance: Experiments and Insights

1. What were the key steps in Mendel's experiments on pea plants?

Mendel's experiments followed a systematic procedure to study inheritance. The key steps involved:

  • Selection: Choosing true-breeding pea plants with easily observable, contrasting traits.
  • Emasculation: The removal of anthers from a flower to prevent self-pollination.
  • Pollination: The transfer of pollen from the anther of one selected parent plant to the stigma of the emasculated flower of another parent.
  • Collection and Germination: Collecting the seeds produced from the cross and growing them to observe the traits in the first-generation offspring (F1 generation).
  • Self-pollination: Allowing the F1 generation plants to self-pollinate to produce the second generation (F2 generation) for further observation.

2. What are the seven contrasting characters of the pea plant (Pisum sativum) selected by Mendel for his experiments?

Mendel selected seven pairs of contrasting characters in the pea plant for his inheritance experiments. These were:

  • Stem height: Tall vs. Dwarf
  • Flower colour: Violet vs. White
  • Flower position: Axial vs. Terminal
  • Pod shape: Inflated vs. Constricted
  • Pod colour: Green vs. Yellow
  • Seed shape: Round vs. Wrinkled
  • Seed colour: Yellow vs. Green

3. What is the main difference between Mendel's monohybrid and dihybrid cross experiments?

The primary difference lies in the number of traits being studied. A monohybrid cross involves studying the inheritance of a single character (like flower colour) at a time. In contrast, a dihybrid cross is an experiment that studies the inheritance of two different characters simultaneously (like seed shape and seed colour) in the same cross.

4. Why was Mendel's choice of the pea plant so crucial for the success of his inheritance experiments?

Mendel's selection of the pea plant (Pisum sativum) was a key factor in his success for several reasons. The plant has a short life cycle, allowing multiple generations to be studied in a relatively short time. It produces a large number of offspring, providing statistically significant data. Most importantly, it has several easily distinguishable contrasting traits and its flowers are naturally self-pollinating but can also be easily cross-pollinated, giving Mendel control over the experiments.

5. How does the Law of Segregation explain the reappearance of a recessive trait in the F2 generation?

The Law of Segregation states that during gamete formation, the two alleles for a heritable character separate (segregate) from each other so that each gamete ends up with only one allele. In the F1 generation, heterozygous individuals (e.g., Tt) carry both dominant and recessive alleles, but only the dominant trait is expressed. When these F1 individuals form gametes, 50% of the gametes receive the dominant allele (T) and 50% receive the recessive allele (t). The random fusion of these gametes during F2 generation selfing allows for the recessive alleles to combine (tt), leading to the reappearance of the recessive phenotype.

6. How did Mendel's dihybrid cross experiment lead to the formulation of the Law of Independent Assortment?

The Law of Independent Assortment was derived from the results of the dihybrid cross. When Mendel crossed plants differing in two characters (e.g., seed shape and seed colour), he observed that the inheritance of one character was not dependent on the inheritance of the other. The F2 generation produced a phenotypic ratio of 9:3:3:1, which included two new combinations of parental traits (recombinants). This showed that the alleles for seed shape assorted independently of the alleles for seed colour during gamete formation, leading to the conclusion that genes for different traits are inherited independently of one another.

7. What is incomplete dominance and how does it differ from Mendel's principle of dominance?

Incomplete dominance is a pattern of inheritance where the phenotype of the heterozygous offspring is an intermediate blend between the phenotypes of the two homozygous parents. For example, crossing a red-flowered snapdragon with a white-flowered one produces pink-flowered offspring. This differs from Mendel's principle of complete dominance, where the heterozygous individual expresses only the dominant allele's phenotype, completely masking the effect of the recessive allele.

8. Explain the concept of codominance with the example of the human ABO blood group system.

Codominance is a genetic scenario where both alleles in a heterozygous pair are fully and simultaneously expressed in the phenotype. A classic example is the human ABO blood group. The gene 'I' has three alleles: IA, IB, and i. Alleles IA and IB are codominant. If an individual inherits both IA and IB, their blood type is AB, because both the A-type and B-type antigens are produced on the surface of their red blood cells. Neither allele masks the other's expression.

9. What is a test cross and why is it an important experimental tool in genetics?

A test cross is an experimental cross where an organism showing a dominant phenotype but having an unknown genotype is crossed with an organism that is homozygous recessive for the same trait. Its importance lies in its ability to determine the genotype of the dominant parent. If all offspring from the test cross display the dominant phenotype, the parent must be homozygous dominant. If any offspring display the recessive phenotype, the parent must be heterozygous.


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