Gregor Mendel and His Principles of Inheritance

Gregor Johann Mendel, an Austrian monk (1822-1884), was the father of genetics. He has set down the foundation for understanding heredity with his breakthrough work with pea plants. He has done experiments to establish that the traits of parents are transferred to their offspring in a predictable manner, and nowadays genetics is being based on his study. He formulated three main principles: the Law of Segregation, the Law of Independent Assortment, and the Law of Dominance.

Mendel experimented using Pisum sativum, or pea plants, studying the characteristics over generations. With his quantitative method, he could conclude that every character is controlled by a pair of factors (now called genes) and in gamete formation, members of a gene pair separate from each other. His primary work, Experiments on Plant Hybridization, was presented in 1865 in the Proceedings of the Natural History Society of Brünn. Whereas Mendel's results were largely unknown at the time of its discovery, three biologists rediscovered Mendel's work in the year 1900: Hugo de Vries (of the Netherlands), Carl Correns (of Germany), and Erich Tschermak (of Austria). These rediscovered findings verified Mendel's outcomes and gave rise to classical genetics.

Experiments

For his experiment, Mendel preferred Pisum sativum, which is easy to culture, has a short life cycle, and can self-fertilize. He described seven visible traits in the pea plants and used those that were purely qualitative natures-of-character: traits that may be easily classified into distinct classes of phenotype, such as flower color, seed shape, and stem length. So, part of Mendel's success came from this carefully selected trait.

The Seven Traits Examined by Mendel:

1. Stalk height: Tall (dominant) vs. dwarf (recessive)

2. Placement of flowers: Axial (dominant) vs. terminal (recessive)

3. Color of flower: Violet (dominant) vs. white (recessive)

4. Pod shape: Inflated (dominant) vs. constricted (recessive)

5. Pod color: Green (dominant) vs. yellow (recessive)

6. Seed color: Yellow (dominant) vs. green (recessive)

7. Shape of seed: Round (dominant) vs. wrinkled (recessive)

Each of these characters expressed a unique pattern of inheritance, in which one character was dominant to the other. Mendel proved that traits were governed by factors-or what we now call alleles-that came in pairs: one from each parent. In gamete formation, these factors segregate so that each parent contributes only one allele for every characteristic.

Genes and Alleles

A gene refers to a part of DNA whose function is to determine a particular trait in an organism. The term "gene" was coined by Danish botanist Wilhelm Johannsen in 1909. Mendel's factors are terms now known as genes, which possess the information necessary to express particular traits. The exact location of a gene on a chromosome is termed as the locus.

There are many different forms of a gene, collectively known as alleles. For instance, there might be a gene controlling the color of a flower, one version being an allele for red and another for white. These alleles will occupy the same places on homologous chromosomes. Although each of these alleles controls the same trait, they will have different effects: red vs. white flowers.

Alleles result from gene mutations that change the DNA sequence of a gene. These mutations can lead to several effects such as:

1. The generation of a normal product.

2. The production of a more or less efficient product.

3. The generation of a non-functional product.

4. No product formation at all

Alleles that occur in more than 1% of a population are termed wild-type alleles, while allelles that occur in less than 1% are termed mutant alleles. Typically, wild-type alleles govern common traits of a population, while mutant alleles determine rare traits.

Dominance Relationships

The wild-type and mutant alleles interact with each other into three categories:

1. Complete dominance: One allele completely overshadows the effect of the other allele.

2. Incomplete dominance: Neither allele completely dominates the other, and the phenotype of the offspring is intermediate.

3. Codominance: Neither of the two alleles has any dominance over the other; they express equal to each other in phenotype.

In pea plants of Mendel, dominance was complete for the characteristics studied. For instance, tallness (T) had dominance over dwarfism (t), so a plant with genotype Tt would be tall.

Genotype and Phenotype

The genotype of an organism refers to its genetic constitution or specific combination of alleles it carries. Three types of genotypes are likely for a diploid organism as follows:

Homozygous dominant, such as TT

Homozygous recessive, such as tt

Heterozygous, such as Tt

Therefore, phenotype is the overall observable characteristics of an organism resulting from its genotype. Thus, for example, TT and Tt genotypes should yield the same tall plant, but only tt would give rise to a dwarf plant.

The genotype-to-phenotype relationship needs to be understood to observe inheritance of traits and how these manifest themselves. However, though a genotype directly decides on a specific trait, it is also subject to certain influences from environmental factors concerning some aspects.

Mendel's Laws of Inheritance

Mendel's inheritance laws are the fundamental principles applicable to sexually reproducing eukaryotic organisms. Two important laws were formulated by him based on his experiments of hybridization with Pisum sativum, which is popularly known as pea plants.

Law of Segregation

The law of segregation, based on Mendel's monohybrid cross experiments, explains how the alleles separate during gamete formation. In a monohybrid cross, only one trait such as height is considered. The individual carries two alleles for a specific character. During gamete formation, these alleles separate, so that each gamete carries only one allele. This means that gametes are pure as far as their genetic content is concerned, and hereditary factors do not blend.

Example: In Mendel's cross between tall and dwarf pea plants, the allele for tallness, T, is dominant and the allele for dwarfness, t, is recessive. When a true-breeding tall plant (TT) was crossed with a true-breeding dwarf plant (tt), the F1 progeny were heterozygous tall, but all of these F1 plants looked tall because of the dominance of "T". When these F1 plants were self-crossed, the F2 generation showed a 3:1 phenotypic ratio of tall to dwarf plants. The reappearance of dwarf plants in the F2 generation indicates that alleles also assort independently during gamete formation.

Law of Independent Assortment

He used dihybrid crosses in the inheritance of two traits in order to formulate his law of independent assortment. According to this law, different pairs of alleles segregate independently of each other during gametogenesis. Thus, through independent assortment, new combinations of traits are exhibited in offspring.

Example: In Mendel's dihybrid cross between plants producing yellow, round seeds (YYRR) and plants producing green, wrinkled seeds (yyrr), the F1 generation was entirely made of heterozygous plants (YyRr) that were all yellow and round. When those F1 plants were self-fertilized, the F2 generation exhibited a phenotypic ratio of 9:3:3:1, comprising yellow-round, green-round, yellow-wrinkled, and green-wrinkled seeds. This proves that seed color segregates independently of seed shape.

Incomplete Dominance and Codominance

Two other patterns of inheritance that do not follow Mendel's laws are incomplete dominance and codominance.

Incomplete Dominance

Incomplete dominance is when one allele does not dominate over the other allele. Instead, the phenotype of the heterozygote falls in the middle between the two phenotypes of the two homozygotes. In this case, the genotypic and phenotypic ratios are the same as well.

Example: When the red-flowered Mirabilis jalapa, also known as the Four O'clock plant is crossed with a white-flowered one, then the resulting plants are pink-flowered. This is because the allele that controls red flower colour is not wholly dominant to the white allele; so when the heterozygous plants are obtained, they express an intermediate phenotype of pink flowers. When these pink plants are allowed to self-cross, the F2 generation has the following phenotypic ratio: 1 red : 2 pink: 1 white. There will be a corresponding genotypic ratio of 1:2:1.

Conclusion

When codominance does exist, the two alleles of a heterozygous organism are fully expressed in the phenotype. The intermediate phenotype in incomplete dominance is not characteristic of codominance because it expresses the coexistence of both alleles simultaneously.

Example: In humans, the MN blood group system is codominant. A person with genotype MM expresses only the M antigen, those with genotype NN express only the N antigen, and MN expresses both M and N antigens equal. Thus the phenotypic effect of each allele is distinct and visible in heterozygous individuals.

Test Cross

A test cross is a genetic tool used to determine the genotype of an individual displaying a dominant phenotype whose genotype is unknown. The test cross is carried out by crossing the individual with a homozygous recessive, and then, when the phenotypes of the offspring are observed, the genotype of the parent whose genotype needs to be determined may be deduced.

Case 1: If a tall pea plant of unknown genotype is crossed with a dwarf variety (tt) and only tall offspring are produced, then the tall parent plant is homozygous (TT)

No scenerio 2: When the tall and the dwarf offspring appear simultaneously, then the tall parent variety is heterozygous because it expresses both kinds of gametes.

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