4.2. Inheritance of One Gene

Let us take the example of one such hybridisation experiment carried out by Mendel where he crossed tall and dwarf pea plants to study the inheritance of one gene. He collected the seeds produced as a result of this cross and grew them to generate plants of the first hybrid generation. This generation is also called the Filial 1 progeny or the F1. Mendel observed that all the F1 progeny plants were tall, like one of its parents; none were dwarf. He made similar observations for the other pairs of traits – he found that the F1 always resembled either one of the parents, and that the trait of the other parent was not seen in them.

Mendel conducted a hybridization experiment by crossing tall and dwarf pea plants. The offspring from this cross, known as the F1 generation, all showed the tall phenotype. This means that the tall trait was dominant over the dwarf trait. Thus, Mendel concluded that in a trait pair, one trait can express itself while the other remains hidden. This led him to propose the existence of dominant and recessive traits.

Think of a strong tree in a garden overshadowing a smaller bush. Just like the strong tree (tall trait) dominates the view, the tall plant's trait overshadows the dwarf one, preventing it from being visible in the first generation of offspring.

Mendel then self-pollinated the tall F1 plants and to his surprise found that in the F2 generation some of the offspring were ‘dwarf’; the character that was not seen in the F1 generation was now expressed. The proportion of plants that were dwarf were 1/4th of the F2 plants while 3/4th of the F2 plants were tall. The tall and dwarf traits were identical to their parental type and did not show any blending, that is all the offspring were either tall or dwarf, none were of in-between height.

When Mendel self-pollinated the tall F1 plants, he found that the next generation (F2) included some dwarf plants. This revealed that the dwarf trait, although not visible in the F1 generation, was still present genetically. The Dwarf phenotype reappeared in a 3:1 ratio, indicating that the traits segregated independently during reproduction and did not blend together.

Imagine two colors of paint: blue and yellow. If you mix them (like blending traits), you get green (representing a blend of traits). But if you only see blue paint next, it doesn't mean yellow is gone—it's just not on display. The yellow paint can still reappear later, showing that the original colors still exist.

Based on these observations, Mendel proposed that something was being stably passed down, unchanged, from parent to offspring through the gametes, over successive generations. He called these things as ‘factors’. Now we call them as genes. Genes, therefore, are the units of inheritance. They contain the information that is required to express a particular trait in an organism. Genes which code for a pair of contrasting traits are known as alleles, i.e., they are slightly different forms of the same gene.

Mendel theorized that traits are inherited as discrete units, which he referred to as 'factors' (now known as genes). Each gene can have different forms, or alleles, that dictate the expression of traits. For example, a gene determining flower color in pea plants might have one allele for purple and another for white flowers.

Think of genes as instruction manuals for making a dish. Just like a recipe can have variations—such as using different spices—the same goes for genes with different alleles affecting how traits are expressed.

If we use alphabetical symbols for each gene, then the capital letter is used for the trait expressed at the F1 stage and the small alphabet for the other trait. For example, in case of the character of height, T is used for the Tall trait and t for the ‘dwarf’, and T and t are alleles of each other. Hence, in plants the pair of alleles for height would be TT, Tt or tt.

In Mendel's system of nomenclature, a capital letter represents a dominant allele (like T for tall) while a lowercase letter represents a recessive allele (like t for dwarf). The genotype of a plant can thus be homozygous dominant (TT), homozygous recessive (tt), or heterozygous (Tt). The dominant allele expresses its trait over the recessive in hybrids.

Imagine a character in a movie (dominant trait) overshadowing a background character (recessive trait). The main character's traits (T) are seen in the film, while the background character's traits (t) do not appear unless the background character takes center stage.

Mendel also proposed that in a true breeding plant the allelic pair of genes for height are identical or homozygous. TT and tt are called the genotype of the plant while the descriptive terms tall and dwarf are the phenotype. What then would be the phenotype of a plant that had a genotype Tt? Mendel found the phenotype of the F1 heterozygote Tt to be exactly like the TT parent in appearance, he proposed that in a pair of dissimilar factors, one dominates the other and hence is called the dominant factor while the other factor is recessive.

The Law of Dominance states that in a pair of contrasting traits, one (the dominant trait) will always be expressed in the phenotype. For example, a heterozygous plant (Tt) will appear tall, similar to a homozygous dominant plant (TT) due to the dominance of T over t.

Consider an art competition where the primary colors shadow the effects of the pastels. If a bright red and a soft pink are mixed on canvas, the bold red (dominant) takes over the overall look while the pink (recessive) is hardly visible unless a different technique is used.

The production of gametes by the parents, the formation of the zygotes, F1 and F2 plants can be understood from a diagram called Punnett Square. It is a graphical representation to calculate the probability of all possible genotypes of offspring in a genetic cross.

A Punnett Square is a tool used to predict the genotypes of offspring. By placing the possible gametes of each parent along the top and side of a grid, all possible combinations can be calculated, allowing us to determine the probability of each genotype appearing in the offspring.

Using a Punnett Square is like creating a menu for a potluck dinner where you list what each guest will bring. By looking at what everyone is contributing, you can see how many combinations of dishes will likely appear on the dinner table.

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