Q.In a monohybrid cross of plants with red and white flowered plants, Mendel got only red flowered plants. On self-pollinating these F1 plants got both red and white flowered plants in 3:1 ratio. Explain the basis of using RR and rr symbols to represent the genotype of plants of parental generation.

In Mendel’s monohybrid cross, the parental generation consisted of true-breeding red-flowered plants and true-breeding white-flowered plants. True-breeding means that when self-pollinated, they always produce offspring identical to themselves — so their genetic makeup must be pure for that trait.

Mendel used the symbol R to represent the dominant allele (red flower colour) and r for the recessive allele (white flower colour). Since the red-flowered parent is true-breeding, it must carry two copies of the dominant allele — hence its genotype is written as RR. Similarly, the white-flowered parent, being true-breeding for the recessive trait, carries two copies of the recessive allele — its genotype is rr. …

To understand why Mendel chose symbols like RR and rr, we need to step back into the world of mid-19th century biology. Before Mendel, most people believed that traits from parents blended together in offspring — like mixing red and white paint to get pink. Mendel, working with pea plants, noticed something different. When he crossed a pure red-flowered plant with a pure white-flowered plant, all the first-generation (F1) offspring had red flowers. No blending occurred. The white colour seemed to disappear entirely.

But when he let these F1 red plants self-pollinate, white flowers reappeared in the next generation (F2) in a clear pattern: roughly three red for every one white. This 3:1 ratio was the key. Mendel reasoned that each plant must carry two “factors” (what we now call genes) for a trait — one inherited from each parent. In the pure red parent, both factors were for red; in the pure white parent, both were for white. He called these factors “dominant” (the one that shows up, red) and “recessive” (the one that hides, white).

Now, why use RR and rr? The letter R stands for the dominant factor (red), and r for the recessive factor (white). The pure red parent has two identical dominant factors, so its genotype is RR. The pure white parent has two identical recessive factors, so its genotype is rr. This notation is not arbitrary — it tells us two crucial things. First, that the plant is “true-breeding” or homozygous: it will always pass on the same factor to all its offspring. Second, it sets up a clear, predictable pattern for the cross.

When RR is crossed with rr, every F1 offspring gets one R from the red parent and one r from the white parent, making them all Rr. Since R is dominant, all F1 plants have red flowers. But notice: the white factor (r) is still present, just hidden. When two Rr plants self-pollinate, the factors combine randomly: RR, Rr, rR, and rr. The first three combinations all produce red flowers (because at least one R is present), while only rr produces white. That gives the 3:1 ratio.

Argue from what would happen if the parents were NOT pure-breeding: if the red parent were Rr instead of RR, it would itself show some white offspring on selfing, meaning it wasn't a stable "true-breeding" starting point at all. The fact that Mendel's parental stocks bred true for generations is exactly what forces their g …