In a Mendelian monohybrid cross, the F2 generation shows identical genotypic and phenotypic ratios. What does

NCERT Exemplar · Q34

Q. In a Mendelian monohybrid cross, the F2 generation shows identical genotypic and phenotypic ratios. What does it tell us about the nature of alleles involved? Justify your answer.

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Concept understanding — Mendelian Genetics Basics

Mendelian Genetics Basics

Imagine you have a box of coloured beads — red and white. If you pick one bead from the box, you get either red or white. Now imagine that the colour of your eyes, or the shape of your earlobe, is decided by something like that: a tiny "packet" inside your cells that comes in two versions, and you inherit one from each parent. That is the core idea of Mendelian genetics.

The everyday intuition

You have probably noticed that children often look like their parents — same hair colour, same dimples, same height. But they are never exact copies. Why? Because each parent contributes half of the "instructions" for building a child. Those instructions come in pairs, one from mother and one from father. Sometimes one instruction overrides the other; sometimes they blend. Gregor Mendel, a 19th-century monk, figured out the rules by watching pea plants — tall vs short, yellow vs green seeds — and counting what appeared in the next generation.

The precise meaning

Mendelian genetics is the study of how traits are passed from parents to offspring through genes. A gene is a unit of heredity — a stretch of DNA that codes for a specific characteristic, like flower colour. Each gene comes in different versions called alleles. For every gene, you inherit two alleles: one from your mother, one from your father.

If the two alleles are identical, you are homozygous for that trait. If they are different, you are heterozygous. In a heterozygous pair, one allele may be dominant — it shows up in the appearance — and the other recessive — it stays hidden unless both alleles are recessive.

Note

Mendel's key insight was that traits are not blended like paint. Instead, alleles remain separate and are passed on intact. A recessive allele can skip a generation and reappear later, unchanged.

Why it matters

Mendelian genetics is the foundation of modern biology. It explains:

Why some diseases run in families (like cystic fibrosis or sickle-cell anaemia)
How plant and animal breeders create new varieties
Why you might have your grandmother's eyes but not your mother's

The NCERT textbook states that Mendel's work established the laws of inheritance — the Law of Dominance, the Law of Segregation, and the Law of Independent Assortment. These laws describe how alleles separate during the formation of eggs and sperm, and how different genes are inherited independently of one another.

Key terms at a glance

Gene: a unit of heredity on a chromosome
Allele: a variant form of a gene
Dominant: the allele that expresses itself even when paired with a different allele
Recessive: the allele that expresses itself only when paired with an identical recessive allele
Homozygous: having two identical alleles for a gene
Heterozygous: having two different alleles for a gene …

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When the F2 generation of a monohybrid cross shows identical genotypic and phenotypic ratios, it indicates that the alleles involved exhibit either incomplete dominance or co-dominance, meaning the heterozygous genotype produces a distinct phenotype.

Mendelian genetics, as established by Gregor Mendel, forms the bedrock of our understanding of heredity. A monohybrid cross involves studying the inheritance of a single trait by crossing two true-breeding parents that differ in that trait. For instance, crossing a true-breeding tall pea plant with a true-breeding dwarf pea plant. The first filial (F1) generation consists of hybrids, and when these F1 individuals are self-pollinated, they produce the second filial (F2) generation.

Under the principle of complete dominance, which Mendel primarily observed and described, one allele completely masks the expression of the other in the heterozygous state. In a monohybrid cross exhibiting complete dominance, the F1 generation would uniformly express the dominant trait. When these F1 individuals are self-pollinated, the F2 generation typically shows:

A phenotypic ratio of 3:1 (three individuals expressing the dominant trait for every one expressing the recessive trait).
A genotypic ratio of 1:2:1 (one homozygous dominant, two heterozygous, and one homozygous recessive).

Notice that in complete dominance, these ratios are not identical. The reason for this difference is that the heterozygous genotype (e.g., Tt for tallness) expresses the same phenotype (tall) as the homozygous dominant genotype (TT). This means two different genotypes (TT and Tt) result in the same observable trait, leading to the 3:1 phenotypic ratio despite the 1:2:1 genotypic distribution.

However, the question describes a scenario where the F2 generation shows identical genotypic and phenotypic ratios. This is a crucial deviation from typical Mendelian complete dominance and tells us something fundamental about how the alleles interact.

Important

For the genotypic and phenotypic ratios to be identical in the F2 generation of a monohybrid cross, each distinct genotype must correspond to a distinct phenotype.

This condition is met when the alleles involved exhibit either incomplete dominance or co-dominance.

Incomplete Dominance: In this case, neither allele is completely dominant over the other. The heterozygous individual expresses an intermediate phenotype that is distinct from both homozygous parents. A classic example is the snapdragon flower colour, where a cross between a true-breeding red flower (RR) and a true-breeding white flower (rr) produces F1 offspring that are all pink (Rr). When these pink F1 plants are self-pollinated, the F2 generation will show:
- 1 Red (RR) : 2 Pink (Rr) : 1 White (rr) as the genotypic ratio.
- 1 Red : 2 Pink : 1 White as the phenotypic ratio. Here, the genotypic ratio (1:2:1) is identical to the phenotypic ratio (1:2:1) because the heterozygous genotype (Rr) produces a unique, intermediate phenotype (pink) that is distinguishable from both homozygous phenotypes (red and white). …

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Q.In a Mendelian monohybrid cross, the F2 generation shows identical genotypic and phenotypic ratios. What does it tell us about the nature of alleles involved? Justify your answer.

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