Occasionally, a single gene may express more than one effect. The phenomenon is called: (a) multiple allelism

NCERT Exemplar · Q12

Q. Occasionally, a single gene may express more than one effect. The phenomenon is called:

(a) multiple allelism

(b) mosaicism

(d) polygeny

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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 a single gene influences multiple, seemingly unrelated traits, the phenomenon is called pleiotropy.

To understand pleiotropy, you first need a clear picture of how genes normally work. In Mendelian genetics, we often think of a one-to-one relationship: one gene controls one trait, like seed colour or flower position. But nature is rarely that simple. A gene does not exist in isolation — it codes for a protein, and that protein may play a role in different parts of the body or at different stages of development. When a single gene affects two or more distinct phenotypic traits, that is pleiotropy.

The word itself comes from Greek — pleion meaning "more" and tropos meaning "way" or "effect". So literally, "many effects". This is exactly what happens: one gene, many outcomes.

A classic example from your NCERT textbook is the gene for sickle-cell anaemia. The same genetic mutation (a change in the beta-globin gene) causes the red blood cells to become sickle-shaped, leading to anaemia. But that same mutation also confers resistance to malaria. So one gene influences both blood cell shape and disease resistance — two very different traits. That is pleiotropy in action.

Another textbook example is the gene for phenylketonuria (PKU). A single defective gene leads to the inability to metabolise the amino acid phenylalanine. This one defect causes multiple effects: mental retardation, reduced hair and skin pigmentation, and even a musty odour in urine. Again, one gene, many effects.

Now, why is pleiotropy different from the other options in the question?

Note

Multiple allelism refers to a gene having more than two allelic forms in a population (like the ABO blood group system, where three alleles — I^A, I^B, and i — determine blood type). That is about variation at a single locus, not about one gene affecting multiple traits.

Mosaicism is a condition where an individual has cells with different genetic makeups, often due to mutations occurring after fertilisation. It is not about a single gene having multiple effects.

Polygeny (or polygenic inheritance) is the opposite of pleiotropy. Here, multiple genes (often many) contribute to a single trait, like skin colour or height in humans. So instead of one gene → many traits, it is many genes → one trait. …

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Q.Occasionally, a single gene may express more than one effect. The phenomenon is called:

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