Biology · Principles of Inheritance and Variation · NEET
Pleiotropy means one single gene controls many different traits. Normally we think one gene gives one trait. But sometimes a single gene changes several body features at once. That single gene is called a pleiotropic gene. This is important for NEET because it is often asked in match-the-column questions.
Pleiotropy is ONE gene affecting MANY traits. Do not mix it up. It is the opposite of polygenic inheritance, which is MANY genes controlling ONE trait (like human skin colour). NEET loves to swap these two, so remember: pleiotropy = one gene, plenty of traits.
NCERT gives phenylketonuria (PKU) as the example of pleiotropy in humans. One gene that codes for the enzyme phenylalanine hydroxylase is mutated. This single mutated gene causes several problems together: mental retardation, and reduced hair and skin colour (lighter hair and skin). So one gene = many traits.
Because that one gene usually controls a metabolic pathway. A metabolic pathway is a chain of chemical steps in the body. When the gene is faulty, the whole pathway is disturbed. That disturbed pathway then affects many different body features (phenotypes) at the same time. This is the mechanism NCERT states for most pleiotropy cases.
A normal single gene we study affects only one trait or phenotype (like seed shape or flower colour in Mendel's peas). A pleiotropic gene is special because its effect is not limited to one feature. NCERT says: we usually see a gene affect a single trait, but there are instances where a single gene shows multiple phenotypic expression.
The phenomenon of pleiotropism refers to
Match the following: Column I: (a) Dominance (b) Codominance (c) Pleiotropy (d) Polygenic inheritance Column II: (i) Many genes govern a single character (ii) In a heterozygous organism only one allele expresses itself (iii) In a heterozygous organism both alleles express themselves fully (iv) A single gene influences many characters
Match List I with List II: List I: A. Incomplete dominance B. Co-dominance C. Pleiotropy D. Polygenic inheritance List II: I. Human skin colour II. Inheritance of flower colour in Antirrhinum sp. III. Phenylketonuria disease in humans IV. ABO blood groups
Try the real previous-year questions from this chapter — each with the answer and a full solution.
A pleiotropic gene is a single gene that controls or affects many different traits (phenotypes) at the same time, instead of just one.
Phenylketonuria (PKU) in humans is the NCERT example. Sickle cell anaemia is also a well-known pleiotropy example, but for NEET, phenylketonuria is the one directly given in the NCERT text.
In most cases the pleiotropic gene controls a metabolic pathway. When the gene is faulty, the whole pathway is disturbed, and this affects several different phenotypes at once.
No. Pleiotropy is one gene affecting many traits. Polygenic inheritance is many genes controlling one trait, such as human skin colour. They are opposites and NEET often tests this difference.
The gene that codes for the enzyme phenylalanine hydroxylase is mutated. This single mutation causes several effects, which is why PKU is a pleiotropy example.
