Which of the following is a classic example of polygenic inheritance, where a trait is controlled by many genes and shows continuous variation in a population?

Introduction / Context:
Polygenic inheritance occurs when a trait is determined by the cumulative effect of many genes, each contributing a small additive effect. Such traits often show continuous variation, forming a range rather than discrete categories. This question asks which listed example fits polygenic inheritance, an important concept in modern genetics.

Given Data / Assumptions:

• Several traits and conditions are listed, including skin pigmentation, Huntington disease, flower colour in peas, ABO blood groups, and colour blindness.

• We assume basic knowledge of Mendelian and non Mendelian inheritance patterns.

• Polygenic traits typically show many intermediate phenotypes.

• Only one option describes such a continuously varying trait.

Concept / Approach:
Traits like human skin colour, height, and weight are classic polygenic traits and show a wide spectrum of phenotypes due to additive effects of multiple genes and environmental influences. By contrast, Huntington disease is a single gene dominant disorder, purple and white flower colour in peas follow simple Mendelian ratios, ABO blood groups are determined by multiple alleles of one gene, and colour blindness often follows a sex linked single gene pattern. Therefore, the correct approach is to identify which trait shows continuous variation and involves many genes acting together.

Step-by-Step Solution:
Step 1: Consider skin pigmentation in humans. It shows a gradient of shades from very light to very dark, with many intermediates. Step 2: Genetic studies indicate that several genes influence the amount and type of melanin produced, so this trait is polygenic. Step 3: Huntington disease is a monogenic disorder caused by a mutation in a single gene and does not show continuous variation of normal phenotype. Step 4: Flower colour in peas as studied by Mendel involves simple dominant and recessive alleles with discrete phenotypes (purple or white). Step 5: ABO blood group is controlled by multiple alleles of one gene, resulting in four main blood groups rather than a continuous spectrum. Step 6: Colour blindness is usually caused by mutations in single genes on the X chromosome, leading to clear affected or unaffected categories. Step 7: Therefore, skin pigmentation in humans is the only option that clearly represents polygenic inheritance.

Verification / Alternative check:
Genetics textbooks commonly list human skin colour as a textbook example of polygenic inheritance. Population distribution graphs often show a bell shaped curve of skin shades in mixed populations, typical of polygenic traits. Such references do not describe Huntington disease, ABO blood groups, or colour blindness as polygenic; these are treated under patterns such as dominant disorders, multiple alleles, and sex linked traits. This confirms the identification of skin pigmentation as the correct answer.

Why Other Options Are Wrong:

Huntington disease in humans: Caused by a mutation in one gene and inherited as an autosomal dominant trait, not a polygenic trait with continuous variation.

Purple versus white flower colour in peas: A classic Mendelian monogenic trait with clear dominant and recessive phenotypes.

ABO blood groups in humans: Determined by three alleles of one gene, giving distinct groups, not a continuous spectrum.

Colour blindness in humans: Commonly an X linked recessive trait related to single genes affecting colour vision.

Common Pitfalls:
A frequent confusion is between multiple alleles and many genes. Multiple alleles involve different versions of a single gene, while polygenic traits involve many genes each contributing a small effect. Another pitfall is to assume that any trait with more than two phenotypes is polygenic, which is not always true. The key clue for polygenic inheritance is continuous variation rather than a few discrete categories.

Final Answer:
The classic example of polygenic inheritance among the options is Skin pigmentation in humans.