In classical Mendelian genetics, which example best describes a simple Mendelian trait?

Introduction / Context:
Gregor Mendel studied simple traits in pea plants where a single gene with two forms controlled a clear either or characteristic. Such traits are called Mendelian traits. In humans, some traits follow a similar simple pattern, while many others involve multiple genes or alleles. This question asks you to identify which example fits the classical Mendelian model.

Given Data / Assumptions:

• Mendelian traits are controlled by a single gene with two contrasting alleles.
• They typically show clear dominant and recessive patterns.
• Many human traits are more complex and controlled by multiple genes.

Concept / Approach:
A simple Mendelian trait is one where a single gene determines a trait and the phenotype shows discrete categories. Earlobe attachment is a classic textbook example, where earlobes are described as either attached or free. In contrast, blood type involves multiple alleles, plant height can be polygenic and continuous, and body mass is influenced by many genes plus environmental factors.

Step-by-Step Solution:
Step 1: Recall the definition of a Mendelian trait, involving one gene and two alleles. Step 2: Examine each option to see whether it describes a simple either or pattern. Step 3: Notice that attached versus detached earlobes fits an either or category consistent with one gene. Step 4: Recognise that blood type uses multiple alleles and results in several possible types, not just two. Step 5: Observe that plant height and body mass are continuous traits influenced by many genes and environment.

Verification / Alternative check:
Genetics examples in many school books mention earlobe attachment, tongue rolling and some disease traits as typical Mendelian traits. These traits show simple dominant and recessive inheritance patterns in family pedigrees. On the other hand, blood type is a classic example of multiple alleles and codominance, illustrating concepts that go beyond simple Mendelian inheritance.

Why Other Options Are Wrong:
Option b, blood type in humans, is controlled by three alleles and shows codominance between some of them, so it is not a simple two allele Mendelian case.

Option c, plant height with a wide range of heights, suggests polygenic inheritance where many genes contribute small effects.

Option d, human body mass, depends on multiple genes and environmental influences such as diet and lifestyle, making it a complex trait rather than a simple Mendelian one.

Common Pitfalls:
Students sometimes think that any genetic trait is automatically Mendelian. Another pitfall is forgetting that Mendelian traits show distinct categories, not gradual variation. Learning to distinguish single gene traits from polygenic and multi allele traits is important for understanding more advanced genetics topics.

Final Answer:
A good example of a simple Mendelian trait is that humans either have attached earlobes or detached earlobes.