Which one of the following statements about alleles is NOT TRUE?
(1) They may occupy different loci in the same chromosome
(2) There may be several at one locus
(3) One may be dominant over another
(4) They may show co-dominance

Understanding Alleles: Their Dominance loci and co-dominance

In the world of genetics, alleles are fundamental to understanding heredity and trait variation. They are the different versions of a gene that determine specific characteristics—like eye color or blood type. However, there’s often confusion about their behavior and location.

Let’s analyze the question:

Which one of the following statements about alleles is NOT TRUE?

1. They may occupy different loci in the same chromosome

2. There may be several at one locus

3. One may be dominant over another

4. They may show co-dominance

✅ Correct Answer: (1) They may occupy different loci in the same chromosome — NOT TRUE

What Are Alleles?

Alleles are different forms of the same gene found at the same locus (specific position) on a pair of homologous chromosomes. Every individual inherits two alleles for each gene—one from each parent.

Alleles can influence how traits are expressed. For example:

• A person with two alleles for brown eyes will have brown eyes.

• A person with one brown eye allele and one blue eye allele might still have brown eyes if the brown allele is dominant.

Why Statement (1) Is NOT TRUE

“They may occupy different loci in the same chromosome.” ❌

This statement is false because alleles of the same gene must occupy the same locus (position)—but on homologous chromosomes, not on the same chromosome.

Here’s why:

• Genes are arranged in a fixed order on chromosomes.

• The same gene locus exists on both copies of a homologous chromosome pair.

• Alleles refer to alternative versions of that gene at the same locus—not scattered across different locations or on the same chromosome.

So, if alleles were found at different loci on the same chromosome, they would no longer be alleles—they’d be entirely different genes.

Why the Other Statements Are TRUE

✅ (2) There may be several at one locus

Yes, this refers to multiple alleles. While any individual only carries two alleles per gene (one from each parent), the population can have many variations of that gene.

Example: The ABO blood group system has three alleles—A, B, and O—but individuals only inherit two.

✅ (3) One may be dominant over another

Absolutely. In classic Mendelian genetics:

• A dominant allele masks the expression of a recessive one.

• For example, in pea plants, the allele for tall (T) is dominant over the short (t) allele.

✅ (4) They may show co-dominance

Correct. In co-dominance, both alleles are expressed equally in the phenotype.

Example: In human AB blood type, both A and B alleles are co-dominant, so both antigens appear on red blood cells.

Summary Table

Final Thoughts

The false statement about alleles is that they can exist at different loci in the same chromosome. In reality, alleles are alternate forms of the same gene found at the same location on homologous chromosomes. Understanding this concept is essential to mastering the principles of genetics, inheritance patterns, and how traits are passed down.

Whether it’s dominance, co-dominance, or multiple alleles, the study of alleles offers insights into the beautiful complexity of biological diversity.