Explain the terms Similarity, Identity and Homology with suitable examples.

Introduction

In bioinformatics and molecular biology, understanding the differences between similarity, identity, and homology is important for interpreting sequence alignment results. These three terms are often used when comparing DNA, RNA, or protein sequences, but they have distinct meanings. This answer explains each term with simple definitions and examples.

1. Similarity

Similarity refers to the degree of resemblance between two sequences. It includes both exact matches and similar (but not identical) characters, especially in protein sequences where different amino acids may have similar properties.

Example:
Consider two protein sequences:
Sequence A: ARKTLM
Sequence B: ARKTLV

In this case, five out of six amino acids are exactly the same, and one (M in A vs V in B) is similar because both are hydrophobic. So, these sequences are highly similar.

Note: Similarity can be expressed as a percentage, such as “the two sequences are 90% similar.”

2. Identity

Identity refers to the exact match between characters in the same position of two sequences. It does not consider whether the differences are chemically similar, only whether they are exactly the same.

Example:
Using the same sequences:
Sequence A: ARKTLM
Sequence B: ARKTLV

Here, five out of six positions match exactly (A, R, K, T, and L), but M ≠ V. So, the identity is 5/6 = 83.3%. This is less than the similarity percentage, which may include similar amino acids.

Identity is usually reported in percentage format and is used to measure how closely two sequences match exactly.

3. Homology

Homology refers to a biological relationship between two sequences or genes, meaning they come from a common ancestor. Homology is a qualitative concept—it either exists or it doesn’t. You do not say “70% homologous” — instead, you say “homologous” or “not homologous.”

Types of Homology:

• Orthologs: Genes in different species that evolved from a common ancestral gene through speciation. They usually retain the same function.
• Paralogs: Genes that arise by duplication within the same species and may evolve new functions.

Example:
The hemoglobin gene in humans and the hemoglobin gene in mice are considered orthologs. They are homologous genes because they descended from a common ancestor and have similar functions.

Summary of Differences

Conclusion

Similarity, identity, and homology are important concepts in sequence comparison. While similarity and identity are numerical values used to describe how closely two sequences match, homology is a biological term that refers to a shared ancestry. Understanding these differences helps researchers interpret sequence alignment results and study genetic relationships between organisms or genes.