This document defines and compares heterologs, homologs, analogs, orthologs, and paralogs. Heterologs differ in origin and activity, while homologs have a common origin but not necessarily common activity. Sequence similarity is a quantitative measure of how many bases match between two aligned sequences. Analogs have common activity but different origins, evolving convergently. Orthologs are homologs that evolved from a common ancestral gene through speciation, often retaining the same function. Paralogs are homologs produced through gene duplication within a genome, and may evolve new functions.

1. Heterologs & Homologs
Course: B.Sc Biochemistry
Subject: Basic of Bioinformatics
Unit: III

2. Heterologs & Homologs
Heterologs :- Heterologs differ in both origin and activity.
• Genes that are "unique" in activity and sequence are said to
be heterologous. Note that genes initially defined as
heterologous by syntax (letter matching) may actually be
homologous by activity.
Homologs :- Homologs have common origins but may or may
not have common activity.
• Genes that share an arbitrary threshold level of similarity
determined by alignment of matching bases are termed
homologous. Homology is a qualitative term that describes a
relationship between genes and is based upon the
quantitative similarity.

3. Sequence Similarity
Similarity is a quantitative term that defines the degree of
sequence match between two compared sequences.
• For example, two aligned genes or segments of sequence
that are homologous may have varying degrees of similarity
based upon identical base matches in the alignment. In the
first sequence alignment in the following figure, the
sequences are obviously identical and therefore exhibit 39
matches out of 39 positions aligned, or 100% similarity. In the
second alignment the aligned sequences contain 28 matches
out of 39 possible. The quantitative match or degree of
similarity is then 28/39 or 72%. In both cases the sequences
are homologous.

4. A
• atgcctgaaggcctattgtttcccagtcgattggctgct...
||||||||||||||||||||||||||||||||||||||| 39 of 39 matches
atgcctgaaggcctattgtttcccagtcgattggctgcg...
B
• atgcctgaaggcctattgtttcccagtcgattggctgct...
|||||| |||||| |||||||| |||||| || 28 of 39 matches
atgccteggcttatattgtatcccagtccattggcagcg...

5. Analogs & Orthologs
Analogs: - Analogs have common activity but not common origin.
• Genes or proteins that display the same activity but lack sufficient similarity to
imply common origin are said to have analogous activity. The implication is that
analogous proteins followed evolutionary pathways from different origins to
converge upon the same activity. Thus, analogous genes or proteins are
considered a product of convergent evolution. Analogs have homologous
activity but heterologous origins.
• Example :- Brain & Computer
Orthologs :- Orthologs are homologs produced by speciation.
• Orthologs are genes in different species that evolved from a common ancestral
gene by speciation. Normally, orthologs retain the same function in the course of
evolution.
OR
• any gene pairwise relation where the ancestor node is a speciation event. Often
have similar function

6. Paralogs
Paralogs: - Paralogous are homologous produced by gene
duplication.
• Paralogs are genes related by duplication within a genome.
Orthologs retain the same function in the course of evolution,
whereas paralogs evolve new functions, even if these are
related to the original one.
OR
• any gene pairwise relation where the ancestor node is a
duplication event. Paralogs tend to have different functions

7. 1.

8. 2.

9. Sequence Alignment – PHYLOGENY
3.

10. Books and Web References
• Books Name :
1. Introduction To Bioinformatics by T. K. Attwood
2. BioInformatics by Sangita
3. Basic Bioinformatics by S.Ignacimuthu, s.j.
• http://en.wikipedia.org/wiki/Sequence_alignment
• http://en.wikipedia.org/wiki/Homology_%28biology%29
• http://en.wikipedia.org/wiki/Heterologous
10

11. Image References
• 1.http://gorbi.irb.hr/files/5712/7497/9729/Slide09.jpg
• 2.http://www.ensembl.org/info/genome/compara/tree_exa
mple1.png
• 3.http://www.nature.com/nature/journal/v496/n7445/image
s/nature12027-f1.2.jpg