Orthologs are homologous sequences that descended from a common ancestral sequence after a speciation event separated two species. Paralogs are homologous sequences related through a gene duplication event in a common ancestor. Xenologs are homologous sequences resulting from horizontal gene transfer between two organisms. The document discusses these three types of sequence homology - orthologs, paralogs, and xenologs - which arise from different evolutionary events involving speciation, duplication, and horizontal transfer of genes.

1. Orthologs,
Paralogs, &
Xenologs
Muhammad Osama Zafar
BS-BioTech (4th – Semester)
Gulab Devi Educational Complex
(GDEC), Lahore
CourseTitle:
Molecular Evolution

2. Sequence Homology
 Sequence homology is the biological homology between DNA,
RNA or Protein Sequences
 Defined in terms of shared ancestry in the evolutionary
history of life
 Homlogous genes(Homologues) are derived from a common
ancestors
 Homology among DNA, RNA, or proteins is typically inferred
from their nucleotide or amino acid sequence similarity.

3. Types of Sequence Homology
 Significant similarity is strong evidence that two
sequences are related by evolutionary changes from a
common ancestral sequence.
 Alignments of multiple sequences are used to indicate
which regions of each sequence are homologous.
 Two segments of DNA can have shared ancestry because of
three phenomena:
 a Speciation event (ORTHOLOGS)
 a Duplication event (PARALOGS)
 a horizontal(lateral) gene transfer event (XENOLOGS)

4. Orthologs
 Homologous sequences are orthologous: if they are inferred
to be descended from the same ancestral sequence separated
by a speciation event: when a species diverges into two
separate species
 The copies of a single gene in the two resulting species
are said to be orthologous.
 Orthologs, or orthologous genes, are genes in different
species that originated by vertical descent from a single
gene of the last common ancestor.
 The term "ortholog" was coined in 1970 by the Molecular
Evolutionist Walter Fitch.

5. Example (ORTHOLOGS)
EyeLess Gene: Controls the Eye development & formation

6. Paralogs
 Paralogous genes are genes that are related via duplication
events in the last common ancestor (LCA) of the species
being compared.
 They result from the mutation of duplicated genes during
separate speciation events.
 When descendants from the LCA share mutated homologs of the
original duplicated genes then those genes are considered
paralogs
 These are the homologous genes in one species derived from
the gene duplication

7. Example (PARALOGS)
Drospophilla

8. Xenologs
 Homologs resulting from horizontal gene transfer between
two organisms.
 Xeno is a Greek word that means “Foreigner ”.
 Can have different functions if the new environment is
vastly different for the horizontally moving gene.
 In general, though, xenologs typically have similar
function in both organisms.
 The term was coined by Walter Fitch
 Results from a Horizontal Gene Transfer(HGT) between two
organisms. A direct transfer between two species.

9. Example (XENOLOGS)

10. Summary