Homology
- Homology refers to the study of similarity due to
shared ancestry between a pair of structures
- Structures or genes might have evolved from origin but they
have different functions
- A common example of homology is the hands of human being and
wings of bat
- It is caused due to the continuity of information
- Sequence homology is the similarity between
DNA, RNA or protein sequences based on shared ancestry
Criteria for
being homologous
- Two sequences are homologous if they are more than 30%
identical over its entire length
- Homologous genes can be similar in
sequence
- Similar sequences are not necessarily
homologous
so the sequence need not be identical to be
homologous.
Homologous genes are broadly divided into:
Orthologous genes:
- These are homologous genes that diverged after evolution gives
rise to different species by the process known as
speciation.
- It maintains a similar function to that of the ancestral gene
that they evolved from.
Paralogous genes:
- These are homologous genes that have diverged within
one species.
- These genes are new genes that hold new
function and these genes arise during gene
duplication where one copy of the gene receives a mutation
that gives rise to a new gene with a new function
- The function of these genes is often similar
to the role of ancestral gene
Mutation can also lead to 2 genes having 80% of
their sites homologous and 20% of their sites non-homologous.