text: user3119: I'm having a bit of confusion about how can I retrieve all the genes that are associated to a specific gene ontology.
  Lets say I need to find all the genes that are associated with this GO entry:
  
  GO:0030098 lymphocyte differentiation
  
  This is the graph representing the parents of my GO. In the same page I find the Child Terms that represent the children of my term.
  Now, I can download the goa file (related to the organism I'm interested into) containing the GO associations and grep my term to extract a list of associated genes but, as far as I can understand, this list does not represent the whole graph that actually describe my GO term. 
  Should I also get the genes in the childred terms of my term? Or in the parents terms? If yes, then again I have to go back in the graph and find the parents/children of the found parents/children and so on, till the whole graph is visited.
  Is this the correct approach? Or should I only focus on those genes related to my GO:0030098, and that's it?
  I found this:
  Retrieving a list of human genes having GO associations
  and this but I think they don't address my question since they do not describe the parent-children association.
  Also I found this, on the GO FAQ, but what I need to do is quite the opposite.
  Summarising: I need to find the genes associated to a specific GO term. Should I also look for its parents or children or not?
  Any clues?
  
  user1332: You can use BioMart to filter by your GO term and get the genes as attributes. BioMart is ontology-aware so will pull out all genes associated with your term and with any of its child terms.
  There is no need to look up the GO child terms as BioMart already deals with this. A term may have more than one parent term, but if it's associated with a gene then everything about that term is associated with the gene. The term is not associated with the gene via parent X or parent Y, it means that both are relevant. So if the child term is associated with the gene, then the parent term is by proxy.
  If you're not already familiar with BioMart, there's a help video.
  Here's a BioMart query that gets you exactly what you need in human using the web interface
  If you want to work with the R interface to biomaRt, here's some documentation and this is the identical query to above:
  library(biomaRt)
  ensembl=useEnsembl(biomart="ensembl", dataset='hsapiens_gene_ensembl')
  getBM(attributes = c('ensembl_gene_id','external_gene_name', 'go_parent_term', 'name_1006'), 
        filters = 'go', 
        values = 'GO:0030098', 
        mart = ensembl)
  
  
original_text: 3119: <p>I'm having a bit of confusion about how can I retrieve all the genes that are associated to a specific gene ontology.</p>
  
  <p>Lets say I need to find all the genes that are associated with this GO entry:</p>
  
  <blockquote>
    <p>GO:0030098 lymphocyte differentiation</p>
  </blockquote>
  
  <p><a href="https://www.ebi.ac.uk/QuickGO/GTerm?id=GO:0030098" rel="nofollow noreferrer">This</a> is the graph representing the parents of my GO. In the same page I find the <strong>Child Terms</strong> that represent the children of my term.</p>
  
  <p>Now, I can download the goa file (related to the organism I'm interested into) containing the GO associations and <code>grep</code> my term to extract a list of associated genes but, as far as I can understand, this list does not represent the whole graph that actually describe my GO term. </p>
  
  <p>Should I also get the genes in the childred terms of my term? Or in the parents terms? If yes, then again I have to go back in the graph and find the parents/children of the found parents/children and so on, till the whole graph is visited.</p>
  
  <p>Is this the correct approach? Or should I only focus on those genes related to my GO:0030098, and that's it?</p>
  
  <p>I found this:
  <a href="https://bioinformatics.stackexchange.com/questions/2533/retrieving-a-list-of-human-genes-having-go-associations">Retrieving a list of human genes having GO associations</a>
  and <a href="https://www.biostars.org/p/52101/" rel="nofollow noreferrer">this</a> but I think they don't address my question since they do not describe the parent-children association.</p>
  
  <p>Also I found <a href="http://www.geneontology.org/faq/how-can-i-programmatically-get-list-go-terms-associated-gene-identifier" rel="nofollow noreferrer">this</a>, on the GO FAQ, but what I need to do is quite the opposite.</p>
  
  <p>Summarising: I need to find the genes associated to a specific GO term. Should I also look for its parents or children or not?</p>
  
  <p>Any clues?</p>
   1332: <p>You can use <a href="http://www.ensembl.org/biomart/martview" rel="nofollow noreferrer">BioMart</a> to filter by your GO term and get the genes as attributes. BioMart is ontology-aware so will pull out all genes associated with your term and with any of its child terms.</p>
  
  <p>There is no need to look up the GO child terms as BioMart already deals with this. A term may have more than one parent term, but if it's associated with a gene then everything about that term is associated with the gene. The term is not associated with the gene via parent X or parent Y, it means that both are relevant. So if the child term is associated with the gene, then the parent term is by proxy.</p>
  
  <p>If you're not already familiar with BioMart, there's a <a href="https://www.youtube.com/watch?v=QvGT2G0-hYA" rel="nofollow noreferrer">help video</a>.</p>
  
  <p><a href="http://www.ensembl.org/biomart/martview/dc43680a090b326feaf65bec5d81dcef?VIRTUALSCHEMANAME=default&amp;ATTRIBUTES=hsapiens_gene_ensembl.default.feature_page.ensembl_gene_id|hsapiens_gene_ensembl.default.feature_page.external_gene_name|hsapiens_gene_ensembl.default.feature_page.go_id|hsapiens_gene_ensembl.default.feature_page.name_1006&amp;FILTERS=hsapiens_gene_ensembl.default.filters.go_parent_term.%22GO:0030098%22&amp;VISIBLEPANEL=resultspanel" rel="nofollow noreferrer">Here's a BioMart query that gets you exactly what you need in human using the web interface</a></p>
  
  <p>If you want to work with the R interface to biomaRt, <a href="https://bioconductor.org/packages/release/bioc/vignettes/biomaRt/inst/doc/biomaRt.html#retrieve-all-entrezgene-identifiers-and-hugo-gene-symbols-of-genes-which-have-a-map-kinase-activity-go-term-associated-with-it." rel="nofollow noreferrer">here's some documentation</a> and this is the identical query to above:</p>
  
  <pre><code>library(biomaRt)
  ensembl=useEnsembl(biomart="ensembl", dataset='hsapiens_gene_ensembl')
  getBM(attributes = c('ensembl_gene_id','external_gene_name', 'go_parent_term', 'name_1006'), 
        filters = 'go', 
        values = 'GO:0030098', 
        mart = ensembl)
  </code></pre>
  