def is_exon_in_mRNA(gff_in, exon, mRNA):
    """
    Check if the exon is in mRNA based on genomic coordinates.

    Arguments:

    - exon: gff record of exon
    - mRNA: gff record of mRNA
    """
    mRNA_id = mRNA.get_id()
    if mRNA_id not in gff_in.exons_by_mRNA:
        return False
    
    for curr_exon in gff_in.exons_by_mRNA[mRNA.get_id()]:
        # Exon in mRNA if it has the same coordinates and strand
        # as one of the mRNA's exons
        if curr_exon.start == exon.start and \
           curr_exon.end == exon.end and \
           curr_exon.strand == exon.strand:
            return True
    return False