def isHeteroplasmy(variant, depth_min = 40, depth_strand = 0, depth_ratio_min = 0.0, freq_min = 0.01):
	""" 
	determine whether a variant is a heteroplasmy according to the filers specified
	
	Attributes
	----------
	variant: MTVariant
	depth_min: the minimum depth
	depth_strand: the minimum depth on either the forward and the reverse strand
	depth_ratio_min: the minimum ratio of reads passing QC
	freq_min: the minimum minor allele frequency
	
	Returns
	----------
	True: is a heteroplasmy
	False: not a heteroplasmy
	None: does not pass QC
	
	"""
	if (variant.depth_qc >= depth_min and variant.depth_fwd >= depth_strand and variant.depth_rev >= depth_strand and variant.depth_ratio >= depth_ratio_min):
		if (variant.alt_freq >= freq_min):
			return True
		else:
			return False
	else:
		return None