def estimate_phi(x):
    """ Estimate proportion of genotypable reads.

    Args:
        x (numpy.array): major, minor, and total read counts

    Returns:
        numpy.array: estimate of proportion of genotypable reads.

    """

    phi = x[:,0:2].sum(axis=1).astype(float) / (x[:,2].astype(float) + 1.0)

    return phi