def sequence_sampling_probability(sequence_list):
    """Calculate genome size and the relative sizes of the the sequences in
    the multi-fasta file. Relative sizes are further used as probabilities
    for random sampling."""

    # Calculate genome size
    genome_size = 0
    for f in sequence_list:
        genome_size = genome_size + int(f[1])

    # Calculate the size percentage of each sequence
    probabilities = []
    for f in sequence_list:
        probabilities.append(int(f[1]) / genome_size)

    return (probabilities, genome_size)