def summed_chr(chr_length_dict):
    """Create a dictionary where each value is the cumulative sum of all bp in each chromosomes

    Parameters
    ----------
    chr_length_dict : dict
        A dictionary describing the length of each chromosome. 

    Returns
    -------
    dict
        A dictionary where each value corresponds to the cumulative sum of the previous chromosomes lengths. 
    """
    
    chrom_list = ['I', 'II', 'III', 'IV', 'V', 'VI', 'VII', 'VIII', 'IX', 'X', 'XI', 'XII', 'XIII', 'XIV', 'XV', 'XVI']
    summed_chr_length_dict = {}
    summed_chr_length = 0
    for c in chrom_list:
        summed_chr_length_dict[c] = summed_chr_length
        summed_chr_length += chr_length_dict.get(c)    
     
    return summed_chr_length_dict