Mass of a protein

[buran]

aa_mass = {
    'A': 71.03711,   'G': 57.02146,   'M': 131.04049, 'S': 87.03203,
    'C': 103.00919,  'H': 137.05891,  'N': 114.04293, 'T': 101.04768,
    'D': 115.02694,  'I': 113.08406,  'P': 97.05276,  'V': 99.06841,
    'E': 129.04259,  'K': 128.09496,  'Q': 128.05858, 'W': 186.07931,
    'F': 147.06841,  'L': 113.08406,  'R': 156.10111, 'Y': 163.06333
}

gen_id = input('Enter gen ID:')
sequence = input('Enter amino acid sewuence:')
mass=sum(aa_mass.get(aa.upper(), 0) for aa in sequence)
print(f"Protein {gen_id} has a mass of {mass:.2f} Da.") # better use f-string

if you prefer ordinary loop instead of list comprehension

mass = 0
for aa in sequence:
    mass += aa_mass.get(aa.upper(), 0)