Complex sentence: If both parents are carriers, there is a 25% chance the child will have BGS, a 50% chance the child will have one mutant copy and be asymptomatic and a 25% chance the child will be asymptomatic and not a carrier.
Split into simpler sentences: If both parents are carriers, there is a 25% chance the child will have BGS. There is also a 50% chance the child will have one mutant copy and be asymptomatic and a 25% chance the child will be asymptomatic and not a carrier.