The final 6 volt systems remark in error. One of the most popular cars of the 1960s, the Volkswagen Beetle, continued to use 6 volt systems until 1967.
David took a 12 volt/6 cell battery out of the 1970s Chevrolet Blazer, and replaced it with a 6 volt/3 cell battery. A 6 volt system can usually be used with a 12 volt battery in an emergency, but not the opposite. Only vehicles made before the '60's use 6 volt electrical systems.
In the letter left by Barbara, she says protection from the solar flares is inherited via a gene which is "probably recessive". For children to inherit a genetic attribute possessed by only one parent, it would have to be dominant, not recessive.