Why are some tissues more vulnerable to inhibition of PDC activity, and how does this selective tissue vulnerability explain the cardinal clinical manifestations of the disease?

Some tissues are more vulnerable to PDC deficiency as they have high demand of oxygen and ATP like brain cells or neurons and muscles. Due it's deficiency both oxygen and ATP demand is not fulfilled and these cells show signs earlier then other cells

Homozygous B means that the female posses 
Heterozygous A means that the male posses  (A and B are both dominant while O is recessive to the two so if it is heterozygous A or B then O must be the other allele.

View the Punnett Cross below:

Thus based on the cross, there is a 50% chance that the offspring would be blood type AB and 50% chance the offspring would be blood type B.

Note: the blood type is the superscript on the base I.