Why is a parent usually a better match for transplanted tissue to a donor than a random individual of the same species?

For decreased chances of rejection

Explanation:

The three higly polymorphic MHC 1 genes in human beings are HLA-A, HLA-B and HLA-C.

These determine the compatibility of the organ or tissue in the recipient's body each by the help of many alleles that segregates in a population.

There are very less chances that a random chosen donor will match the a recipient six allele genotype.

This is the reason parent  may be the best donor for organ transplantation or tissue transplantation.

b - the respond is innate and varies over time.

good luck!