Say both parent and child are genotyped for all SNPs. In this setup we are only looking at variant positions between one parent and child - so neither parent nor child are homozygous reference (no AA/AA) in the millions of positions we are studying.
Assume all variants are biallelic and there is no de novo mutation (no AA/BB or BB/AA)
P C AA AB AB AA AB AB AB BB BB AB BB BB
What is the fraction of AB/AB among these possibilites?
We are looking at all positions in the genome so the minor allele frequency (MAF) of B allele varies. You can choose whatever B frequency you like.