recessive.awk

function allele_in_parent(allele, parent_alleles) {
	return allele == parent_alleles[1] || allele == parent_alleles[2];
}

/^#/ { print $0; }
/^[^#]/ {
  split($10, mfields, ":");
  split(mfields[1], mother, "/");
  split($11, ffields, ":");
  split(ffields[1], father, "/");
  split($12, cfields, ":");
  split(cfields[1], child, "/");
  if ((child[1] != 0 && child[2] != 0) &&
      ((allele_in_parent(child[1], father) && allele_in_parent(child[2], mother)) ||
       (allele_in_parent(child[1], mother) && allele_in_parent(child[2], father)))) print $0;
}