You can conclude that the father was either heterozygous for that trait or fully homozygous.
White blood cells, also know as macrophages or phagocytes
Use binomial distribution with p=0.25, n=5, x=2
P(X=x)=C(n,x)p^x (1-p)^(n-x)
P(X=2)=C(5,2) 0.25^2 0.75^(5-2)
=10*0.0625*0.421875
=0.26367
So the probability of 2 of 5 children having type-O blood from these parents is 0.26367.