血型

ABO gene versions	Blood type
AA	A
BB	B
AO	A
BO	B
AB	AB
OO	O

Both A and B alleles (基因) are dominant over O. As a result, individuals who have an AO genotype will have an A phenotype. People who are type O have OO genotypes.

www2.palomar.edu/anthro/blood/ABO_system.htm

Genotype: the genetic constitution of an individual organism

Phenotype: the set of observable characteristics of an individual resulting from the interaction of its genotype with the environment

話說姐姐的血型是 O pos，而父母都是 B pos，所以父母的 genotype 一定是 BO.