In a divorce proceeding, a husband (type O) claims that his wife (type A) must have been unfaithful, since their child is type O. Could he be the biological father

Yes, he could possibly be the biological father of the child.

Explanation:

Blood grouping system is a system of grouping individuals into different blood groups, and the most widely used grouping system of medical importance is the ABO blood grouping system, which classifies individuals into blood groups A, B, AB or O, and the ABO blood groups are hereditary.

In the ABO blood group, blood groups alleles A and B are dominant, while O is recessive, so the genotypes of the various groups will look like this;

Group Genotype

A AA or AO

B BB or BO

AB AB

O OO

In this case, a father with group O must have the genotype 'OO' and a wife with type A, can either have genotype 'AA' or 'AO'. If their child is type 'O' it means that the child's genotype is 'OO', hence, among the possible occurrence of the mother, the one that validates this result is the mother having a genotype of 'AO', because when this is crossed with the father's genotype of 'OO' the result below will be gotten

O O (father)

(mother)

A AO AO

O OO OO

meaning that out of four offsprings, there is a 2 in 4 (50%) chance of getting an offspring that is genotype AO (group A) and a 50% chance of giving birth to a child with genotype 'OO' (group O), hence, there could be a possibility of the man being the biological father of the child.