In general, type O blood is recessive, so both of the person's parents would have had to had type O, and the person would be "OO". Heterozygous for type B blood could mean either "AB" or "BO". So if the spouse is "AB" each of their children would have an equal chance of being "AO" or "BO". If the spouse is "BO" each of their children would have an equal chance of being "BO" or "OO". To get an accurate answer, you'd have to factor in the existing proportions of "AB" and "BO" blood types. Apply those ratios to the otherwise equal chances of "AO" and "BO" for a "AB" parent, and "BO" and "OO" for a "BO" parent. For example, if the proportion of "AB" to "BO" in the general population is 1 in 4, the results would be as follows: From a "AB" parent with an "OO" spouse: — "AO" 12.5% — "BO" 12.5% From a "BO" parent with an "OO" spouse: — "BO" 37.5% — "OO" 37.5% Since there would be two ways to have a child with "BO" blood, the final ratios would be: — "AO" 12.5% — "BO" 50.0% — "OO" 37.5% Of course the exact answers would depend on the actual proportions of "AB" and "BO" in the general population. However, it might be hard to separate the "BO"s from the "BB"s in the general population to determine the "BO" frequency. They'd both test as "B" because the "O" in "BO" would be masked. One in four is just used as an example for the calculation.