If a mother has blood type plus and the father a- can offspring have o negative blood?

Yes, it is possible for offspring of a mother with blood type positive and a father with blood type A- to have O negative blood. This can be seen in the following scenario:

The mother has the genotype PP (heterozygous for the positive blood type allele)

The father has the genotype AA (homozygous for the A blood type allele)

During meiosis in the mother, the P allele and the p allele (which codes for the O blood type) can segregate and be passed onto the offspring.

Similarly, during meiosis in the father, the A allele can be passed onto the offspring.

If the mother contributes the p allele and the father contributes the A allele, it is possible for them to have offspring with the genotype AO (which results in A- blood type).

If the mother contributes the p allele and the father contributes the A allele, it is possible for them to have offspring with the genotype pO (which results in O negative blood type).

Therefore, although the parents have positive and A- blood types, it is possible for them to have offspring with O negative blood type. This is because the inheritance of blood types follows specific genetic principles and can result in different combinations of alleles in the offspring.