Two people who are both O-types are OO plus OO, so two O-types can only produce O-types of children. Human blood types are hereditary and conform to Mendel’s law of inheritance. the ABO blood group system can be divided into four blood types, AB, B, A and O, which are controlled by three alleles, A, B and O, respectively, of which A and B are controlled by dominant genes and O is controlled by recessive genes. The blood type of a fetus is determined by the combination of genes inherited from half of each parent, for example, if the father’s blood type is O, his gene can only be OO because it is controlled by a recessive gene, and the recessive gene can show O only when both of them are. the gene of a person with A blood type is AA, or AO, and the gene of a person with B blood type is BB or BO heterozygous, so according to Mendel’s law of inheritance, the blood type of a child should Free combination of AB, A, B, and O types are possible. So a child born to a type A and a type B has all four blood types possible.