1.What is a blood group? There are many antigen systems on the surface of human red blood cells, and the phenotype of these antigens is the blood type. Different systems have different ways to classify the blood type of red blood cells, for example, we can classify chopsticks into steel chopsticks, wooden chopsticks, bamboo chopsticks, plastic chopsticks, etc. by material, or black chopsticks, red chopsticks, yellow chopsticks, white chopsticks, etc. by color, and long chopsticks, medium chopsticks, and short chopsticks by length. And there are different categories in each antigen system, such as steel and wood, black and red, long and short in the above classification. The ABO blood group that we often talk about is one of these systems and is the most commonly used one. 2. What is an allele? Human genes exist in pairs. Alleles generally refer to a pair of genes located at the same position on a pair of homologous chromosomes that control relative traits, like a pair of chopsticks. If two members of a pair of alleles are identical, the individual is pure for this trait. The ABO blood grouping system is based on the presence of the A or B antigen at the allele. If both genes of the allele contain the A antigen gene or one contains the A antigen gene and the other is null, we call it A blood type; if both genes of the allele contain the B antigen gene or one contains the B antigen gene and the other is null, we call it B blood type; if both points of the allele are null, we call it O blood type; if one contains the A antigen gene and the other contains the B antigen gene, we call it AB blood type. We compare the colors of chopsticks black, red and white to the alleles of ABO blood group with A, B and O. Then there are six possible combinations of allele loci of ABO blood group, namely AA, AO, BB, BO, OO and AB, as shown in the figure: 3. Each parent’s gene is given half to the child. As mentioned above, all human genes exist in pairs, however, only half of the genes of the father’s sperm cells and the mother’s egg cells are combined to form a pair of genes for the child. The child has no chopsticks to eat with, but when each parent gives one to the child, the child has a pair of chopsticks. This pair of chopsticks is the allele in the child, and the combination in the table below is created. Thus, the relationship between the child’s blood type and the parent’s blood type, excluding phenotypic mutations of very low probability, gives us the following 16 conclusions: Father’s blood type A, mother’s blood type A: the child may be type A, may be type O, and never type B or AB. Father’s blood type A, mother’s blood type B: the child has all four blood types possible. Father’s blood type A, mother’s blood type O: the child may be type A, may be type O, never type B or AB. Father’s blood type A, mother’s blood type AB: the child is never type O, all other blood types are possible. Father’s blood type B, mother’s blood type A: All four blood types are possible for the child. Father’s blood type B, mother’s blood type B: the child may be type B, may be type O, never type A or type AB. Father B, mother O: Child may be B, may be O, never B or AB. Father B, mother AB: The child is never type O, but any other blood type is possible. Father’s blood type O, mother’s blood type A: the child may be type A, may be type O, never type B or type AB. Father O, mother B: The child may be B, may be O, never A or AB. Father O, mother O: The child can only be type O. Father’s blood type O, mother’s blood type AB: the child may be type A, may be type B, never type O or type AB. Father’s blood type AB, mother’s blood type A: The child will never have blood type O. Any other blood type is possible. Father AB, mother B: The child will never have O blood type, but all other blood types are possible. Father’s blood type AB, mother’s blood type O: the child may be type A, may be type B, never type O or type AB. Father’s blood type AB, mother’s blood type AB: the child will never have blood type O, other blood types are possible.