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Cis AB is a rare ABO Blood group antigen genotype which is phenotypically almost same as AB. In Cis AB inheritance A and B antigen seem to be segregating as if they are in cis in very close loci on the same chromosome. These can happen due to mutant alleles at ABO locus (multiple variants are available). Usually it is a single allele making a single enzyme which can make both A and B antigens^[1]. Antigen expression is weaker than A1 or B.

Cis AB versus regular (trans) AB

Inheritance of regular ABO for AB & O parents

Cis AB inheritance, scenario 1.1: in the AB parent, one allele is AB and other is O

[edit] Scenarios

When one parent carries a Cis AB allele, the other allele can be any of O, A or B and the phenotype of this parent is anyway AB, but the children will inherit either the AB or the other allele from this parent.

1. If the other parent is O phenotype (OO genotype) there can be three likely scenarios for blood group of children of a Cis AB carrier (and a 4^th very unlikely scenario):
   1. The second allele is O: children are either AB or O
   2. Second allele is A: Children are either AB or A
   3. Second allele is B: Children are either AB or B
   4. A very rare 4th possibility exists: if the other allele is also Cis AB then the children will be always AB irrespective whatever the other parent is, because they will have one cis AB allele from this parent.
2. If the other parent is A, depending on whether this parent is genotypically AA or AO and what the other allele is in the Cis Ab carrying parent, there are following possible scenarios:
   1. Other parent is AO and second allele is O: The children are either AB or A or O
   2. Other parent is AA and the second allele is O: The children are either AB or A
   3. Other parent is AO and second allele is A: The children are either AB or A
   4. Other parent is AA and the second allele is A: The children are either AB or A
   5. Other parent is AO and the second allele is B: The children are either AB or B
   6. Other parent is AA and the second allele is B: The children are always AB
   7. Rare situation: If the other allele is also cis AB:The children are always AB
3. Like wise there will be similar scenarios for the other parent being B:
   1. Other parent is BO and second allele is O: The children are either AB or B or O
   2. Other parent is BO and second allele is A: The children are either AB or A
   3. Other parent is BO and the second allele is B: The children are always AB or B
   4. Other parent is BB and the second allele is B: The children are either AB or B
   5. Other parent is BB and the second allele is O: The children are either AB or B
   6. Other parent is BB and the second allele is A: The children are always AB
   7. Rare situation: If the other allele is also cis AB:The children are always AB

(Caution: ABO inheritance is generally derived assuming the children are not the very rare Bombay phenotype which would require both parents to be carriers of it.)

[edit] Real life implications

[edit] Maternity and paternity disputes

There can be paternity or maternity disputes if tested by ABO blood grouping. Those of the above scenarios where a child's phenotype is written in bold will be such a situation. For example (scenario 1.1 above) a child of a cis AB (who will apparently look as a regular AB phenotype) individual and an O individual will be either AB or O instead of the usual A or B (see diagram above).

[edit] Differential diagnosis

If the child of a marriage between AB and an O individual is O (the green colored offspring in the scenario 1 image above), then a rare alternative possibility is that the parents were carriers (heterozygous) for the Bombay phenotype (Hh) and the child is a Bombay (hh) homozygous by genotype thus expressing Bombay phenotype also called O_h where irrespective of the presence of the ABO alleles the substrate from which those antigens are made is not made and thus A, B and even O antigen is completely absent.

[edit] References