国家临床技能大赛医患沟通题目：十七岁患儿急需肝移植，父亲坚持活体供肝，患儿O型血，母亲A型，父亲AB型，如何进行医患沟通？
关于这道题，很多人张口就是“喜当爹”。诚然，各位迅捷的反应速度证明了你们的高中生物知识学得很扎实，而对医学生或者学习遗传学的人而言，他们并不会忽略掉mutation的存在。
其实这样的情况是完全可能发生的，只是很罕见(rare)而已。
如果患儿的父亲是cis AB的表型（cis AB是变异的结果而正常AB表型是trans AB），母亲是A（AO），那么患儿的血型可以是AB、A或O（注意看下文红字部分）。关于Cis AB，详见参考文献4（http://www.sciencedirect.com/science/article/pii/S0887796306000137）文章的最后一句话这么写的：Particularly, the possibility of cis-AB is extra important to keep in mind when an AB child is born to an O parent, and vice versa.
Cis AB is a rare ABO Blood group antigen genotype which is phenotypically almost the same as AB. In Cis AB inheritance A and B antigen seem to be segregating as if they are 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. Antigen expression is weaker than A1 or B.
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 4th 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
参考文献：
【1】Yamamoto F, McNeill P D, Kominato Y, et al. Molecular Genetic Analysis of the ABO Blood Group System: 2. cis‐AB Alleles[J]. Vox sanguinis, 1993, 64(2): 120-123.
【2】Tzeng C H, Chen Y J, Lyou J Y, et al. A novel cis‐AB allele derived from a unique 796C> A mutation in exon 7 of ABO gene[J]. Transfusion, 2005, 45(1): 50-55.
【3】Yoshida A, Yamaguchi H, Okubo Y. Genetic mechanism of Cis-AB inheritance. II. Cases associated with structural mutation of blood group glycosyltransferase[J]. American journal of human genetics, 1980, 32(5): 645.
【4】Yazer M H, Olsson M L, Palcic M M. The cis-AB Blood Group Phenotype: Fundamental Lessons in Glycobiology[J]. Transfusion medicine reviews, 2006, 20(3): 207-217.