You are talking about Rena and Eisuke I presume. It is not necessarily true.Solera wrote: err well even if you don't have the same blood type... you could be relatives[pls remember that o.o]
If you are going to read Detective Conan, it behooves you to know a bit about Mendelian genetics of blood types.
The blood type of an individual is determined by the proteins expressed on the surface of his/her red blood cells. The proteins themselves are encoded by genetic information in their chromosomes (chromosome 9 to be specific).
Both A and B are derivatives of the same H antigen (Blood type O) and are therefore mutually exclusive on each chromosome, which means that each chromosome can only encode from A, B or O antigen. However, because chromosomes exist in pairs that are passed down from the parents, the two Chromosome 9 can code for different ABO proteins, which culminates in the different blood types (A and B can be co-expressed and O is always "recessive" due to the antigen nature of the blood types [i.e. O is expressed but does not result in an immune reaction]).
Genotype AA = Blood Type A
Genotype AO = Blood Type A
Genotype BB = Blood Type B
Genotype BO = Blood Type B
Genotype AB = Blood Type AB
Genotype OO = Blood Type O
Shinichi's explanation that a parent with AB blood type (Genotype AB) cannot have an O blood type child even with another O blood type (Genotype OO) parent:
AB + OO + AO or BO (A biological child of the parents MUST have type A or type B blood)
Using this interesting tidbit, we can not only determine the blood types of Rena and Eisuke's parents but also their genotypes :
Eisuke was biologically type O (Genotype OO) and Rena is biologically type AB (Genotype AB). Since they are biological siblings, their parents MUST have blood type A (with Genotype AO) and blood type B (with Genotype BO).
AO + BO + OO, AB, AO, BO (or Type A, B, AB, O with 25% probability of each blood type)
The Rhesus factor on the other hand is more or less a case of a genetic mutation that renders the genetic codon (on Chromosome 1) incapable of producing functionally proteins. Due to the functional/non-functional nature RhD(-) is autosomal recessive, which means a Rh(-) person has BOTH his/her Chromosome 1 incapable of producing functional RhD proteins.
Because of the Founder effect (and the Recessive nature of RhD(-)), RhD(-) blood types are predominantly found in populations of European and African origin and are almost unheard of in Asian populations. Even more so with the Japanese, since early Asian population migrations to the island is small and infrequent enough to create another Founder effect on the subsequent Japanese population.