Twins and Genetics

Mod­ern genet­ics some­times has a bit of a “believe it or not” feel­ing to it, things are repor­ted that sound like they come out of Greek myth­o­logy. One of the recent news items on the BBC had that feel­ing for me, where they write about semi-identic­al twins. These babies, who come from one egg fer­til­ized by two sperm, have the same mater­nal genet­ic mater­i­al and each has some of the paternal genet­ic mater­i­al from each sperm. So they’re both chi­mer­as, and one is also a herm­aph­rod­ite. Search­ing through the web for inform­a­tion about soci­ety’s treat­ment of herm­aph­rod­ites through the ages is a good way of spend­ing a lot of time read­ing inter­est­ing art­icles; vari­ous cul­tures and soci­et­ies have differed greatly in how they treated these people. But I digress. 

In myth­o­logy chi­mer­as are those beasts that are partly one anim­al or mon­ster, and partly anoth­er. In the mod­ern med­ic­al usage of the term, a chi­mera has two sets of genet­ic mater­i­al, so a DNA test taken from one part of the body will give a dif­fer­ent res­ult to a DNA test taken from some oth­er part of the body. Some people with eyes of dif­fer­ent col­ours are chi­mer­as, for example, the dif­fer­ent col­ours com­ing from the dif­fer­ing genet­ic material.

Anoth­er genet­ic­ally inter­est­ing con­di­tion that involves twins is the van­ish­ing twin syn­drome, where one twin absorbs all the genet­ic mater­i­al of the oth­er, some­times lead­ing to a chi­mera. The per­son, if born before early ultra­sounds were com­mon, prob­ably won’t even know that he or she star­ted off as one of twins. There was a well-known case of a moth­er who could only accept a kid­ney trans­plant from one of her three sons because the oth­er two had genet­ic mater­i­al from the van­ished twin rather than the moth­er who gave birth to them (see the above link or this one for more details).

These things give rise to some inter­est­ing issues, such as what about DNA test­ing? If one per­son can have the DNA of two people, with one set of DNA in some tis­sues, and the oth­er set in oth­er tis­sues, DNA test­ing becomes prob­lem­at­ic. Cur­rently there’s no way of know­ing the real pro­por­tion of van­ished twins, since there is only sel­dom a reas­on to test any giv­en per­son thor­oughly enough to find out (one example being need­ing to prove par­ent­age of chil­dren, as in the rel­at­ively recent Lydia Fairchild case). Chi­mer­as can have two blood types as well, mak­ing them ideal blood recip­i­ents, but less than ideal blood donors. That’s anoth­er way people may find out, I guess, when they get their blood type tested, but again it’s hard to know what pro­por­tion of chi­mer­as have two blood types.

I hope the par­ents of those babies (now tod­dlers) suc­ceed in keep­ing their names and oth­er inform­a­tion private. The interest in their unusu­al genet­ic make-up could badly affect their lives if the par­ents aren’t strong enough to with­stand pres­sure from the press, or sci­ent­ists, or the curi­ous pub­lic. I also hope they’re treated with com­pas­sion and empathy as they grow older, rather than intrus­ive nosiness.