4/15/2006: Mixed-race couple has black and white twins

Originally Posted by sktzofrenic

Identical twins does indeed mean the same sperm and therefore the same DNA, but please keep in mind that not all the genotypes in the DNA will express itself as a phenotype. In this case they both have the genes for both black and white skin but only one phenotype in this case expressed itself.

Identical twins WILL have the same skin color. You are thinking of incomplete dominance or genes that can switch on and off. Multiple alleles (genotype) contribute to skin color (phenotype) in human beings. If you have the identical set of alleles, you will have the identical skin color of your twin. Explanation for those with a scientific bent.

[Pie]

Not quite true. It is possible to change the expression of genes without changing the underlying sequences -- see this Wikipedia article on twins

Identical twins have identical DNA but differing environmental influences throughout their lives affect which genes are switched on or off. This is called epigenetic modification. A study of 80 pairs of human twins ranging in age from 3 to 74 showed that the youngest twins have relatively few epigenetic differences. The number of differences between identical twins increases with age. 50-year-old twins had over three times the epigenetic difference of 3-year-old twins. Twins who had spent their lives apart (such as those adopted by two different sets of parents at birth) had the greatest difference. (Fraga, et al., 2005).

DNA methylation, histone modification and other processes related to gene activation occur throughout an individual's lifetime. The study of these changes is called epigenetics.

[Kagen4o4]

but this does not mean a persons skin colour will change.

[xoxoxoBruce]

Originally Posted by John

... and as soon as I post that, I realise that you're responding to someone else, and suddenly I feel dumb.

Ha ha ha, I'm glad I'm not the only one that happens too.

[Pie]

Vitiligo and liver spots are mostly age-related pheonomena. Natural eye color and hair color definitely change over time.

...And have you ever gotten a tan?

[tippy]

Not sure who is right and who is wrong as to whether this is one of the genes that can be switched on and off in identical twins but:
-there was a case of a white woman in SA during apartheid era whose skin changed colour virtually over night due to some kind of condition that changed the way her melanin receptors worked. she was immediatley ostracised by everyone. (I've googled this but can't find it- saw it on a programme about genetics and how melanin works)
- cases of twins and siblings of different skin colours although rare has happened before. i saw a whole programme on it, inlcuded a scene where one twin complained that the main source of racist taunts at school regarding his skin colour was in fact his own brother (nice). and both parents do not necessarily need to be of mixed race.
- you also get cases where a white/light skinned couple will have a black/dark skinned child because they have mixed race genes several generations back, often unknowingly. There were cases of this happening in America. In times past some families separated off any family members who were not obvioulsy "white" and then buried the whole thing, so thet when the genes re-emerged several generations later everyone was mystified... (from same programme on twins)

you can imagine the accusations!!!

the whole issue of how genes express themselves is totally fascinating.

[Kagen4o4]

like in family guy.

[AureliusVin]

I think what ashke was saying is that you can never have identical twins with different skin tones, which is 100% correct.

[Pie]

Originally Posted by AureliusVin

I think what ashke was saying is that you can never have identical twins with different skin tones, which is 100% correct.

Again, look at environmental factors -- one twin can't get a tan?!?

[Kagen4o4]

no, once one twin gets a tan it automatically transfers to the other twin.

[wolf]

I'm having a problem with my racial math ... assuming mom and dad were both half-black and half-white, then the kids are both half-black and half-white. If the ratio were one-quarter, then they'd be quadroons, but I can't find the specific term for half-and-half.

[ashke]

Eh, a tan is different from a natural skin tone. And there's a limit on how dark you can get. It's bad for the skin too -__-.

[xoxoxoBruce]

Originally Posted by wolf

I can't find the specific term for half-and-half.

Mermaid?

[Kagen4o4]

nice