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Just what a shocking situation ! Having a twin of the opposite race! What is the world comming to now?

They are not 'opposite'(?) races. They are both half-breeds. By chance it is easier seen in one. They will still produce similar colour offspring (regression to the mean). Dizygotic twins are no more genetically similar than ordinary siblings. It is essentially two separate pregnancies that occur at the same time. Large variation amongst siblings is common. In the case of hybrid offspring resulting from miscegenation, melanisation is one more, particularly complex,  phenotypical manifestation.

Everyone has a relatively similar number of melanocytes but the amounts and ratio of eumelanins and pheomelanin produced is governed by a complex combination of allelomorphic, physiological and biochemical processes. Melanogenesis begins with a catalytic enzyme containing copper (sorry, can't recall the name) that causes the progressive oxidation of an amino acid that synthesises the variable melanin polymers within the melanocyte cells.

Despite the complexities skin colour can be reliably predicted by DNA analysis.

I've uploaded a few papers from the journal Human Genetics (e.g., Genetics of skin colour variation in Europeans; Global skin colour prediction from DNA; etc.) -
https://ulozto.net/folder/UJbPv3IIJpy0

[video width=480 height=380]https://thv1.uloz.to/d/8/d/d8d04bce308ad689669410e85aa8bb1f.360.mp4?fs=ASX7U5RHNZkY&fide=a4wfzo6&fileId=289903861&vt=1591394399&sg=r7naUbcadefRAN1nXUPFRw&bl=