| | Re: Twin couples' offspring are siblings not cousins?
Interesting question. Genetically speaking, the cousins would have the same proportion of similar genes as siblings would. Normally, with diploid organsims (i.e. containing two sets of chromosomes), parents each donate 50% of their genetic make-up to their offspring - but of course the 50% is a random sampling. Therefore, the children share 50% of their genes with both parents and 50% with any siblings. Cousins, that is offspring of siblings, share 25% of their genes (assuming outbreeding) because their parents shared 50% and half of these were passed on to the cousins.
In the example, the parents of the two cousins are identical twins and therefore share 100% of their genes. So this is in fact genetically equivalent to the cousins having the same parents. The cousins thus share 50% of their genes, rather than the normal 25%. This means they are as closely related as siblings are, though of course they are under normal definitions considered cousins.