QotW: how alike are the kids of identical twins?

This week Eva Higginbotham has been looking for the answer to this question from Sam...

Eva - It was a love story like no other! In 2017, identical twins Briana and Brittany Deane met identical twins Joshua and Jeremy Salyers and fell in, as they put it, ‘double love at first sight’. The potential for awkward mix-ups aside, the double-couples are now married, and planning to get pregnant at the same time and raise their children under one roof. But, will their children be like twins? I put the question to Tessa Bertozzi, a geneticist at the University of Cambridge. 

Tessa - No, the children won’t be like twins, but this is still an interesting question from a genetics point of view. Even though their family tree makes them cousins, they will be just as related as siblings. Think of it like this: identical twins have the exact same DNA, so two sets of identical twins each having a child is equivalent to a single couple having two children. In both cases you end up with two children who share approximately 50% of their DNA.

Eva - So you share about half your DNA with each of your brothers or sisters, but how does that come about? Our DNA is organised into structures called chromosomes, and most people have 23 pairs. We inherit half of them from our biological father and half from our biological mother. And this is where it gets interesting.

Tessa - The set of 23 chromosomes that each parent passes on is different every time thanks to the biological process that produces eggs and sperm, known as meiosis. There are a couple of reasons for this. Firstly, the pairs of chromosomes, one from your mother and one from your father, find each other in the cell and, amazingly, swap portions of their DNA. We call this crossing over. This means, for example, that mom’s chromosome 5 now has bits of dad’s chromosome 5 too.

Eva - So when your cells are making eggs or sperm, some of the DNA you got from Dad gets mixed up with some of the DNA you got from Mum. And then...

Tessa - After crossing over, the chromosome pairs all line up in the middle of the cell. When the time comes to divide, they split up randomly - so an egg cell might get chromosome 5 from mom and chromosome 6 from dad, instead of receiving all of mom’s or all of dad’s chromosomes. This means that every sperm or egg produced by a single person has a completely unique combination of genes. In other words, the half of our DNA that we get from each parent will be a different half than the one our siblings inherit, but since both siblings receive DNA from the same genetic pool, on average 50% of their DNA is overlapping.

Eva - So there you have it - the Salyers twins kids will be taking genes from the same gene pool, but won’t end up with ALL of the same genes and so won’t be identical. At least, until we can get cloning off the ground! Next week we’ll be answering this question from Neerav...

Neerav - I have a question as to how coffee granules dissolve so well compared to soup-in-a-pack, for example. The difference is amazing, coffee dissolves with minimal or no stirring!