kids timberland Ask an Expert ABC Science
Tom and Tim both eye Jen and Jan. Tom falls for Jen, Tim falls for Jan. Soon they all start having babies.
It might sound like the beginning of a joke, or perhaps a romantic comedy starring the Olsen twins, but if a set of identical twins married another set of identical twins what would their children look like? Since each pair of identical twins has identical DNA would both couple’s children look the same?
“[Two sets of] identical twins will not have identical offspring,” says Professor Rodney Scott, head of medical genetics at the University of Newcastle.
The only way we get identical offspring is when a fertilised egg splits into two cells.
“Those identical cells grow into identical twins,” says Scott.
These types of twins are called monozygotic twins, which literally means they come from the same cell: ‘mono’ meaning one, and ‘zygotic’ meaning a fertilised cell. Since monozygotic twins developed from the same cell, they have identical DNA.
So if they’re not identical, what would Tom and Jen’s, and Tim and Jan’s children look like?
Each of Tom and Jen’s children would look different to the next just like any brother or sister and the same goes for Tim and Jan’s children.
Their children wouldn’t inherit identical traits from their parents because of a process called ‘genetic recombination’.
Genetic recombination occurs during a special type of cell division that forms egg or sperm cells, when the chromosomes swap their genes in a unique way, forming genetically unique eggs or sperm.
“All the chromosomes get mixed up,” says Scott.
for example, because of this process of genetic recombination, all of Jen’s eggs would be different from each other.
When one of Jen’s unique eggs comes together with one of Tom’s unique sperm during fertilisation (another form of genetic recombination), this produces a unique child.
Considering identical twins have the same genes, is there a possibility that the genes could recombine in the same way in an egg or a sperm so that the identical twin pairs have identical children?
Not really. According to Scott there is “an infinite variety of genomes that you can come up with”. The likelihood that the genes will recombine identically is the same as that of parents having two children, at different times, which are identical a likelihood so small, we call it zero.
So genetic recombination helps prevent siblings from looking the same, but that’s not its only function. The process is important for the survival of a species. By reassembling or recombining genes, it ensures that harmful genes are not automatically passed down to future generations.
“It’s an inbuilt safety mechanism,” explains Scott.
So Tom and Jen’s, and Tim and Jan’s children wouldn’t look the same, but their situation would still be a little strange: even though Tom and Jen’s children would be cousins to Tim and Jan’s children, all the children would be as closely related or have the same genetic relationship as brothers and sisters.
What’s more, some of the children might also be identical twins: this is because identical twin pairs like Tom and Jen,
and Tim and Jan carry genes that mean they have a higher likelihood than non twin couples of having children that are identical twins.