Why are children all so different?

14 August 2012


Hi Chris,

Firstly Love the show. A question that has been bugging me thought it might be good for Question of the Week

I was wondering what mechanism causes children born to the same parents all have different characteristics from each other. I understand that half the Chromosomes come from the father and half the Mother so you'll always get a mixture of both parents characteristics. However, this simple understanding doesn't explain why they aren't all identical twins (albeit born at different times). So are the Chromosomes somehow jumbled further say chromosomes no. 2 from Mum and no. 4 from Dad? What mechanism controls individuality?

Ta Cameron Lapworth


Dr. Zegerman:: So to understand real individuality really have to understand how you and I were made. It really starts when a sperm from your dad and an egg from your mum got together and made you as an individual. Sperm and egg are made by a special form of cell division, which is called meiosis. Meiosis is special because it turns a diploid, which has two copies of every chromosome, into a haploid, which has one copy of every chromosome. So, when cells goes through the process of meiosis, you take this 46 human chromosomes and you turn it to a cell with one copy of every chromosome, which now means has only 23 chromosomes.

And very importantly, this process of meiosis is random. Every cell that goes through meiosis will inherit one of every chromosome, but whether inherits one from your mother or father is random. So for example, when your father's cells went through meiosis to make sperm, it was random whether that you inherited chromosome 2 let's say, from his mother or chromosome 2 from his father. So now, you can see that when you get an egg and a sperm fusing together, as happened for you and your brothers and sisters, you now see this is now a random collection of chromosomes that aren't just from your parents but random collection of chromosomes they inherited themselves from their parents.

But it's actually much more complicated than just random assortment. During the process of meiosis, chromosome 2 will align next to chromosome 2 and chromosome 4 will align next to chromosome 4 and so on and so forth. And this process is absolutely essential to ensure that each haploid sperm or egg inherits exactly one copy of all the different chromosomes. More than just being essential for the inheritance of every single chromosome, it also allows a very special process to occur whereby chromosomes that are similar but not identical that line up can now exchange pieces of DNA. And this results in making completely unique chromosomes. So now you've swapped a bit of chromosome 2 that your father inherited from his mum with a bit of chromosome 2 that he inherited from his dad. And the end result is a chromosome 2 that is neither your dad's, nor is it your grandma's nor is it your grandfather's. It's completely unique. It's got a bit of both grandma's and granddad's and that is the chromosome that your sperm that made you inherited. It's completely unique to that sperm.

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