Not all genes are used in every cell.
Imagine I gave 3 people a general housekeeping book (this is DNA in my metaphore) which contains: recipes, cleaning tips and, sewing instructions etc.
Then I asked one person to bake a cake, one person to do laundry and one person to re-sew a button onto a shirt using the info in the book.
They all have the same total information, but they look at different parts of the book for instructions.
It’s called epigenetics!
Your genes are made of DNA. Your DNA is shaped like a really long strand, but it’s coiled *real* tight. Because of this coiling, only some parts of the DNA are turned outwards, and are thus able to tell the cells what to do.
There are chemicals that affect how your DNA coils, and which bits are facing outward – and so change how the cell behaves, what it looks like, what it does, and so on.
It’s like a manual, where you’re only allowed to read the open page – but different things tend to leaf through and leave different pages open. Sometimes, it’s page 2 and 3, and sometimes is page 46 and 47.
Epigenetic changes can also come about because of your hormones, lifestyle, food, activity, environment, and many other things.
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