Chromosomes are like libraries for our genes.
We have thousands of genes that instruct our bodies on how they should develop, determining things like hair color, eye color, and body parts. These genes are organized into chunks called chromosomes.
The X chromosome is one of two sex chromosomes. Sex chromosomes help determine whether someone has the reproductive body parts of a boy or a girl. Typically, girls have two X chromosomes, and boys have an X and a Y chromosome.
Someone with XY sex chromosomes is typically a man, and XX a woman!
Chromosomes are passed down from generation to generation. One set comes from the egg, and one set comes from the sperm. Women (who have 2 X chromosomes) always pass down an X to their children. Men (who have an X and a Y) either pass down the X or the Y to their kids. If he passes down the X, the child will be a girl. If he passes down the Y, the child will be a boy.
If the dad passes the X chromosome down, the child will be female, and a Y, the child will be male! The sex of a baby is determined by the dad’s chromosomes, since the mom passes an X down.
There is one gene that is responsible for Fragile X syndrome, called FMR1. This gene is on the X chromosome. Usually this gene helps to make an important protein that helps with brain development.
People with Fragile X syndrome inherit a version of FMR1 that does not work. People with Fragile X do not make the protein correctly. This lack of protein causes the condition.
But why doesn’t the gene work in some people?
The FMRI gene has a pattern called CGG repeats. In a typical FMR1 gene, there are less than 45 CGG repeats. When there are over 200 repeats, the FMR1 gene shuts down and does not work. So people with more than 200 CGG repeats have Fragile X syndrome.
Boys with the full mutation will have Fragile X syndrome. Since they only have one copy of the X chromosome, they only need one full mutation to have Fragile X.
Usually girls do not have the condition, even with the full mutation. This is because girls have 2 X chromosomes. Even though one of the genes is not working, she still has another on the other X that works and makes the protein. But some girls who have a full mutation may have difficulties learning or socializing.
Repeats of DNA can expand
These repeats are unstable. This means that the number of repeats can change between a parent and a child! Sometimes the number of repeats will expand, and a child will end up with more repeats than their parents. The more repeats a person has, the more likely it is to expand. People with 45-54 CGG repeats have an intermediate form of the gene, but this does not cause Fragile X syndrome.
When there are 55-200 CGG repeats, someone has a “premutation” which means that their FMR1 is not stable. This could expand to more repeats and could cause Fragile X syndrome in their children or grandchildren.
The chance of the premutation expanding to the size of a full mutation is related to how big the premutation is. The larger the CGG repeat number, the higher the chance it will expand to a full mutation.
The premutation is more likely to expand when it is passed on by a woman. A woman with a premutation often passes down a larger amount of repeats on to her children. But since typically only one of her X chromosomes has the premutation, she only has a 50% chance of passing it on to her kids.
When a father passes down a premutation to a daughter, it does not usually expand to a full mutation and cause Fragile X syndrome. A father cannot pass down the premutation to his sons, since he only passes his Y chromosome to them (remember, the Fragile X gene is on the X chromosome!).
If a woman has a premutation and it is passed on to her kids, it can remain a premutation or it can expand to a full mutation. If she has a full mutation and it is passed on, it will remain a full mutation. The chances that it will expand all depend on how many CGG repeats the person has.
But why do the repeats expand?
Now this is where it gets REALLY complicated. When there are an unstable amounts of repeats, the DNA has trouble copying when it makes new cells. When there are lots of repeats, the DNA may make loops which expand the new DNA. If you want to read more, the term to look up would be “replication slippage” or “trinucleotide repeat expansion”.