Let's not get too technical here. I promise I will not use words like "messenger-RNA" or "recessive heterozygotes" .
Every human cell contains a cookbook describing how to make a little person, in duplicate. The cookbook is contained in chromosomes, little cell parts that you can see under a good microscope. One of the chromosomes comes in two forms, one that looks like the letter "X", and one that looks like the letter "Y". Cells in a man's body have one X and one Y chromosome, cells in a woman's body have X and X. Having a Y chromosome makes you a man. Every man gets his X chromosome from his mother, and his Y chromosome from his dad.
The Y chromosome, by the way, does not only determine gender. According to some (woman) scientists, it also predisposes for excessive tv channel flipping, drinking from milk bottles, and other, typically "male" traits.
Every sex cell (male sperm and female eggs) has the cookbook, but only one copy for it. They carry either an X or a Y chromosome. Put the two together and you get a cell with either XY chromosomes (it's boy!) or XX chromosomes (it's a girl!).
To put it in a tic-tac-toe schedule:
X | Y
--|---------------|---------------|
M | X from mom, | X from mom, |
o X | X from dad, | Y from dad, |
t | makes XX | makes XY |
h | (girl) | (boy) |
e --|---------------|---------------|
r | X from mom, | X from mom, |
X | X from dad | Y from dad, |
| makes XX | makes XY |
| (girl) | (boy) |
-----------------------------------
The chromosomes are like chapters of the cookbook. The words are called genes. There is a gene that tells the body to make your eyes brown, or blue, or green. There is another one that codes for curly red hair, or straight black hair, or whatever grows on your head. The genes are made up of DNA, incredibly long spiral chains of four amino acids, with call letters G, C, T, and A. Those four are the letters of the cookbook of life.
The cookbook of your cells obviously is not chiseled in marble. Lots of things, like radiation, smoking, aging, certain drugs, or pure chance can alter the code, or wipe parts out. If that happens to your body cells, you'll be the only one with the changed code. If your sex cells have a code change, your children may have it, too.
There is a gen on the X chromosome (gen Xp21, to be exact) that codes for muscles to produce a protein called dystrophin. We do not know yet what exactly dystrophin is good for, but we do know that we need it; that without it, most of voluntary muscles will deteriorate.
The code in the Xp21 gen can be altered, or sometimes the whole gen gets deleted. If you are a woman, you may have such an altered gen, and never know it. The other X chromosome will have an unchanged dystrophin gen, and so your muscles will be producing dystrophin. You carry the (altered) DMD gen, but you are not afflicted.
(There is some disagreement about this. Some scientists say that you can always find DMD symptoms in carriers, if you only look hard enough. A small percentage of carriers will develop clear symptoms.)
The altered Xp21 gene may have been passed on to you by your parents. Or, and this happens in about one-third of all cases, the gene may have altered during your lifetime, so that there isn't any other case of DMD in your family history.
If you have a DMD gen on one X chromosome, you may pass it on to your children. If we write the affected X chromosome as (X), then the tic-tac-toe schedule looks like:
Father
X | Y |
--|---------------|--------------|
M | X from mom, | X from mom, |
o X | X from dad, | Y from dad, |
t | makes XX | makes XY |
h |(regular girl) | (regular boy)|
e --|---------------|--------------|
r |(X)from mom, | (X) from mom,|
(X)| X from dad | Y from dad, |
| makes (X)X | makes (X)Y |
| (girl carrier)|(boy with DMD)|
-------------------------------
That may look like a pretty straightforward proposition: you have a chance of one in two of having healthy children, and a chance of one in four of having either a girl carrier, or a boy with DMD. In reality, every child is like a lottery ticket: you may get 15 healthy sons in a row, your first child may have DMD, anything can happen.
If you are a boy with a DMD gen, there won't be any doubt about it. You won't have a second X chromosome with a good copy of the dystrophin gen to compensate, you muscles will not produce dystrophin, and your parents will find out about it.
This also explains why they always only call the mothers carriers, and why you don't find many girls with full-blown DMD. A guy with DMD will know he has it, and will not often be willing or able to pass it on. In the (hopefully hypothetical) case his partner is a carrier, the genetic tic-tac-toe game ends up like:
Father
(X) | Y |
--|---------------|--------------|
M | X from mom, | X from mom, |
o X |(X) from dad, | Y from dad, |
t | makes (X)X | makes XY |
h |(girl carrier) | (regular boy)|
e --|---------------|--------------|
r |(X)from mom, | (X) from mom,|
(X)|(X) from dad, | Y from dad, |
| makes (X)(X) | makes (X)Y |
|(girl with DMD)|(boy with DMD)|
-------------------------------
If you take it one step further, to "boy with DMD meets girl with DMD", every child ends up with altered dystrophin genes.Fathers of boys with DMD have no cause to blame the mothers for their defective genetic material, and the mothers have no cause to blame themselves. After all, the fathers contributed a Y chromosome. Even in the best of cases, that never has a dystrophin gen on it. Every man is a "deleted dystrophin gen" carrier!
