It’s usually a mutation that doesn’t allow the gene to work properly. An important part of the gene can be deleted or the DNA sequence is changed in some other way. Sometimes a change in just one letter of the code can break the functionality of the gene. When the cell tries to make a protein based on the mutated sequence, it produces a protein that cannot be used for any purpose in the body or it cannot produce the protein at all.
I should add that you can have many dysfunctional genes in your DNA without having any health problems if the genes regulate something optional like hair color and not vital like proper muscle production. So you can have lots of dysfunctional genes without having any medical dysfunction.
It’s usually a mutation that doesn’t allow the gene to work properly. An important part of the gene can be deleted or the DNA sequence is changed in some other way. Sometimes a change in just one letter of the code can break the functionality of the gene. When the cell tries to make a protein based on the mutated sequence, it produces a protein that cannot be used for any purpose in the body or it cannot produce the protein at all.
I should add that you can have many dysfunctional genes in your DNA without having any health problems if the genes regulate something optional like hair color and not vital like proper muscle production. So you can have lots of dysfunctional genes without having any medical dysfunction.