Adrenoleukodystrophy is a heritable disease caused by a mutation in the ABCD1 gene located on the X chromosome.
Chromosomes in males and females
Chromosomes are packaged DNA and contain all the information that determines the genetic makeup of an individual. Each human cell contains 23 chromosome pairs, with one chromosome in each pair inherited from one parent.
Twenty-two of the chromosome pairs are so-called autosomes. The two chromosomes in an autosome pair are identical and carry the same genes. So, men and women have two copies of all the genes in the autosomes. The 23rd pair is called the sex chromosomes, and it differs in men and women. Men have one X and one Y chromosome, while women have two X chromosomes. The X and Y chromosomes contain different genes.
Therefore, with two copies of the X chromosome, a female will have two copies of every gene on this sex chromosome, just like the autosomes. But for men, the 23rd pair does not have identical genes, and men have only one copy of the genes present on the X and Y chromosomes.
ALD is an X-linked condition because the gene affected (ABCD1) is on and can be inherited through the X chromosome only.
Inheriting ALD
The ABCD1 gene is located on the X chromosome at position Xq28. Since men only have one X chromosome, they carry only one copy of this gene. Women have two copies of the gene because they have two X chromosomes.
Consequently, men develop ALD if they inherit a mutated ABCD1 gene on their only X chromosome.
On the other hand, if women inherit one X chromosome with the mutated ABCD1 gene, the normal copy on the second X chromosome can balance out the defect. Women with one healthy and one mutated copy of the ABCD1 gene are called ALD carriers. Girls very rarely inherit two copies of the mutation from each parent, although this can happen.
Men inherit the Y chromosome from their father. Since ALD is an X-linked condition, men with ALD cannot pass the condition to their sons, but will pass it to their daughters. Thus, the daughters of men with ALD will all be ALD carriers or develop the disease if they also inherit a faulty copy of the ABCD1 gene from their mother.
A mother passes an X chromosome to both her sons and her daughters. Therefore, there is a 50 percent chance that both the son and daughter of an ALD carrier mother will inherit the defective ABCD1 gene. But, because boys only have one X chromosome and thus only one copy of the ABCD1 gene, inheriting it from an ALD carrier mother means that they will develop the condition.
Acquiring novel ALD-causing mutation
Infrequently (in about 5 percent of cases), a de novo mutation in the ABCD1 gene can also occur. A de novo mutation is a new change in a gene that is only present in a child but not in his or her parent. It can happen in the mother’s egg or the father’s sperm. It can also occur just after the egg and sperm unite to form a fertilized egg. As the fertilized egg divides, the mutation is carried over to all the cells that form the embryo and fetus.
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