The genes within our DNA hold the codes for thousands of traits – from eye color to height. They also play a role in the chances of inheriting many diseases.
Many primary immunodeficiency diseases (PIDD) are genetically inherited, meaning that they are passed from parent to child. Some PIDD, however, can occur spontaneously when a mistake or mutation occurs in a gene, usually during a baby's development in the womb.
Genes are organized on chromosomes inside cells. In humans, each cell normally contains 23 pairs of chromosomes, for a total of 46. Twenty-two of these pairs are called autosomes and look the same in both males and females. The other pair, called the sex chromosomes, determines whether we are male or female.
PIDD can be linked to problems with the sex chromosomes or the autosomes.
In addition to determining whether we are male or female, the sex chromosomes carry important genes for our immune system's function. X-linked recessive inheritance means that the disease is caused by a mistake or mutation that is inherited on the X chromosome, one of the sex chromosomes.
X-linked recessive diseases are much more common in males than females. Here’s why:
Females have two X chromosomes and males have one X and one Y chromosome. Therefore, any male who inherits a defective X chromosome will have the disease. A female must receive two defective X chromosomes, one from each parent, to inherit the disease. Females with one defective X are only carriers of the disease and will not exhibit the symptoms.
Often family history is helpful for the diagnosis of X-linked recessive conditions as there may be a history of sickly male babies, or early infant deaths in males with undiagnosed disease in the family tree. The family history is also important in identifying family members who may need genetic counseling for future pregnancies.
In autosomal recessive conditions, the autosomes rather than the sex chromosomes are affected.
Recessive means that two abnormal copies of a gene must be inherited, one from each parent. If only one gene for the disease is inherited, then the person who inherited it is a carrier.
Males and females can be affected equally with this type of inheritance. Disease only occurs when each parent is a carrier and each passes on a defective copy of the associated gene.
The chances of this happening are one in four, since the parents can also pass on normal copies of the gene in question. Therefore, there is also a one in four chance of a healthy baby, and a 50% chance of having a baby who is a carrier.