The Inheritance of Platelet Disorders
Almost all platelet disorders are caused by autosomal recessive genes. This means that a child has to get the gene for the disorder from both parents. That is why these disorders are so rare. Because the genes are on the autosomes and not on the sex chromosomes, the disorders occur in both men and women.
With a recessive inheritance pattern, if a child gets a normal gene from one parent and a gene for a platelet disorder from the other parent, the child will not have the disorder. He or she will be a carrier. This is because the platelet disorder gene is recessive – it is not stronger, or dominant, over the normal gene. For the child to have the platelet disorder, he or she must get the abnormal gene from both parents. Figure 3-4 shows the possible gene combinations for these autosomal recessive types of platelet disorders.
If both parents carry a defective recessive gene for a platelet disorder, with each pregnancy there is:
- a 25% (one in four) chance of having a child who is unaffected
- a 25% (one in four) chance of having a child who has the platelet disorder
- a 50% chance of having a child who does not have the platelet disorder but carries the defective gene.
Figure 3-4. An autosomal recessive inheritance pattern – the possible gene combinations with most platelet disorders. The recessive gene is shown with a little r. To have the disorder, a child must inherit a recessive gene from both parents.