How is achondroplasia inherited?
Although achondroplasia is a genetic condition, when a person has achondroplasia, it is not always inherited from a parent. In fact, only about 20% of people with achondroplasia inherited it from either their mom or dad. The other 80% of the time, achondroplasia is a new diagnosis in the family and was not inherited from either parent. In this case, there was a spontaneous change in the FGFR3 gene carried by the egg or sperm cell that helped conceive that individual. That gene change is present from the time the baby is conceived and causes the abnormal bone development seen in achondroplasia. Because the affected individual carries that gene change in all of the cells of their body, their own future offspring are at risk for achondroplasia.
If a person with achondroplasia has a baby with a person who is unaffected, there is a 50% chance that any offspring will have achondroplasia. This means that when one parent is affected there is also a 50% chance that the baby will not have achondroplasia.
When both parents have achondroplasia, there is still a 50% chance that any child they have will have achondroplasia. There is a 25% chance that any child will not inherit achondroplasia. This means that the child inherited neither parent's changed copy of the FGFR3 gene. There is also a 25% risk that any child whose parents both have achondroplasia could inherit both parents' changed copy of the FGFR3 gene. Unfortunately, babies with two copies of the FGFR3 gene mutation do not survive.