All eggs contain one X chromosome ... children Individuals with Down's syndrome have reduced muscle tone and learning difficulties. It is caused by an extra chromosome at chromosome 21.

The Barth Syndrome Foundation (BSF), the only patient advocacy organization solely dedicated to saving lives through education, advances in treatments, and finding a cure for Barth syndrome ...

Barth syndrome is an ultra-rare, life threatening pediatric mitochondrial disease which is known to affect less than 150 individuals in the United States and less than 300 individuals worldwide.

Down syndrome ... for the proximal half of chromosome 21 does not induce the Down syndrome phenotype. In patient AP, the normal Jackson score probably results from X inactivation spreading ...