Is it inherited?
Rett Syndrome (RTT) is a rare genetic disorder, namely, it results from an alteration in one or more genes. However, less than 1% of the recorded cases are inherited or passed from one generation to the next. So, mostly all cases are spontaneous, meaning that the mutation (alteration) occurs randomly and it’s not inherited.
Our body is formed by cells, and in each single cell there is information about how we are. This information is called DNA, which contains genes and it’s passed from parents to offspring. DNA in cells is forming a structure called chromosome. Humans have 46 chromosomes, and two of them determine our sex, X and Y chromosomes. Boys have one X chromosome and one Y chromosome, but girls have two X chromosomes and only one of them is active in any given cell.
As explained in the page What’s the cause?, the MECP2 gene is found on a person’s X chromosome. In a girl with RTT, some cells will have active the X chromosome with the defective genes, and some other cells will have this altered X chromosome inactive. This means that although some of the girl’s brain cells use the healthy gene and express normal amounts of protein, some of the cells use the defective gene.
RTT is a dominant disorder; it means that having just a mutation in one of the X chromosomes will cause the disease. The severity of RTT in part depends on the percentage of the cells that express a normal copy of the MECP2 gene. If larger percentage of cells has the X chromosome with the normal MECP2 gene turned off, the symptoms may be more severe.
Women with classic RTT do not reproduce but mildly affected females and asymptomatic female carriers (have the mutation but do not show clinical symptoms) have reproduced and the risk to her offspring of inheriting the MECP2 gene mutation is 50%.
Boys with mutations that cause RTT in females do not show clinical features of RTT but experience severe problems and die before or soon after birth.
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