Fascinating. Would you predict a similar mechanism for Fragile X syndrome (with milder symptoms for females, presuming that the "fragile" X would be the likely candidate for "deactivation" early in prenatal development)?
Linda Tollefsrud Professor of Psychology University of Wisconsin - Barron County 1800 College Drive Rice Lake, WI 54868 (715) 234-8176 [EMAIL PROTECTED] -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Sent: Saturday, March 22, 2008 10:39 AM To: Teaching in the Psychological Sciences (TIPS) Subject: RE:[tips] Turner/X-inactivation (was: Identical twins) I described the phenomenon of X-inactivation in women (random inactivation of one of the two X-chromosomes in each somatic cell, to which Linda Tollesfrud replied: > If this is true, why do Turner's syndrome females typically have > deficiencies in both mental and physical development? > Challenging question! I think I can answer it, although it does stretch to the limit my little knowledge of this complex and poorly-understood phenomenon. But I'm going to need a lot of words. I assume the basis for the question is the fact that Turner females have only one X-chromosome in each cell while normal females have two, and therefore Turner deficiencies are explained as due to this loss of genetic material. But if the newer X-inactivation hypothesis is correct (and it is), then for a normal female only one of the two X's is turned on in each cell. In that case, both normals and Turner cases would have the same number of working X-chromosomes, so why would Turner cases have mental and physical deficiencies? In answer, first note that the loss of an entire chromosome is a very serious matter, almost always leading to fetal death long before birth. The primary exception is Down syndrome, because the chromosome lost (the 21st) is the smallest, so very few genes are lost. A male equivalent to Turner ( YO: a Y with no X) has never been observed, most likely because the loss of the X is fatal. But not only is Turner syndrome (XO) viable, but the deficiencies are not severe by genetic standards. Turner women do have some serious physical malformations and do not mature sexually, but overall IQ is usually normal. The mental deficiency referred to by Linda is primarily in visuo- spatial performance. So how can they function so well with the loss of one of the largest chromosomes? The X-inactivation hypothesis explains how. First, more background. Males have only one X-chromosome. Females have two. Consequently, if both X's were active, females would have double the genetic dose for each gene than males, with catastrophic result. Mother Nature (read: evolution) takes care of this by the development of X- inactivation, an epigenetic mechanism. As only one X is active in each female cell, females and males receive the same genetic dose. But not so fast, chromo-breath. There are a small number of genes which appear _both_ on the Y-chromosome of males as well as on their X. So for these few genes, males do receive a double dose, and there is no necessity to have them inactivated in females. So they're not: they "escape" from X-inactivation. But Turner females have only one X. So for these special genes, and only these, they have a deficient dose. This is what gives rise to the problems associated with Turner syndrome. These problems are not inconsequential, but they are not lethal, and they allow a reasonably normal life, with medical help. That's my version. To show that I'm not just making this up, here's an authoritative version, probably harder to understand than mine, but concise: "Turner syndrome is caused by a reduced complement of genes that are typically expressed from both X chromosomes in females. Normally one X chromosome is randomly inactivated during the first week of life (when there are fewer than 200 embryonic cells); therefore, it may seem paradoxical that having a single X chromosome would cause clinical consequences. However, not all genes from the second chromosome are inactivated in Turner syndrome. Some genes escape X-inactivation via a process initiated by the X-inactivation-specific transcript (XIST) gene that is transcribed exclusively from the inactive genes. The loss of these noninactivated X genes causes the phenotypic manifestations characteristic of Turner syndrome, such as short stature" >From Morgan, T. (2007). Turner Syndrome: Diagnosis and Management. American Family Physician. At http://www.aafp.org/afp/20070801/405.html Stephen ----------------------------------------------------------------- Stephen L. Black, Ph.D. Professor of Psychology, Emeritus Bishop's University e-mail: [EMAIL PROTECTED] 2600 College St. Sherbrooke QC J1M 1Z7 Canada Subscribe to discussion list (TIPS) for the teaching of psychology at http://flightline.highline.edu/sfrantz/tips/ ----------------------------------------------------------------------- --- To make changes to your subscription contact: Bill Southerly ([EMAIL PROTECTED]) --- To make changes to your subscription contact: Bill Southerly ([EMAIL PROTECTED])
