Genetics Behind Turner’s
Genetics Behind Turner’s
Some interesting facts: -20% of females only have portions of their X chromosome missing, not the entire chromosome -30% of females with the disease only have a portion of the chromosome missing in some of their cells, lowering the amount of significant symptoms. Mosaicism refers to this type of abnormal distribution of the X chromosome throughout the body. -It is said in 75-80% of Turners Syndrome cases, the missing X chromosome remains in the sperm of the father, while the mothers X chromosome is the only present chromosome throughout the body.
Turner’s Disease is the partial or completely missing X chromosome found in females. We have not talked about the actual process that occurs which results in the missing X chromosome.
As seen in the diagram above, when the sperm and egg come together, each chromosome has a sequential pair, and in the case of Turners Syndrome, the alternate X chromosome is missing. This loss of DNA coding leads to the eventual symptoms of the disease, which we have briefly spoken about in earlier posts with more information to come.
When the egg and sperm meet together, the DNA from each organisms replicate (splitting their chromosomes) and then join back together with the new pair (sperm and egg). When one of the chromosomes does not split to join with the other cell, the chromosome becomes in complete. This phenomenon is referred to as nondisjunction, which is the sole explanation for Turners Disease.
Once this missing chromosome is replicated, each cell from then one within that humans body has the missing (altered) X chromosome, leading to the multiple physiological and internal differences and abnormalities.
As stated before, Turner’s Syndrome is caused by genetic loss from one of the X chromosomes in women. A normal human has twenty three matched pairs of chromosomes, which results in a total of forty six chromosomes. Every cell has twenty two pairs of chromosomes that are called autosomes. According to the Merriam Webster dictionary, an autosome is a chromosome other than a sex chromosome. The remaining pair of chromosomes is the sex chromosomes, which in females are signified as X chromosomes. The two X chromosomes are necessary for normal ovarian development in females. There are actually different types and names for the genetic losses that occur with the X chromosomes in Turner’s Syndrome.
The first type of genetic loss is called X-Chromosome Monosomy. This is when the X chromosome is completely lost when oocytes are formed in the woman. If this were to occur in men, the embryo would die because men need both an X and a Y chromosome. Women can survive with Turner’s because they already have one X chromosome. According to http://turners.nichd.nih.gov/genetic.html the, “karyotype X-monosomy is termed, “45X” meaning that an individual has 44 autosomes and a single X-chromosome. The usual female karyotype is 46, XX.” A karyotype is a test that determines the size, shape, and number of chromosomes present in a cell. For Turner’s, the physician conducts a blood test to examine the chromosomes found in lymphocytes. .
Genetic Features – Figure 1. Loss of a sex chromosome during gametogenesis
A – Normal meiosis leads to production of spermatocytes with a single sex chromosome.
B – Abnormal meiosis leads to production of a spermatocyte with no sex chromosome.
C & D – If the abnormal sperm fertilizes a normal egg, the result will be a 45, X embryo. The same result happens if an egg loses its X chromosome during meiosis, and then gets fertilized by a normal sperm containing a single X
The image above is from http://turners.nichd.nih.gov/genetic_fig1.html and explains X-Chromosome Monosomy
The second type of genetic loss is called X-Chromosome Mosaicism. An X chromosome is lost during the early embryonic stages of development, so that some of the cells receive one X chromosome. In this condition, the clinical features of the syndrome correlate with the percentage of 45X cells in the woman’s body. If only a small percentage of cells are affected, the symptoms of Turner’s will mostly be less severe. An example of this is that a woman with a large percentage may never experience menstruation. A woman with a smaller percentage may have her menstruation, but it may not happen until she is twenty or older. To diagnosis this, a physician must conduct a blood test. The genotype for this is specified as 45X (10)/ 46XX (90).
Genetic Features – Figure 2. Loss of Sex Chromosome During Early Embryo Development
If an X chromosome is lost during an early cell division, daughter cells – some 45, X and some 46, XX are variably mixed together giving rise to all the differentiated tissues of the girl in a mosaic pattern. The manifestations of Turner syndrome will depend on the proportion of normal vs. monosomic cells in the different tissues. The karyotype is specified: 45, X/46, XX
The third type is X chromosome defects. The X chromosome is not completely missing. It can be fragmented, deleted portions, or have structural problems, such as the ring formation. The consequences of having this are very widespread over a variety of different problems. A small deletion will have very small effects, like a short stature or ovarian failure. A larger deletion could cause the woman to have all of the symptoms of Turner’s Syndrome.
The diagram above was found at http://turners.nichd.nih.gov/genetic_fig2.html and explains X-Chromosome Mosaicism.
Genetic Features – Figure 3. X Chromosome Abnormalities
During cell divisions in the process of gamete (egg or sperm) formation, an X or Y chromosome may undergo breakage or rearrangement.
- A chromosome consisting of two long arms, or q segments, is called an “isoXq” chromosome, and it is missing the short arm (p).
- Sometimes just a portion of the distal short (delXp) or long arm (delXq) is deleted.
- Sometimes breakage of the terminal ends of both long and short arms leads to joining together of the broken ends, forming a ring chromosome (rX).
The diagram above was found at http://turners.nichd.nih.gov/genetic_fig3.html and is about chromosomal abnormalities.