When does the Genetic Defect occur?
Turner syndrome is found to occur early during the division of the sex cells. Within this process of dividing cells, an X chromosome is simply lost or absent, leaving only one set of X chromosomes. An egg that is fertilised with only one X chromosome results in Turner syndrome, and a fertilised egg containing only one Y chromosome is incapable of life. A fertilised egg containing only one X chromosome is a sporadic event and occurs in about 1 in every 2500 children born [1].
Figure 1.1 demonstrates the absence of a whole X chromosome within the process of meiosis. When both sperm and egg join to begin fertilisation, the Genetic information (DNA) from each sex cell is split as shown in the diagram. However, this diagram shows both outcomes of meiosis, the first being the normal separation which occurs in the majority of cases. Meiosis is the process in which a sex cell undertakes in order to produce gametes. This process is essentially divided up into to main generalised sections; Meiosis I and Meiosis II. Meiosis I is responsible for the reduction or separation of the total amount of chromosomes along the Metaphase (the equator of the cell) then these homologues separate it's called Anaphase [refer to Figure 1.2] . Essentially the original number of chromosomes are split and separated into different cells. Meiosis II is responsible for separating the sister chromatins into their own gamete cells. "Normal Separation" [Figure 1.1] is what is considered to be the general process obtained during Meiosis, "Non-disjunction" is the beginning process of the defect (this occurs on the far right of the figure in Meiosis I, where there are no chromosomes present as well as in Meiosis II at the far bottom left). Turner syndrome occurs when one of the chromosomes during replication do not split as they should, leaving the chromosome incomplete. This is called Non disjunction; the failure of sister chromatins to separate during and after mitosis. [2] As shown in Figure 1.1 non-disjunction within Meiosis I will result in a two trisomic gametes and two monosomic gametes (as shown on the right side of Figure 1.1). As oppose to Meiosis II if non-disjunction were to occur in this stage of the process it would result in two of the gametes being normal, one gamete being trisomic, and the other being monosomic [4]. Thus concluding non-disjunction that occurs in meiosis II does decrease the chance of having a child with Turners syndrome as two out of the four gametes are recognised as normal. As opposed to non-disjunction that occurs in Meiosis I, which results in a 50% chance of a trisomic gamete and 50% chance that it will be a monosomic gamete. |
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Who is responsible, Mum or Dad?This defect is in fact not passed down through the parents, it is simply not inherited from anyone. This is because Women with Turner Syndrome are infertile due to lack of development in the ovaries (the are commonly sterile). Therefor it would be impossible for a child to inherit this genetic defect from their mothers. It would also be impossible to inherit this defect from the father, as Turner syndrome only exists in females "It's a girl thing".
However statistics show that 75-80% of cases of girls with Turner syndrome, the single X chromosome comes from the mother's egg and the the father's sperm that fertilises the egg is missing its sex chromosome. [5] There is no known biological explanation for this, aside from it being a random occurrence. |
Reference List
[1] Health Grades Inc.. (2014). Statistics by Country for Turner Syndrome.Available: http://www.rightdiagnosis.com/t/turner_syndrome/stats-country.htm. Last accessed 9th August 2014.
[2] CASPION. (2008). Nondisjunction. Available: http://www.biology-online.org/dictionary/Nondisjunction. Last accessed 10th August 2014.
[3] Utoronto. (2006). Index of /~w3bio380/picts/lectures/lecture4.Available: http://www.utm.utoronto.ca/~w3bio380/picts/lectures/lecture4/. Last accessed 13th August 2014.
[4] University of Washington, Seattle. (2004). Karyotype Analysis of Human Chromosomes. Available: http://www.biology.iupui.edu/biocourses/N100/2k4csomaldisordersnotes.html. Last accessed 10 August 2014
[5] Biology Exams 4 U. (2014). What happens if Non-disjunction.Available: http://www.biologyexams4u.com/2012/09/what-happens-if-non-disjunction-occurs.html#.U-3e4oCSxuA. Last accessed 12th August 2014.
[6] University of Utah, Health Sciences. (2014). Turner Syndrome.Available: http://learn.genetics.utah.edu/content/disorders/chromosomal/turner/. Last accessed 10th August 2014.
[2] CASPION. (2008). Nondisjunction. Available: http://www.biology-online.org/dictionary/Nondisjunction. Last accessed 10th August 2014.
[3] Utoronto. (2006). Index of /~w3bio380/picts/lectures/lecture4.Available: http://www.utm.utoronto.ca/~w3bio380/picts/lectures/lecture4/. Last accessed 13th August 2014.
[4] University of Washington, Seattle. (2004). Karyotype Analysis of Human Chromosomes. Available: http://www.biology.iupui.edu/biocourses/N100/2k4csomaldisordersnotes.html. Last accessed 10 August 2014
[5] Biology Exams 4 U. (2014). What happens if Non-disjunction.Available: http://www.biologyexams4u.com/2012/09/what-happens-if-non-disjunction-occurs.html#.U-3e4oCSxuA. Last accessed 12th August 2014.
[6] University of Utah, Health Sciences. (2014). Turner Syndrome.Available: http://learn.genetics.utah.edu/content/disorders/chromosomal/turner/. Last accessed 10th August 2014.