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Bio Project

Edward's Syndrome:Trisomy 18A Mitotic Condition

Trisomy 18, otherwise known as Edward's Syndrome In order to understand this, we must first discuss meiosis.Meiosis is the processes of specialized sex cells, broken into meiosis one and two. Through these two parts of meiosis which include prophase, anaphase and telephase. In these phases one specialized cell will split and form four sperm or egg cells, called gametes. Meiosis can lead to genetic variation in many ways. One major example is crossing over, or genetic recombinaiton, is when in Prophase I, two homologous (or a pair of similar chromosomes) cross over and form a tetrad, and where the two chromosomes meet, is called a chiasma. At these points, the DNA flip flops and is switched to the opposite chromosome. Another factor that also determines genetic variation is independent assortment. This occurs during Metaphase I and is based on how the tetrads line up at the cell's equator before anaphase, and the pulling apart of chromosomes. The last most common factor to affect genetic variation is random fertilization, in which it is the simple variable of which sperm will fertilize which egg. When all these factors come together the calculations end up at (2^23)^2, with the 2 doubling for each crossing over, which occurs frequently. Edward's syndrome is caused by a nondisjunction, or chromosomes becoming "stuck" together, and travel to one gamete cell,causing it to have an extra chromosome, in this cause, and extra chromosome 18. This can also occur when an extra chromosome 18 is attached to the end of another chromosome. The resulting condition is possible spontaneous misscarriage, cleft lip and pallette,slow growth, short life span, abnormal head shape and overlapping fingers. Those that live past a month have severe learning disabilities.




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