INTRODUCTION:-A trisomy is a genetic abnormality in which there are three copies, instead of the normal two, of a particular chromosome. A trisomy is a type of aneuploidy (an abnormal number of chromosomes) .Full trisomy" means that an entire extra chromosome has been copied. "Partial trisomy" means that there is an extra copy of part of a chromosome.Trisomies are sometimes characterized as "Autosomal trisomies" (trisomies of the non-sex chromosomes) and "Sex-chromosome trisomies." Autosomal trisomies are described by referencing the specific chromosome that has an extra copy. Most common types of autosomal trisomy that survive to birth in humans are: Trisomy 21 (Down syndrome) Trisomy 18 (Edwards syndrome) Trisomy 13 (Patau syndrome) ,Trisomy 12 (Chronic Lymphocytic Leukemia) ,Trisomy 9, Trisomy 8 (Warkany syndrome 2) ,Trisomy 22
 CAUSES:-The number of chromosomes is different for different species. Human beings have 46 chromosomes (i.e. 23 pairs of chromosomes). Human gametes have only 23 chromosomes. If the chromosome pairs fail to separate properly during cell division the egg or sperm may have a second copy of one of the chromosomes. (See non-disjunction.) If such a gamete results in fertilization and an embryo, the resulting embryo may also have an entire copy of the extra chromosome. Trisomy can also be caused by a rearrangement of the chromosomes inside the person's cells. This is known as translocation and it is inherited from one of the parents
SYMPTOMS:-Cleft lip or palate, Clenched hands (with outer fingers on top of the inner fingers)Close-set eyes -- eyes may actually fuse together into one, Decreased muscle tone ,Extra fingers or toes (polydactyly),Mental retardation, severe, Scalp defects (missing skin),Seizures, Small head (microcephaly),Undescended testicle (cryptorchidism)
DIAGNOSIS:-Before birth MRI, Ultrasound, CT, X-ray, and genetic testing can detect the presence of trisomy 18.Chromosome studies show trisomy 13, trisomy 13 mosaicism, or partial trisomy.
 TREATMENT:-Treatment of this syndrome focuses on the particular physical problems with which each child is born. Many infants have difficulty surviving the first few days or weeks due to severe neurological problems or complex heart defects. Surgery may be necessary to repair heart defects or cleft lip and cleft palate. Physical, occupational, and speech therapy will help individuals with Patau syndrome reach their full developmental potential.
 PREVENTION:-Parents of infants with trisomy 13 that is caused by a translocation should have genetic testing and counseling, which may help them avoid having another child with the condition.

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