Chromosomal Disorders
What are the most common chromosomal disorders? There are numerous chromosomal disorders. The actual disorder arises from some type of malformation during mitosis. There is usually an extra chromosome or a part missing of a chromosome. Trisomy 13, Trisomy 18, and Trisomy 21 are the most common chromosomal disorders.Trisomy 13 Syndrome is a chromosomal disorder resulting in a syndrome characterized by specific (midline) dysmorphic features and organ malformations. Trisomy 13 occurs in one out of every 4,000 to 10,000 live births and affects the thirteenth set of chromosomes. Advanced maternal age is the contributing risk factor to Trisomy 13. Genetics contribute to seventy-five percent of all cases. A single defect during the first three weeks of development of the prechordal mesoderm can lead to morphologic defects of the midface, eyes, and forebrain and induction defects on the prosencephalon (cerebral hemispheres, diencephalon, hypothalamus, thalamus). Some facial abnormalities generally associated with Trisomy13 are: microcephaly with sloping forehead, aplasia cutis congenital, cleft lip and low set ears. Very often there are problems with the central nervous system like: holoprosencephaly, apn
Trisomy of only the bottom 1/3 of chromosome 21 is sufficient to account for Down's phenotype. Risk factors are increased maternal age (although 80% are born to mothers <35 years). Skeletal abnormalities include: webbed neck and chest, short sternum, widely spaced nipples, small pelvis, congenital dislocation of the hips, limited hip abduction. In conclusion, Trisomy 13, Trisomy 18 and Trisomy 21 are the most common chromosomal disorders. Seventy-five of fetuses with Trisomy 21 abort spontaneously. Trisomy 18 occurs in every one in eight thousand live births, affects the eighteenth set of chromosomes, most die in embryonic or fetal life, second most common autosomal aberration, and second most common multiple malformation syndrome. Facial abnormalities include: microcephaly with prominent occiput, narrow bifrontal diameter, short palpabral fissures, low-set malformed ears, cleft lip, and micrognathia. Trisomy 21 occurs in an amazing one of every 800 births. Trisomy 21, often known as Down's Syndrome, is a chromosomal disorder resulting in a syndrome characterized by specific dysmorphic features (facies) and organ malformations. Trisomy 21 cases usually suffer from: respiratory tract infections, acute and chronic airway obstruction, sleep apnea (small airways, large tonsils/adenoid, obesity), and cor pulmonale. Organ malformations often seen in the Central Nervous System causing severe mental retardation, hypotonia -> hypertonia, neural tube defects, poor suck and weak cry, failure to thrive, ocular anomalies. The one contributing factor seen present in all of these syndromes is advanced maternal age. This disorders occurs in one of every eight hundred live births. Trisomy 18 usually have a very poor prognosis with thirty percent dying by one month of age, fifty percent dying by two months of age, and ninety percent dying by twelve months of age. Principle Clinical Features in the Newborn (Clin Peds 5:4 (1966))o 100% contain at least 4 of these o 89% contain at least 6 of these: "X flat facial profile (90%) "X poor Moro reflex (85%) "X infantile hypotonia (80%) "X hyperflexibility of joints (80%) "X excess skin on neck (80%) "X slanted palpebral fissures (80%) "X dysplasia of pelvis (70%) "X anomalous auricles (60%) "X dysplastic 5th midphalanx (60%) "X 45 simian crease (45%) Listed are just a few of the many traits associated with Down's Syndrome: wide-spaced first toe, excess skin on back of neck, brushfield spots, seizures, learning disabilities, borderline to moderate IQ range (from 80 -> 40) , Global Developmental Delays, gross motor due to hypotonia, speech and language delay , 75% have an expressive language disability, behavioural problems, aggression, depression, inappropriate, and Alzheimers.
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