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Autosomal trisomies: Pathology review
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A 1 day old newborn boy, named Nikolas, is brought to the emergency department due to frequent vomiting of a "greenish liquid” immediately after meals. Physical examination shows a flat nasal bridge, small mouth with a protruding tongue, and a single palmar crease on each hand. A plain abdominal x-ray reveals a double bubble appearance on the upper abdomen, with no gas seen distally. The infant was born at home to a 41 year old mother who received no prenatal care and is unable to provide any medical history. Some days later, a 39 year old mother gives birth to a female baby, named Taylor, through emergency cesarean section at 36 weeks of gestation. Taylor is found to have a punched out lesion on the left side of her scalp, where skin is missing. On further examination, her head is smaller compared to infants of the same age and gender, and she has an extra finger on her right hand. The mother lives in a remote area and was not able to receive any prenatal care. Finally, 37 year old Annita visits the prenatal clinic at 16 weeks of gestation for the quadruple screen test. Results show a low level of maternal serum alpha-fetoprotein or AFP for short, low human chorionic gonadotropin or hCG, low unconjugated estriol, and normal inhibin A. She has not undergone any first trimester screening.
Based on the initial presentation, all cases seem to have some form of autosomal trisomy. This is where the baby ends up with three copies of an autosomal chromosome instead of two. For your exams, remember that, in most cases, this results from a process called nondisjunction. This typically occurs during meiosis 1, where a chromosome pair in the egg or sperm cell doesn’t split apart. So the child of this individual could receive 2 chromosomes from that parent and 1 more from the other parent. The resulting zygote will have three autosomal chromosomes or an autosomal trisomy.
Another topic examiners love to focus on is Robertsonian translocation, which means that a piece of one chromosome translocates over to another chromosome. The result is a hybrid chromosome with both long arms and one hybrid with both short arms. The one with the short arms is typically lost by the end of meiosis. Having both long arms leads to “balanced carriers”, since most of the genes are still there. Now, the translocation can also be unbalanced, if one normal chromosome ends up with the short arm, and the other normal chromosome with the long arm. And since the long arms carry most of the genetic material, cells with the long arm will basically have one extra chromosome, which, when combined with the other parent’s again, will result in trisomy, while cells with the short arm are basically missing a chromosome and can result in monosomy.
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