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Correct Answer: nondisjunction.
down syndrome, also known as trisomy 21, is a genetic disorder characterized by the presence of an extra copy of chromosome 21. this additional genetic material disrupts the normal course of development, causing the characteristic facial features, intellectual disability, and other health issues associated with the syndrome. the primary cause of down syndrome is nondisjunction, a chromosomal error that occurs during the cell division process known as meiosis.
nondisjunction is a failure in the proper separation of chromosomes during the anaphase stage of meiosis. normally, each chromosome pair should separate into two daughter cells, each receiving one copy of the chromosome. however, in nondisjunction, both chromosomes are pulled into the same daughter cell, resulting in one cell with an extra chromosome (trisomy) and one cell with a missing chromosome (monosomy). when the gamete (sperm or egg) with an extra chromosome 21 fuses with a normal gamete during fertilization, the resulting embryo has three copies of chromosome 21, leading to down syndrome.
although nondisjunction is the most common cause of down syndrome, other genetic mechanisms can also result in the same condition. one such mechanism is through a robertsonian translocation, where part of chromosome 21 is translocated onto another chromosome, typically chromosome 14. this translocation doesn't increase the total amount of genetic material but can lead to a trisomic condition if the translocated chromosome segregates with a normal chromosome 21 during meiosis.
it is important to distinguish these mechanisms from other genetic errors such as duplication or non-reciprocal crossing over. duplication involves the doubling of a chromosome segment rather than an entire chromosome, while non-reciprocal crossing over involves an unequal exchange of genetic material between chromosomes, which does not usually result in a trisomic condition like down syndrome.
in summary, down syndrome is primarily caused by nondisjunction during meiosis, leading to the presence of an extra chromosome 21 in the cells of the individual. this genetic error can occur spontaneously and is influenced by several factors, including maternal age. understanding the causes and mechanisms of down syndrome helps in the diagnosis, management, and potential prevention of this disorder.
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