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Down syndrome or Down's syndrome, [12] also known as trisomy 21, is a genetic disorder caused by the presence of all or part of a third copy of chromosome 21. [3] It is usually associated with developmental delays, mild to moderate intellectual disability , and characteristic physical features.
Current strains of Marek virus, decades after the first vaccine was introduced, cause lymphoma formation throughout the chicken's body and mortality rates have reached 100% in unvaccinated chickens. The Marek's disease vaccine is a "leaky vaccine", which means that only the symptoms of the disease are prevented. [12]
In some cases, the signs and symptoms are similar to those of Down syndrome. Changes to chromosome 21 include a missing segment of the chromosome in each cell (partial monosomy 21) and a circular structure called ring chromosome 21. A ring chromosome occurs when both ends of a broken chromosome are reunited.
An anaphase lag of a chromosome 21 in a Down syndrome embryo leads to a fraction of euploid cells (2n cells), phenomenon described as "aneuploidy rescue". There is considerable variability in the fraction of cells with trisomy 21, both as a whole and tissue-by-tissue. This is the cause of 1–2% of the observed Down syndromes. [4]
Brushfield spots are a characteristic feature of the chromosomal disorder Down syndrome or trisomy 21. They occur in 35–78% of newborn infants with Down syndrome. [ 2 ] Brushfield spots tend to be obscured by pigmentation of the anterior border layer of the iris in patients with darker irides.
The average IQ of a young adult with Down syndrome is 50, equivalent to the mental age of an 8- or 9-year-old child, but this number varies widely. [3] At the same time, they enjoy a rich social and emotional awareness. [4] Down syndrome is the most common chromosome abnormality in humans, [3] occurring in about one per 1000 babies born each ...
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Research has found that there is a correlation between advanced paternal age and risk of birth defects such as limb anomalies, syndromes involving multiple systems, and Down syndrome. [74] [35] [93] Recent studies have concluded that 5-9% of Down syndrome cases are due to paternal effects, but these findings are controversial. [74] [75] [35] [94]