In a sample of 1000 patients with Down syndrome, a geneticist discovers that \(95 \%\) of them are trisomic, while \(5 \%\) have diploid number of chromosomes. Explain this discrepancy.

Down syndrome is a genetic disorder caused by the presence of an extra copy of chromosome 21. This extra chromosome can occur in three forms: trisomy, translocation, and mosaicism. Trisomy is when an individual has three copies of chromosome 21, instead of the usual two copies. This can happen when chromosome 21 fails to separate properly during the formation of reproductive cells (eggs and sperm). This form of Down syndrome is the most common and is also the one mentioned in the exercise with 95% of the patients with Down syndrome being trisomic. The diploid number of chromosomes is the normal number of chromosomes that should be present in a human cell, which is 46 (23 pairs). In the context of Down syndrome, 5% of the patients display a diploid number of chromosomes, implying that they have the normal number of chromosomes, yet still exhibit Down syndrome symptoms. This could be due to cases like translocation or mosaicism.

The main cause of the discrepancy in the given percentages (95% trisomic and 5% diploid) lies in the different ways Down syndrome can occur. Most cases of Down syndrome are due to trisomy 21, where the individual inherits an extra chromosome 21 from one of the parents unintentionally. In such cases, individuals will have a total of 47 chromosomes instead of the normal 46 chromosomes. The likelihood of trisomic occurrence increases with parental age, particularly in women older than 35. On the other hand, Down syndrome can also happen with a diploid set of chromosomes (46 chromosomes) in rare cases. This is found in patients with translocation or mosaicism. In translocation, a part of chromosome 21 attaches itself to another chromosome, which results in the same effects as trisomy 21. In mosaicism, some cells in the body have trisomy 21, while others have the normal diploid number of chromosomes. Individuals with mosaic Down syndrome usually exhibit milder symptoms compared to those with trisomic Down syndrome.

The discrepancy in the percentages of trisomic and diploid cases in Down syndrome patients is due to the different ways Down syndrome can manifest. While the majority of patients have trisomy 21, a smaller portion of cases involves the diploid number of chromosomes due to rare situations such as translocation or mosaicism. These different manifestations lead to variations in the severity of Down syndrome symptoms and explain the given discrepancy.