Question

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.

Short answer

Question: Explain the discrepancy between the 95% of Down syndrome patients who are trisomic and the 5% who have a diploid number of chromosomes. Answer: 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.

Step-by-step solution

Understanding Trisomy and Diploidy in Down Syndrome

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.

Explaining the Discrepancy

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.

Conclusion

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.

Key concepts

Understanding Trisomy

Trisomy is a genetic condition where an individual has an extra chromosome, leading to three copies instead of the usual two. This happens most commonly with chromosome 21, which is the basis for Down syndrome. Imagine our genetic code as a set of encyclopedias. Normally, each chromosome represents one volume in a matched pair, similar to having two copies of the same volume. However, in trisomy 21, there is an extra copy of this genetic 'volume.'

The most common form of trisomy is Trisomy 21, causing Down syndrome, where there are three copies of chromosome 21. During cell division, if chromosomes do not separate correctly, one cell may end up with an extra chromosome, leading to trisomy. This mistake is more likely as parents age, particularly if the mother is over 35.

Because it affects almost every cell, Trisomy 21 causes a range of symptoms and characteristics that are typical in Down syndrome. This includes developmental delays, specific facial features, and sometimes other health issues. It's this widespread presence of an extra chromosome that explains why 95% of those with Down syndrome have trisomy.

• Occurs when there are three copies of chromosome 21.
• Most common in Down syndrome patients.
• Risk increases with parental age.
• Leads to characteristic symptoms found in Down syndrome.

What is Translocation?

Translocation is another genetic feature intimately involved with Down syndrome but is significantly less common than simple trisomy. In translocation, a piece of chromosome 21 detaches and attaches to another chromosome. This can lead to similar symptoms as seen in Trisomy 21, despite individuals having the "normal" 46 chromosomes.

Consider translocation like rearranging paragraphs between different volumes of an encyclopedia. A part of the content from chromosome 21 gets misplaced into another chromosome. Even though the number of chromosomes remains typical at 46, the added genetic material from chromosome 21 disrupts normal development leading to characteristics of Down syndrome.

Unlike typical trisomy, translocations can be inherited. This means a parent might carry a rearranged chromosome without any symptoms, but they can pass the translocated chromosome to their child, causing Down syndrome. Genetic counseling is crucial for families with a history of this form of Down syndrome.

• Part of chromosome 21 attaches to another chromosome.
• Results in the appearance of Down syndrome symptoms.
• Individual still has 46 chromosomes.
• Can be inherited, requiring genetic counseling.

Exploring Mosaicism

Mosaicism in Down syndrome is a unique phenomenon where not all cells have the extra chromosome 21. Instead, there is a mix of normal (46 chromosomes) and trisomic (47 chromosomes) cells throughout the body. Think of it as parts of an "encyclopedia" where some volumes have duplicated chapters while others do not. Therefore, the symptoms may vary in their severity and may be less pronounced than in full trisomy 21.

This mixed-cell condition usually arises during early cell division shortly after conception. A mistake in splitting the chromosomes occurs in some cells but not others. As a result, this leads to a mosaic pattern where some parts of the body have normal chromosome numbers and others do not.

Mosaicism might cause milder versions of Down syndrome, and it can present a broad spectrum of effects. Genetic testing is essential to confirm mosaicism and understand its implications on an individual's development and health.

• Presence of both normal and trisomic cells.
• Symptoms of Down syndrome may be milder.
• Occurs due to error in early cell division.
• Requires specific genetic testing for diagnosis.