Question

In a recent cytogenetic study on 1021 cases of Down syndrome, 46 were the result of translocations, the most frequent of which was symbolized as \(t(14 ; 21) .\) What does this symbol represent, and how many chromosomes would you expect to be present in \(t(14 ; 21)\) Down syndrome individuals?

Short answer

Answer: Individuals with translocation Down syndrome t(14;21) have 46 chromosomes, but with extra genetic material from chromosome 21.

Step-by-step solution

Understand the translocation symbol

The symbol \(t(14; 21)\) represents a chromosomal translocation. In this case, it indicates that a portion of chromosome 14 has exchanged places (translocated) with a portion of chromosome 21. This is the most frequent type of translocation that leads to Down syndrome.

Analyze normal chromosome count

In normal human cells, there are 46 chromosomes, which are organized into 23 pairs. One chromosome from each pair is inherited from the mother and the other from the father.

Analyze Down syndrome chromosome count

Down syndrome typically occurs when there is an extra copy of chromosome 21, resulting in a total of 47 chromosomes instead of 46. This additional chromosome 21 leads to the characteristics associated with Down syndrome.

Analyze \(t(14; 21)\) chromosome count

In the case of translocation Down syndrome \(t(14; 21)\), the extra chromosomal material from chromosome 21 gets attached to chromosome 14. As a result, these individuals still have a total of 46 chromosomes, but with extra genetic material from chromosome 21.

Calculate the number of chromosomes in \(t(14; 21)\) Down syndrome individuals

Since in translocation Down syndrome, no extra chromosomes are present, and given that the individuals still have the normal count of 46 chromosomes, we can conclude that individuals with \(t(14;21)\) Down syndrome have 46 chromosomes, but with extra genetic material from chromosome 21 that leads to the manifestation of Down syndrome.

Key concepts

Cytogenetic Study

A cytogenetic study involves the examination of chromosomes under a microscope to identify their number and structure. This process helps researchers and healthcare professionals detect chromosomal anomalies that may lead to genetic disorders.

For example, in the context of Down syndrome, a cytogenetic study can reveal whether the condition is caused by a common trisomy 21, where there's an extra copy of chromosome 21, or by a chromosomal translocation, such as the one indicated by the symbol t(14; 21). In the translocation form of Down syndrome, parts of chromosomes 14 and 21 swap places, potentially disrupting gene function without necessarily changing the overall chromosome count. The study of these chromosomal patterns provides crucial information for diagnosis, management, and counseling in cases of genetic abnormalities.

Chromosome Count Analysis

Chromosome count analysis is a fundamental aspect of cytogenetics, where the total number of chromosomes in a cell is determined. Typically, humans have 46 chromosomes, comprising 23 pairs, with one set inherited from each parent.

In chromosome count analysis for Down syndrome, especially in translocation cases like t(14; 21), although the chromosome count remains normal at 46, the structural rearrangement results in the manifestation of Down syndrome characteristics. The analysis not only accounts for the number of chromosomes but also their integrity and any structural changes, which are critical for understanding genetic conditions and potential impacts on health.

Genetic Abnormalities

Genetic abnormalities refer to any mutation, alteration, or anomaly in the genetic material of an organism that may lead to developmental issues, diseases, or syndromes such as Down syndrome. These abnormalities can range from small mutations within DNA to larger chromosomal abnormalities like deletions, duplications, inversions, or translocations.

Translocations, such as the one that leads to t(14; 21) Down syndrome, are a type of chromosomal abnormality where segments of chromosomes break off and reattach to different chromosomes. This can affect gene expression and function. Understanding the nature of these abnormalities is key to diagnosing and treating various genetic disorders and to providing genetic counseling for affected families.