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 designation represent, and how many chromosomes would you expect to be present in \(t(14 ; 21)\) Down syndrome individuals?

Short answer

Answer: The designation "t(14; 21)" represents a translocation between chromosomes 14 and 21 in Down syndrome individuals. In these cases, there is still extra genetic material from chromosome 21, resulting in the Down syndrome phenotype. However, due to the translocation, these individuals would have a total of 46 chromosomes.

Step-by-step solution

1. Understand the translocation designation

Translocations of chromosomes occur when a part of one chromosome breaks off and attaches to a non-homologous chromosome. The designation t(14; 21) indicates that there is a translocation between chromosomes 14 and 21. This means that a part of chromosome 14 has broken off and attached itself to chromosome 21, or vice versa.

2. Determine the normal number of chromosomes in a human

Normally, humans have 46 chromosomes in their cells, arranged in 23 pairs. One chromosome of each pair comes from the mother, and the other comes from the father.

3. Relate translocations to Down syndrome

Down syndrome is usually caused by an extra copy of chromosome 21, leading to a total of 47 chromosomes. In the majority of the cases, this extra copy is due to nondisjunction during meiosis, leading to trisomy 21. However, in some cases, like the t(14; 21) translocation, this extra material is on another chromosome due to a translocation event.

4. Understand the chromosome count in t(14; 21) Down syndrome individuals

In individuals with t(14; 21) Down syndrome, there is a translocation between chromosomes 14 and 21. Because the extra copy of chromosome 21 material is present in the translocated chromosome, individuals with this form of Down syndrome still have the extra genetic material associated with Down syndrome. However, since the translocation combines two chromosomes into one, the total chromosome count is still 46, not 47.

5. Conclusion

Thus, the designation t(14; 21) represents a translocation between chromosomes 14 and 21 in Down syndrome individuals. In these cases, there is still extra genetic material from chromosome 21, resulting in the Down syndrome phenotype. However, due to the translocation, you would expect these individuals to have a total of 46 chromosomes.