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?

Verified step by step guidance

1. Understand that the symbol t(14;21) represents a chromosomal translocation between chromosome 14 and chromosome 21. This means a segment of chromosome 21 is abnormally attached to chromosome 14, or vice versa.
2. Recall that in Down syndrome caused by translocation, the extra genetic material from chromosome 21 is present due to this rearrangement, rather than having a full extra chromosome 21 as in trisomy 21.
3. Recognize that a normal human karyotype has 46 chromosomes, arranged in 23 pairs. In translocation Down syndrome, the total chromosome number can be different depending on whether the translocation is balanced or unbalanced.
4. Determine that in the common t(14;21) translocation causing Down syndrome, the individual typically has 45 chromosomes because two chromosomes have fused into one (the translocation chromosome), but there is still an extra copy of the critical part of chromosome 21.
5. Summarize that the t(14;21) symbol indicates a translocation between chromosomes 14 and 21, and individuals with this translocation Down syndrome usually have 45 chromosomes instead of the typical 46.

Key Concepts

Chromosomal Translocations

A chromosomal translocation occurs when a segment from one chromosome breaks off and attaches to another chromosome. In Down syndrome, a common translocation involves chromosomes 14 and 21, denoted as t(14;21), where part of chromosome 21 is translocated to chromosome 14. This can lead to an extra copy of chromosome 21 material, causing the syndrome.

Karyotype and Chromosome Number

A normal human karyotype has 46 chromosomes arranged in 23 pairs. In translocation Down syndrome, the total chromosome count can remain 46 because the extra chromosome 21 material is attached to another chromosome rather than existing as a separate chromosome. Thus, individuals with t(14;21) typically have 46 chromosomes, but with an abnormal structure.

Down Syndrome Genetic Mechanism

Down syndrome is caused by trisomy 21, meaning three copies of chromosome 21 material. In translocation cases like t(14;21), the extra chromosome 21 material is attached to chromosome 14, resulting in functional trisomy 21 despite a normal chromosome count. This explains the presence of Down syndrome traits without the typical 47 chromosomes.