What is the modal number of chromosomes in a Down syndrome translocation carrier?

In the context of Down syndrome, particularly the translocation type, understanding the modal chromosome number is crucial. A person with Down syndrome often has 47 chromosomes due to an extra chromosome 21, resulting in three copies instead of the usual two. However, translocation Down syndrome occurs when a portion of chromosome 21 is attached to another chromosome.

When a person is a translocation carrier, they carry a normal complement of chromosomes but have a rearrangement, typically resulting in a total of 45 chromosomes. This happens because one chromosome is involved in the translocation, thereby reducing the total count. Consequently, the normal complement of chromosomes (46) is disrupted, leading to the modal number being 45 for carriers who may not express the phenotype of Down syndrome but can still pass on the condition.

This understanding highlights the significance of translocations in chromosomal integrity and the implications for genetic counseling and diagnostics.