If a portion of a chromosome containing a specific gene is deleted, the amount of the gene product will be determined only by the remaining homologue. For example, people with an interstitial deletion of part of the long arm of chromosome 13 may have serum levels of esterase D that are 50% of normal. When several such individuals were also found to have retinoblastoma, it was suggested that both the esterase and the retinoblastoma genes are located in the missing segment. In contrast to the reduced activity caused by a deletion, a duplication may produce 150% of normal activity of a given gene product, as a result of either a chromosomal trisomy or a triplication of a specific chromosomal segment. Gene dosage appears to be a mechanism of disease in anterior segment dysgenesis, caused by duplication or deletion of the FOXC1 gene; both 50% and 150% of the transcription factor lead to this dysgenesis.
Excerpted from BCSC 2020-2021 series: Section 2 - Fundamentals and Principles of Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.