Question

under what conditions do duplications or deletions of individual genes sometimes cause significant phenotypic effects?

Answer 1

Gene duplication causing significant phenotypic effect:​

1. Position effect: The expression of the gene is modified based on its position in the chromosome. When the duplicated genes from euchromatin region are placed near the heterochromatin region, their expression level reduces. Thus the position of gene affects the gene expression. Eg: 'Bar-eye' in Drosophila.
2. Gene dosage: Duplication of gene might lead to increased product of gene expression which can cause altered physiological functions. For example, in Down syndrome, the genes in chromosome 21 is expressed more which contributes to the disease.
3. Enzyme activity: Gene duplication causes increased or reduced activity in certain enzymes. Eg.Duplication of a segment of chromosome 6 in Barley resulted in α-amylase with higher activity.
4. If the duplicated gene (gene1) is inserted in the coding region of another gene(gene2), then the expression of both the genes (resulting in a inactive protein) or one gene is lost (only gene1 is expressed).

Gene deletion causing significant phenotypic effect:​

Gene deletion mostly results in no expression of that particular gene. Consequently, the protein coded by that gene is not produced and causes changes in cellular function, unless the effect of deletion is nullified by the expression of the same gene in the other pair of chromosome.