An in-situ hybridization is used to determine where a certain gene is active. To translate genetic information into a protein, RNA is needed. This RNA can be fused with a probe and through a biochemical process, a pattern becomes visible. The picture depicts an in-situ hybridization of a Drosophila melanogaster ( = fruit fly) embryo, the expressed gene is called toy ( = twin of eyeless) and is here shown to be expressed in the nervous system.
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Picture was taken by me