Answer the following question given this paragraph: 

One example of a gap gene is the Krüppel gene, which regulates the activity of a number of other genes. Krüppel literally means "cripple" in German, named for the crippled appearance of mutant larva. Gap genes are defined by the effect of a mutation in that gene, and Krüppel is one such effect. Other gap genes are known as tailless and knirps (a "squirt or whippersnapper"). Gap genes encode transcription factors which directly effect the expression of additional genes involved in embryo segmentation, called the pair-rule genes . Pair-rule genes are expressed in alternating segments within the developing embryo. Pair-rule genes have very creative names, such as even-skipped, hairy, odd-skipped, odd-paired, sloppy paired and fushi tarazu , which is Japanese for “few segments. ” See My favorite silly gene names at http://www. itsokaytobesmart. com/post/82318778810/funny-gene-names for a list of other interesting gene names.


Q: Gap genes are defined by the effect of what in that gene?


A:
mutation