mutation is like a mistake in a knitting pattern

An example of an analogy used in popular science writing:

"Until quite recently, mutations in gene sequences were thought to be important not of because of the change in the DNA itself but because of their downstream consequences. It's a little like a mistake in a knitting pattern. The mistake doesn't matter when it's just a notation on a piece of paper. The mistake only becomes a problem when you knit something and end up with a hole in your sweater or three sleeves on your cardigan because of the error in the knitting code.

A gene (the knitting pattern) ultimately codes for a protein (the sweater).

… The DNA gene is the original knitting pattern. This pattern can be photocopied multiple times, akin to producing the RNA. The copies can be sent to lots of people who can each knit the same pattern multiple times, just like creating the protein. It's a simple but efficient operating model and it works – one original pattern resulted in lots of soldiers with warm feet in the Second World War.

…Researchers showed that when cells contained this expanded repeat, they stopped producing the messenger RNA encoded by the gene. Because they didn't make messenger RNA, they couldn't make the protein either, if you don't send out the copies of the knitting patterns, the soldiers don't get socks."

Nessa Carey (2015) Junk DNA. A journey through the dark matter of the genome. London: Icon Books Ltd.

Carey makes explicit the mapping in the analogy.

Read about analogy in science

Read examples of scientific analogies

Many examples of science analogies are listed in 'Creative comparisons: Making science familiar through language. An illustrative catalogue of figurative comparisons and analogies for science concepts'. Free Download.

'Downstream' can be considered as a metaphor (as there is no actual stream, so this is a figure of speech).