How repeatedly mating the same domesticated wolves gave us all these different dog breeds?
In: Biology
This is not exactly ELI5, but gets at the core of your question. *One* of the many reasons is that dogs have a lot of Short Interspersed Nuclear Elements (SINEs). SINEs are short bits of DNA that because of the way they are coded, get copied and reinserted elsewhere in the genome. They are not genes because they don’t make a protein, they are just sequences with instructions that say “duplicate me and put me back somewhere else on the genome” but don’t specify where. As a result you will have multiple copies of different SINEs floating around. Over generations these SINEs will also “evolve” away from one another and become slightly different in their sequences and lengths.
When these SINEs get inserted into genes, they will affect the gene based on where in the gene it is inserted. So you could have the same SINE inserted into different locations on the same gene in different dogs, which leads to different physiological outcomes. For example, all of the different merle colorations (think leopard dogs, australian shepherds, cardigan welsh corgies, etc.) are due to the SILV SINE. There is variation in the merle coloration because of the length of the different SILV SINE inserts and their placements on one gene responsible for depositing melanin (PMEL).
Because dogs have an abundance of these SINEs, there is also more occasion for SINEs to be copied and inserted. Then, when you couple that with breeding and the fact that each individual gets one copy of DNA from each parent, you can get a lot of variation.
Fun fact about SINEs, primates have a SINE family called the ALU sequence, and the number of copies and their positions on the genomes are different between primate species and also within populations of the same species, so they can be used as population markers.
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