As stated previously, Hox genes are the family of genes that control development of body plan. They determine how different areas of the developing embryo become different body parts: where does the head go, where do the legs go — that sort of thing.
The Hox genes are responsible for body patterning in most animals. Even though different animals look completely different, the underlying genes are clearly related. You have them, a mouse has them, a fly has them. Although humans are extremely different from fruit flies, the genes responsible for body patterning are identifiable in both species. The copies in mammals are different from the ones in flies, but not so different in DNA sequence that scientists can't see their common origin in a distant ancestor.
A fly and a mouse have similar Hox genes, for example, but they don't have exactly the same collection of these genes; the exact sequences are different. Also, Hox genes can become duplicated just as can other genes in the imperfect DNA replication process, so different animals have different numbers of Hox genes. Looking across different animals, scientists see cases where some animals have several copies a very similar Hox genes — evidence of past gene duplications — while others will have a different number of copies of related genes.
The fact that the genes for body patterning would be recognizably similar across such different animals was quite a revelation — and a major breakthrough in the evo-devo field. It gives scientists deep insights into how changes at the level of the DNA can result in changes in animal body plan.
Here's the big take-home message: Developmentally you're not all that different from a fly. Understanding Hox genes takes us a long way toward understanding how small changes at the level of the DNA, besides the things we actually know to happen, could result in large differences in animal form.
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