Certain genes are responsible for some major aspects of animal development. One of the most important discoveries in developmental biology (or in evolutionary biology, biology in general, and medicine, for that matter) is a set of genes called the Hox genes. These genes are responsible for the determination of body pattern — a sort of design plan for items such as where the legs go and where the head should be. Pretty important stuff! In effect, these genes control the process whereby the embryo is divided into segments, and then they determine the specific fates of different segments.
The initial research on Hox genes involved a fruit fly, Drosophila. Like other arthropods (invertebrates such as spiders, crustaceans, and insects), the fruit fly is made up of a series of segments, some that are the same and some that are different.
If you know what a millipede looks like, you're familiar with an arthropod with a lot of segments that are all pretty much the same. After the head each segment has two pairs of legs — a very simple design and the fossil record tells us that millipedes are a very ancient group of land animals.
Now consider a fly or other insect. The segments are more differentiated:
1 Head segments: This is where . . . well, where the head is. What can I say?
i Thoracic segments: The thoracic segment includes the internal organs. It's also where the legs are.
1 Abdominal segments: No legs, but this is where the reproductive structures are.
For an up-close-and-personal look at sections, treat yourself by performing a tasty and very informative dissection of a lobster. Find the head and thoracic segments, and then the repeating segments of the tail/abdomen, which are delectable! Note that seeing the segments in the thorax is easier if you take it apart a little bit — externally the thoracic segments are fused together. Not much in the lobster thorax is edible (although some people swear that the liver is a delicacy), but it offers a lot of good biology. Lobsters aren't cheap, of course, so it's important that you get the most for your dollar. Education is priceless!
¿jjjABEft You may be thinking, "Whoa! How can the mammalian body plan be organized by the same family of genes that organized the arthropod body plan, especially when the human body lacks all those repeating segments?" Consider your backbone. It's a structure of repeating segments (all those vertebrae). And at particular points along your spinal column, you have other structures, such as arms and legs.
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