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The abnormal animal featured here is the Shar-Pei dog, whose skin is severely rumpled. It turns out that the molecular basis for this abnormality is similar to that which causes the folding of the human brain. We go on to consider other "ripple" patterns (e.g., fingerprints) and why they're always asymmetric.
The abnormal animal featured here is a frog with its eyes in its mouth. In order to explain how it got that way, the chapter describes how retinas induce lenses. That leads to a discussion of induction in general and to a consideration of the overall strategies that animals use to build their anatomy.
We start with an odd mutation in flies that causes their legs to be double-jointed, but what is even stranger is that the extra joints are upside-down. This leads to a discussion of cell polarity not only in flies but also in the inner ear of humans. Two intercellular signaling pathways are involved: PCP and Notch.
The abnormal animal featured here is a type of whippet that has twice as much muscle mass as the normal breed due to a mutation. Similar mutations have been found in cattle, horses, and people. In order to see how this can happen, we discuss how genes control tissue growth in general via the insulin pathway.
The abnormal animal featured here is a breed of dog with extra toes on its hind feet. We also examine cats with extra toes, and we ask how the number of toes is controlled genetically. The leading theory for that was proposed 70 years ago by a math wizard, Alan Turing. His theory explains a lot of other types of patterns.
The abnormal animal featured here is a fly with four wings instead of two. It was discovered 100 years ago, and the researchers who figured out its etiology went on to win the Nobel prize, so the story is worth retelling. This leads us to ask why the body segments differ at all, so we delve into homeotic genes (a lot).
The abnormal animal featured here is a frog with extra legs. It grew the extra legs because it was infected with a parasite when it was a tadpole, and the parasite caused the growing leg to split into pieces. A similar thing happens in a fly that also has extra legs, except that there there's no parasite, just necrosis.
The abnormal animal featured here is a type of tabby cat with blotches instead of stripes. It turns out that the same mutation that causes this also transforms the spots on cheetahs into blotches as well. As weird as this sounds, it all makes sense in terms of Turing's model, which also explains stripes on zebras and fish.
The abnormal animal featured here is the Siamese cat, whose color pattern is due to a mutation in the tyrosinase gene, which makes the tyrosinase protein sensitive to temperature. We talk about how some species use temperature to dictate their coloration and how this can happen developmentally.
The abnormal animal that introduces this chapter is a fly with no bristles. It turns out that flies "know" where to make bristles based on a GPS system of area codes in their genome. Humans probably have one too, but no one has located it yet. The chapter discusses the evolution of nakedness in humans and the genetics of why.