Vertebrate homeobox genes

I would not be going to the trouble of telling this story if it ended here. The first stirrings that were to rekindle the issue of how vertebrates were related to arthropods came in 1984, about one hundred and fifty years after the original dispute between Cuvier and Geoffroy. It was the time when the identity genes affecting segments in the fruit fly had just been isolated. Recall that these genes are needed for a set of hidden colours that define distinct regions in the fly. The genes code...

Screening the dead

The reverse logic of genetics says that if you want to understand a process, you begin by looking for mutants in which the process is defective or altered in some way. To study early fly development, you might therefore look for mutants in which fertilised eggs do not develop properly. Perhaps you could screen lots of animals, searching for rare mutants in which the embryos do not form normally. By studying the genes that have been altered to give these defective embryos, you might then be able...

Segment identity in flies

About one hundred years after Goethe wrote his treatise on plants, the zoologist William Bateson described a comparable set of abnormalities in the animal world. Bateson was convinced that the only way to understand evolution was by studying how biological forms vary, and he therefore set about cataloguing the principle types of variation in his book Materials for the Study of Variation of 1894 (Bateson was later to become one of the founders of the science of genetics, a term he coined). Among...

Leftright asymmetry in plants and animals

There are many examples of left-right asymmetry in the living world. In some cases, the Land the R-forms occur in equal numbers, and it is largely a matter of chance whether any individual will develop as one form or the other (i.e. there is no overall handedness). Male fiddler crabs, for example, often have an enlarged claw on one side (Fig. 14.10). Whether this is on the left (L-form) or right (R-form) depends on the previous chance experiences of the animal. Young males start off with two...

Symmetry and function

In practice, the symmetry class of an object is often related to the way it is used. Spherical symmetry, for example, predominates in objects that are used with little or no constraint from any specific direction, as with soccer balls that roll around in any way we choose. Radial symmetry, on the other hand, is common in cases where constraints operate along one major axis. Bottles and stools, for instance, are designed to withstand the effects of gravity bottles prevent liquids being spilt...

A pattern of hidden colours

You have been invited to a football stadium with lots of other people to send a birthday message to someone flying overhead in a helicopter. The stadium has already been carefully marked out with a grid of squares, each square being painted a distinct colour, making the stadium look like a colourful patchwork. You are given two pages of instructions and a large piece of card, black on one side and white on the reverse, and told to go and stand on a square. After choosing a square, you look at...

Reproduction through development

We can now return to the problem of how multicellular organisms, such as humans, mice and oak trees, reproduce. Like unicellular organisms, their reproduction is based on cell division but instead of making many separate individuals, the cells stay together to gradually build up a multicellular individual. We can summarise the whole process with a life cycle. In the case of humans, for example, parents each contribute a cell a sperm cell from the father and an egg cell from the mother. These...