Now comes one of the key insights in the whole theory. Although genes for female preference only express themselves in female behaviour, nevertheless they are present in the bodies of males too. And by the same token, genes for male tail length are present in the bodies of females, whether or not they express themselves in females. The idea of genes failing to express themselves is not a difficult one. If a man has genes for a long penis, he is just as likely to pass those genes on to his daughter as to his son. His son may express those genes whereas his daughter, of course, will not, because she doesn't have a penis at all. But if the man eventually gets grandsons, the sons of his daughter may be just as likely to inherit his long penis as the sons of his son. Genes may be carried in a body but not expressed. In the same way, Fisher and Lande assume that genes for female preference are carried in male bodies, even though they are only expressed in female bodies. And genes for male tails are carried in female bodies, even if they are not expressed in females.
Suppose we had a special microscope, which enabled us to look inside any bird's cells and inspect its genes. Take a male who happens to have a longer than average tail, and look inside his cells at his genes. Looking first at the genes for tail length itself, it comes as no surprise to discover that he has genes that make a long tail: this is obvious, since he has a long tail. But now look at his genes for tail preference. Here we have no clue from the outside, since such genes only express themselves in females. We have to look with our microscope. What would we see? We'd see genes for making females prefer long tails. Conversely, if we looked inside a male who actually has a short tail, we should see genes for making females prefer short tails. This is really a key point in the argument.
