 But although the sequences of nucleotides in the genome do not literally ‘code for’ phenotypic traits, they are, of course, amongst the causes of phenotypic traits. Several philosophers and scientists have introduced technical definitions of ‘genetic information’ based on these causal relationships in order to explicate the intuitive idea that genes carry information about phenotypes. These proposals are discussed in more detail in the entry  Biological information.  Here we will simply make two points about these proposals. The first point is that there are some very straightforward senses in which genes ‘carry information’ about phenotypes. The human Y chromosome carries information about sex in the same way that ‘smoke means fire’: one can be predicted from the other. In addition, the SRY region on the Y-chromosome is an adaptation for making organisms into males, so we can apply a version of ‘teleosemantics’, an approach which defines information in terms of adaptation, to this piece of DNA (Sterelny, et al. 1996; Shea 2007 and see the entry on  Teleological Theories of Mental Content).  The second point is that these straightforward senses of ‘information’ also seem to be applicable to environmental causes in development (Oyama 1985; Griffiths and Gray 2005). Mammals have a chromosomal system of sex-determination. But many reptiles use temperature, an environmental signal, to switch genetically identical eggs between male or female developmental pathways. Other reptiles have a genetic system which can be overridden by an environmental signal. Some fish even switch sex in adulthood in response to environmental cues. These environmental signals carry information about sex in the same unproblematic ‘smoke means fire’ sense as the SRY gene. Moreover, the behaviors that parents use to give appropriate cues to their eggs, and some of the products of those behaviors, such as nests of rotting vegetation which maintain a suitable temperature range, are designed by natural selection to ensure the correct sex-ratio in offspring, so the teleosemantic program can be applied to them too. The idea that genes ‘carry information’ about phenotypes in a special sense which distinguishes them from other causes is not the piece of common-sense it is often taken to be, but rather a highly contested idea that is the focus of much work in the philosophy of biology. See the entry on  Biological Information.