After this refresher ofgenetic terms, we can have a closer look at behavior in this context. As already mentioned above, there never is a gene for behavior. Genetic differences between individuals certainly exist, and also affect behavior. But the environment in which a certain gene is expressed, both the internal one, for example, the hormonal or the nutritional state that the animal is in, and the external one, such as the social situation, level ofenviron-mental complexity, or other factors, have an important influence. In rodents, the position of an embryo in the uterus is of tremendous importance for his/her later behavioral strategies. Female embryos between two males, for example, later in life behave more masculine in terms ofterritorial behavior, marking behavior, aggression - at least in Mongolian gerbils male embryos between two sister embryos later on become more philo-patric, tend to stay with their mothers, help them rear new offspring, etc.
Another general problem ofclassical behavioral genetics is that, even in a rather simple organism such as Drosophila, courtship behavior, which has been genetically analyzed since almost 30 years, consists of a whole set of behavioral elements, each of which is constantly influenced by an exchange of olfactory, visual, and tactile signals.
Genes that control production ofenzymes, transcription factors, transmitters, etc., thus often influence a whole array of behaviors; and, in some cases, so-called candidate genes, which are more or less constant over the whole animal kingdom or at least the whole vertebrate subphylum, have been found to be important for behavioral control. One such example is a gene that controls a certain kinase, which in turn influences foraging in Drosophila larvae, and the life cycle stages of worker bees. Another, rather recently found, example is of genes for oxytocin and vasopressin receptors that control a whole set of social and sexual behavior aspects in vertebrates. Another phenomenon that tends to confound simple genetic analysis of behavioral traits is called genomic imprinting. This means that there is a difference in the expression and effect of a gene whether it comes from the maternal or paternal side. A behaviorally relevant example would be the Mest gene in mice, which normally is active only when coming from the paternal side. Mest is expressed in the hypothalamus and the amygdala, two brain regions that, among others, control activity, stress-related behavior, and also maternal care. Combining these phenomena with another recently discovered fact that females can allocate resources differentially to embryos and zygotes in relation to who the father was (e.g., regarding his attractiveness) will certainly make life even more difficult for behavioral geneticists!
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