## From twins to whole genomes

The heredity of certain behavioural characteristics was already known to mankind in prehistoric times which manifested mainly in the domestication of animals. Subsequently, the transmission of various human characteristics have also came to the attention. In the 19th Century, the idea of heredity explained the blood theory, according to which a child is created by the fusion of the parents' blood, thereby acquiring not only their characteristics, but also the characteristics of previous generations. However, the exact mechanism of inheritance was not known. The turning point occurred at the end of the 19th Century, when it was first thought that animals could be a model of human behaviour. The connection between the animal and human minds was described by Charles Darwin, who at the same time decided to test the theory of blood. Together with Francis Galton, they transfused blood into several species of rabbits in order to observe the transmission of various behavioural characteristics. However, their experiments always ended with a negative result, so they did not confirm the popular theory at the time.

From a historical point of view, studies of identical twins provide important data related to biomedical, psychiatric or behavioural questions (differences arising between identical twins were described in Chapter 7 – From epigenetics to human diseases). During the 20th Century, classic twin studies were used extensively to distinguish genetic and environmental sources of variation in human populations. Adoption studies were also widespread, which demonstrated that the cognitive abilities of siblings living in the same household and growing up separately are approximately the same. Thanks to these studies, scientists subsequently began to pay more attention to genetic factors that influence human intelligence. Research in the field of behavioural genetics was (like all other scientific disciplines) greatly influenced by the discovery of the structure of DNA. The polymerase chain reaction became part of the new era of molecular genetics, which enabled the amplification of selected regions of DNA, and other genetic engineering tools became available. Behavioural genetics initially focused on known and available genetic polymorphisms (blood groups or regions of human leukocyte antigens), but the discovery of microsatellite markers, which are distributed along the entire genome at intervals of approximately every 10 million bases, subsequently made it possible to carry out more extensive linkage studies. These provided an approximate localisation of variants responsible for individual differences. Currently, this type of research is mainly focused on analyses of genetic variants within the entire genome, which we call genome-wide association studies, or GWAS for short. When comparing genomes, scientists mainly focus on the possible influence of SNP polymorphisms (you can read more about polymorphisms in Chapter 15 - DNA as evidence) and various other DNA characteristics, which means that not only data from related individuals need to be analysed. The advantage of such studies is that it is not necessary to have pre-selected candidate genes for a certain phenotype. As of 2017, more than 3,000 such studies have been conducted.