||EXPLORING HUMAN HEREDITY 9 each pair cast off, so that at fertilization new combinations arise as if nature were constantly trying out different possibilities in different environments. If you are a parent, a reproductive cell produced in your body and carrying one of each pair of your genes combined with a similar cell from your mate and became the beginning of your child. A set of your genes took on new companions and thus formed new genetic combinations, and a new individual was launched. You and your wife or husband, as the case might be, became involved in the most interesting experiment of all times. You began to live over again in your child. You began your biological eternal life. This same experiment will be repeated each generation so long as your line continues, and you in turn will live over and over again. The statement, "I live again in my children," thus has a good biological basis. Research in Human Heredity Man is not the most suitable organism on which to do research in heredity. Human families are small, many years elapse between generations, and the investigator has no control over matings. Therefore different techniques and methods of investigation from those used with other organisms have to be devised. Usually one of three methods, or some combination of them, is used by most investigators of human genetics. They are the pedigree or family history method, twin studies, and statistical methods. In the pedigree method, a study of the relatives of a known affected person, or a propositus, is made. Usually a pedigree chart is prepared and an effort made to determine the nature of the inheritance. Twin studies are based upon the supposition that identical twins have the same genetic makeup, while fraternal twins are no more alike genetically than ordinary brothers and sisters. Where proper allowances are made for other factors, any differences in the frequency of a trait in identical twins and fraternal twins are due to heredity. There are many different statistical approaches used in the study of human genetics, but they usually attempt to determine the frequency with which some trait or gene occurs in close relatives of affected individuals as compared to the frequency with which they occur in a control group or in the general population, or to estimate, if possible, the gene frequency of certain genes and thus possibly determine the pattern of their inheritance.