Human genetics: methods and hereditary diseases

Genetic experiments cannot be carried out on humans (also for ethical reasons), generations change slowly and families have few children, so special methods are used. In the genealogical (pedigree) method a trait or disease in several generations of a family is drawn in a chart (pedigree): a square is a male, a circle a female, a filled symbol a person who shows the trait; if two healthy parents have an affected child, the trait is recessive. Twin method: twins developed from one egg cell (monozygotic) have the same genotype, so their differences depend on the environment; twins from two egg cells (dizygotic) resemble each other like ordinary brothers and sisters. In the cytogenetic method the karyotype — the set of chromosomes in a cell — is studied under a microscope (46 in humans); the preparation is made from white blood cells in a blood sample taken by a medical worker. The biochemical method detects disorders of metabolism (too little insulin in diabetes, a missing enzyme in phenylketonuria; diabetes is a complex disease depending on many genes and on lifestyle and is not inherited like a simple recessive trait), and the molecular genetic method reads the structure of DNA and the sequence of genes; the human genome has now been fully read. Abilities and character depend on many genes and also on upbringing and environment; they cannot be predicted from one gene or from family history.