Human genetics: methods and hereditary diseases

Hereditary diseases are linked to a change (mutation) in the genetic material; they are divided into gene diseases and chromosome diseases. Dominant gene conditions include polydactyly (extra fingers) and syndactyly (fused fingers); recessive gene diseases include phenylketonuria and albinism (affected only if aa); haemophilia and colour blindness are X-linked recessive. A carrier (Aa) is not ill, but the child of two carriers has a ¼ chance of being affected; in marriage between close relatives the chance that both parents carry the same recessive allele increases. In chromosome diseases the chromosome number changes: Down syndrome — three copies of chromosome 21 (47), Klinefelter syndrome — XXY (47), Turner syndrome — a single X (45); the chance of a child with Down syndrome rises with the mother’s age, yet such children are born to mothers of all ages. What modern medicine can do: detect phenylketonuria early in newborns and prevent brain damage with a special diet; people with Down syndrome, with support, learn, work and live full lives. In genetic counselling a specialist explains the risk to a family, and the decision stays with the family; a hereditary disease is nobody’s fault and these people deserve respect and care. A recent advance: in 2023 a CRISPR-based therapy was approved in some countries for sickle-cell disease and beta-thalassaemia.