It was 1933 when Lionel Penrose made a discovery about Down’s syndrome. While he was working in a hospital for people with learning disabilities, he surveyed several of the patients and their family members. His discovery was that people who have Down’s syndrome were more often to be born from older parents. Penrose made further analysis and figured out it was only the age of the mother that mattered. Now this directly has nothing to do with Genetics, but it is helping to set a path along the medical research aspect of Genetics.
Another discovery made in the name of medical genetics was in 1934 when Asbjørn Følling was examining a brother and sister that developed severe mental retardation. Their mother made a comment of how strongly the children’s urine smelled, and upon further study, Følling discovered that it contained large quantities of phenylpyruvic acid, so his conclusion was that the children could not break it down. He had discovered a metabolic disorder that has effects on the brain. He also learned that children that are affected by this disease inherit a recessive gene that fails to make the enzyme that is responsible for breaking down phenylpyruvic acid.
J.B.S. Haldane, a name that has been mentioned before, made yet another discovery. He was working with Julia Bell in 1937 when they found evidence of linkage in humans. They both found cases of the conditions haemophilia and color blindness in six different families. They then made the conclusion that the genes responsible for these conditions were close to each other on the X chromosome. The significance of this study was that it was the first step toward a genetic map of human inherited conditions.