Prenatal diagnosis and genetic screening : community and service implications / a report of the Royal College of Physicians.

  • Royal College of Physicians of London.
Date:
1989
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Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Prenatal diagnosis and genetic screening : community and service implications / a report of the Royal College of Physicians. Source: Wellcome Collection.

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    Fig 1. The risk of carrying a fetus with a chromosomal abnormality increases with maternal age. Data for Down’s syndrome, the commonest abnormality, are shown separately. The figure is based on a large collaborative European study of results of amniocentesis [64]. About one third of chromosomally-abnormal fetuses abort spontaneously between 16 weeks and term, so the risk of actually delivering a live-born chromosomally-abnormal child is about two thirds that shown in the figure. Fig 2. Patterns of inheritance. a Dominant inheritance. The disorder usually manifests itself in all carriers of a single dose of the abnormal gene. The arrows indicate individuals who have or will develop, the disease, b X-linked inheritance. The disorder manifests itself in males who carry a single dose of the abnormal gene on their only X-chromosome. Female carriers are usually protected by their second, normal X-chromosome. In males, the X-chromosome is always inherited from the mother, so if she is a carrier her sons have a 50% chance of being affected. Daughters inherit one maternal and one paternal X-chromosome: if their mother is a carrier, they have a 50% chance of being carriers, while if their father is affected they are all carriers. The diagnosis of an affected male suggests that his mother is a carrier and that most female relatives are at risk for being carriers. X-linked disorders may be transmitted unnoticed in the female line for several generations (see lower part of Figure). c Recessive inheritance. The disorder occurs only when an individual inherits the abnormal gene from both parents, ie is a homozygote. In fact carriers usually marry non-carriers, so the vast majority of carriers do not have affected children and recessive genes are transmitted undetected in most families for many generations. When by chance two carriers marry, there is a l-in-4 chance in each pregnancy of an affected child (arrow), so most people with recessively-inherited diseases do not have a family history.