A genetic counselor must be referred to if there is a risk of occurrence of a hereditary disease in a family.
– Either one of the parents has a genetic disease, or their parents suffer from a genetic disease
– They belong to a group that is know to have a high risk of genetic disease
– There is a history of recurrent miscarriages
– The woman is over 38 years of age (Down’s syndrome)
– Mother had been exposed to mutagenic or teratogenic conditions during pregnancy(medication; radiation; certain chemicals; infections)
– A pregnancy is high risk.
Reference: Cambridge International AS and A Level Biology, Third Edition
Genetic prognosis assessment includes the following steps:
- Examination(Family anamnesis, verification of genealogy data, DNA diagnosis)
- Recurrence Risk Assessment(Probability that the disease will occur in a member of the family)
- Genetic Counseling and follow-up monitoring(Explaining the nature and consequences of the disease, recurrence risk, assisted reproduction, and future medical help)
The procedure of risk assessment of a particular hereditary disease in a future child is as follows:
- Determine genotype of parents
- Asses possible genotype of gametes
- Asses all possible genotypes of zygotes
- Determine phenotypes for zygotes
- Calculate the probability for the offspring to be affected
- Ultrasound Examination
- Measuring the level of biochemical markers in the mother’s blood serum
- Amniocentesis + Cytogenic or Molecular genetic exam
- Chorionic villus sampling + Cytogenic or Molecular genetic exam
- Cordocentesis (Blood from the umbilical cord is sampled after the 17th week of pregnancy)
- Foetoscopy (Direct observation of the foetus by an instrument inserted through the abdominal wall or the uterine cavity)
The amniotic fluid is sampled(approx. 20 ml) during the 15th or 16th week of pregnancy. Cells of the foetus present in the fluid can be ground in a culture. These can be examined in order to determine the genotype of the foetus. The amniocentesis-related miscarriage is at around 1 in 1600 (0.06%).
Chorionic Villus Sampling:
A small sample from one of the villi in the placenta is taken sometime between the 10th and 13th week of pregnancy. A genetic screening of this sample will be used to determine the genotype of the foetus. The risk of miscarriage is around 0.7%, and it may increase if the baby is smaller than normal for his/her gestational age.
Some new non-invasive methods:
- Isolation of cell-free foetal DNA out of maternal blood (foertal DNA accounts for about 2-10% of the total extracellular DNA in maternal blood)
- This enables the examination of DNA only, and cannot reveal structural chromosomal aberrations
- Available from 2002 in CZ
- Separation of foetal cells out of maternal blood (About 2 to 3 foetal cells are present among 6 million maternal white blood cells in 1 ml of blood).
- This enables molecular genetic analysis, the examination of chromosome aberrations and also a biochemical analysis
During implantation procedures, such as IVF, embryos have to be screened for any genetic abnormalities in order to select the most healthy one. This can be done by:
- A biopsy of the blastomeres (Day 3 after fertilisation, when the embryo consists of 6 to 8 cells) or
- A biopsy of the blastocysts (Day 5 to 6 after fertilisation, when the embryo consists of about 100 cells).
Diagnosis in children and adults:
Individual suspected of genetic disease may undergo either of the following tests:
- Biochemical examinations – Some genetic mutations affect the structure and function of proteins (E.g. Phenylalanine Hydroxilase).
- Metabolic test – Evaluating the speed of metabolic degradation of certain substances (E.g. Phenylanaline in Phenylketuronia or PKU)
- DNA Diagnostics
Detection of Chromosomal Aberrations:
- Karyotype Analysis
- Fluorescent in situ hybridization (FISH)
*PCR – Polymerase Chain Reaction is a technique used to increase the quantity of DNA.