Background and aims It is the wish of every family to have healthy children. Genetic counselling is extremely important for families that are at the risk of transmission of a certain disease. In our country, not many testing procedures are done and the material is most often sent abroad. The price of testing is high, and often goes on the expense of parents, so it is not available to all who might need it. Illustrate the importance of genetic counselling by presenting the cases of the children of two sisters.

Material and methods The family of one of the sisters has two affected children of male sex, aged 4 and 2. The boys cannot walk on their own, do not talk or talk incoherently, cannot control their physiological needs, have numerous neurological issues, suffer from frequent epileptic seizures and psychomotor retardation. The other sister’s family has a healthy 3-year-old boy and a healthy girl of 18 months. The affected 4-year-old boy and his mother were tested using chromosome microarray in a genetic laboratory in Barcelona, Spain, and the healthy girl and her mother, from the other family, were tested using the same method in Ljubljana, Slovenia.

Results Chromosome microarray showed the presence of an Xq28 microduplication around 550 KB long that included the MECP 2 gene, after which a diagnosis of Xq28 duplication syndrome was established in all tested subjects. None of the children had undergone prenatal genetic screening.

Conclusion In both families, the mothers, i.e. the sisters are carriers of a mutation on the X chromosome. In the first family, the boys received a ‘sick’ X chromosome from their mother and are thus phenotypically ‘sick’. The children in the second family are phenotypically healthy. The boy inherited a healthy X chromosome from his mother and the girl inherited a ‘sick’ X chromosome from the mother. The girl is phenotypically healthy, but it is important for her offspring that she knows the following: if her foetus is female, it will be phenotypically healthy with 50% chance of being completely healthy and 50% chance of being a carrier of the pathologic gene, like her mother. If the foetus is male, there is 50% probability that it will be completely healthy and 50% probability that it will be ‘sick’ if it becomes a carrier of the mutated gene. This girl, the carrier of the mutation, needs to know that she will have to undergo genetic screening in the course of her pregnancy. It is crucial that all relatives at risk should be timely informed and tested, for the purpose of welcoming healthy children. Two women, sisters, share the same mutation but with different expression. One of them leads an unhappy life, and so does her entire family. The other sister is happy, even though no prenatal genetic screening was done.