Basics of Medical Genetics - Buzhiyevska T.I. 2001

Medical Genetic Counseling

MEDICAL Genetic Counseling of families is the concluding stage of population-based medical genetic care. It aims to assess the risk of having a child with a hereditary disorder in a family, develop a Prevention plan for specific hereditary conditions, prevent the birth of an affected child, or establish medical supervision and postnatal Treatment protocols. During a counseling session, a medical geneticist—either individually or as part of a multidisciplinary team of highly qualified specialists—explains the Diagnosis, inheritance pattern, prognosis for the patient's life and health, available treatments, prevention strategies, and the recurrence risk to the patient or their parents (upon request).

The goal of medical genetic counseling is to help the family make informed decisions regarding future family planning or better adapt to life when a hereditary disorder is already present. The success of counseling relies heavily on the competence, expertise, and ethical standards of the physician, as well as on how well the counselees understand and accept the information provided. The ultimate decision—whether to have a child or not (to avoid Pregnancy, terminate a pregnancy, place the child for adoption, institutionalize, or pursue other alternatives)—rests entirely with the couple.

Today, family planning options have expanded significantly thanks to modern reproductive technologies, including donor sperm insemination, donor oocytes, In vitro Fertilization with subsequent embryo transfer into the uterine cavity of a specially prepared surrogate, and more.

Before conducting medical genetic counseling, the geneticist must accurately diagnose the hereditary condition, determine its inheritance pattern, and ascertain whether the specific case stems from a de novo mutation or an inherited one. Identifying mutation carriers among family members and determining the parental line through which the mutation is transmitted (maternal or paternal) is of critical importance.

The physician must determine the recurrence risk of the disease within the counseling family—that is, calculate the probability of a specific hereditary disorder manifesting in the proband or their descendants using genetic laws, empirical data, specialized tables, and computer software. This is only possible through a thorough Clinical and Genealogical evaluation of as many relatives as possible, comprehensive genetic and laboratory testing, and the identification of any consanguineous marriages in the family. A risk of up to 5% is classified by geneticists as low and does not warrant restricting family planning.

A risk of 6–20% is considered moderate. When planning a pregnancy in such cases, it is essential to consider the severity and clinical course of the specific disease, other medical and social consequences of the hereditary pathology, and the availability of prenatal Diagnostics.

A risk exceeding 20% is generally regarded as high (for example, in autosomal recessive or autosomal dominant conditions). If precise prenatal diagnosis is unavailable (as maternal ultrasound, fetal cytogenetic, and DNA diagnostics may not yet be sufficiently informative for certain conditions), childbearing is generally not recommended for the family. In such scenarios, alternative reproductive pathways should be explored, such as adoption or assisted reproductive technologies.

The core principle of medical genetic counseling, grounded in medical ethics and the Hippocratic Oath, is to prioritize the well-being of the patient and the proband. Even during prenatal diagnostics, the fetus must be treated as the proband, and its best interests must come first. Information should be communicated to the family and individual members in plain, accessible language and strictly tailored to The Scope of their inquiries. Unsolicited information on topics the family has not asked about is unnecessary and may lead to unwanted social complications. Such sensitive questions often include (among many others): "Is the disease fatal?", "Whose fault is it?", and "Which of us is the mutation carrier?". The geneticist must be prepared to answer all potential questions internally, without burdening the patient with knowledge they do not wish to have.

Risk calculation for families with Monogenic Disorders differs from that for Multifactorial Diseases. Counseling is primarily required for families whose parental genotypes are known or can be predicted with a high degree of probability.

When a monogenic disease occurs within a family—meaning it is inherited—the risk is calculated in accordance with Mendelian inheritance laws and the specific inheritance pattern. In such cases, the penetrance of the mutation is assumed to be complete, and its expressivity sufficient.

In cases of a de novo autosomal dominant mutation in the proband, the risk for their siblings equals the population risk. However, the geneticist's Conclusion that a mutation is novel may be erroneous if biological paternity has not been verified.

In Autosomal dominant inheritance, siblings are considered genetically healthy if they remain symptom-free past the age of onset. Consequently, their children will be both phenotypically and genotypically healthy. An affected individual who is homozygous for a dominant mutation transmits it to all their offspring, whereas a heterozygote paired with a healthy partner has a 50% chance of passing the condition to their descendants. DNA diagnostics make it possible to confirm the presence or absence of a mutation in any tested proband, significantly simplifying medical genetic counseling.

Counseling families with Autosomal Dominant Disorders is often complicated by incomplete penetrance and variable expressivity of the syndrome. To address this, a comprehensive, targeted examination of as many relatives as possible must be performed (searching for subtle clinical or laboratory markers of the syndrome), alongside a review of family photo albums and other historical records.

In Autosomal Recessive Inheritance, the pedigree exhibits a "horizontal" pattern—meaning affected individuals appear within a single generation. Phenotypically healthy heterozygous parents have a 25% chance of bearing affected, homozygous children. Consanguineous marriages within the family elevate the risk of manifesting Autosomal Recessive Disorders. The probability of shared ancestry increases as the genetic distance between parents decreases (e.g., coming from the same village, district, region, or belonging to the same or different ethnic groups). The geneticist collects and evaluates all these data, using specialized tables to determine genetic risk while factoring in the Inbreeding coefficient.

X-linked recessive disorders are transmitted from a grandfather, through a phenotypically healthy mother (who is a heterozygous mutation carrier), to his grandson. There is a 50% chance that granddaughters will be mutation carriers and a 50% chance that grandsons will be affected. Females can develop this pathology only if they inherit the mutation from both an affected father and a carrier mother, resulting in both X Chromosomes carrying the corresponding locus-specific mutation. In the overwhelming majority of cases, this occurs only when the parents of the affected female are consanguineous.

Calculating genetic risk can be considerably more complex, particularly when parental genotypes are unknown and mathematical computations based on probability theory must be applied. When parental genotypes are established, the number of previously born healthy children (in autosomal recessive pathology) or healthy sons (in X-linked conditions) does not affect the recurrence risk for subsequent offspring. However, if parental genotypes are unknown, the risk of an X-linked condition for the next son decreases depending on how many healthy sons have already been born, provided there are no affected siblings in the family. This occurs because successive births of healthy sons reduce the statistical probability that the mother is a carrier of the mutant X chromosome.

The principles and formulas for calculating genetic risk in complex scenarios are detailed in specialized monographs, tables, and computerized software, which medical geneticists routinely utilize. Assessing risk is particularly challenging when a family presents with a single child (a sporadic case) affected by a genetically heterogeneous disorder (characterized by diverse inheritance patterns and multiple mutant genes). In such instances, all potential scenarios must be accounted for, calculating the total probability as the product of the prior probability and the recurrence risk, and subsequently summing the resulting values.

In cases of Chromosomal Disorders within a family, determining the recurrence risk for an affected child involves three main scenarios: 1) Recurrence of aneuploidy when parental karyotypes are normal, which takes into account population frequencies of specific aneuploidies and maternal age; 2) Mosaicism of peripheral Blood lymphocytes in one of the parents. Here, the sibling recurrence risk is calculated using the formula: (X / (2 - X)) ∙ K, where X represents the proportion of the aberrant Cell line, and K is the reduction coefficient for the viability of unbalanced zygotes (for Down syndrome, K = 0.5); 3) Assessing risk when family members carry structural chromosomal anomalies. In such situations, parental cytogenetic test results allow for a theoretical estimation of the relative proportions of abnormal Gametes and zygotes. In practice, however, the actual risk is lower due to the selective advantage of normal gametes and zygotes during Embryogenesis. The risk is higher when a chromosomal rearrangement is carried by the mother (since only a single oocyte matures with no alternative choices) compared to paternal carriage. For the most common translocations, maternal carriage yields an approximate 10% risk, whereas paternal carriage carries a 2% risk. When parents carry a centric fusion of homologous chromosomes, such as t(21q21q), The formation of normal gametes and zygotes is both theoretically and practically impossible (resulting exclusively in trisomies or monosomies). In this case, the recurrence risk for offspring is 100%.

Empirical risk calculations for chromosomal disorders are determined using standard reference tables (S.I. Kozlova et al., 1996).

Empirical risk tables are also utilized during genetic counseling for families with multifactorial pathologies. Specialized software programs are available to calculate risks for atherosclerosis, Diabetes Mellitus, and other multifactorial diseases by integrating both clinical and laboratory data. Furthermore, DNA diagnostic Methods have vastly broadened the scope of medical genetic counseling: restriction fragment length polymorphism (RFLP) analysis—based on linkage between a mutation and specific markers—and Polymerase Chain Reaction (PCR), which allows for the direct detection of Mutations in an individual's genotype. In such cases, medical genetic counseling culminates not merely in a risk assessment, but in a precise genetic diagnosis regarding the individual's status (healthy, carrier, affected, homozygous, or heterozygous).

Modern advancements in medical genetic counseling—leveraging DNA diagnostics, molecular cytogenetics, and related technologies—enable much more precise evaluations of disease risk. Nevertheless, it bears repeating: patients should be provided exclusively with the information they explicitly ask for.