Genetic diagnostic methods

Classical methods of studying heredity

Humans are a complex subject for genetic research due to late sexual maturity, a small number of offspring, and the impossibility of experimental crossings. Therefore, methods based on observation and statistics historically emerged first:

• Genealogical method. Based on the compilation and careful analysis of pedigrees. It helps determine the type of inheritance (e.g., dominant or recessive, sex-linked or autosomal) and predict the probability of a trait appearing in children.
• Twin method. Consists of comparing dizygotic and monozygotic twins. The key indicator here is concordance (degree of similarity). If it is significantly higher in monozygotic twins, this indicates the leading role of the genotype in forming the trait rather than environmental factors.
• Population-statistical method. Studies the distribution of genotype and allele frequencies in populations, relying on the Hardy-Weinberg principle. It is essential for assessing the prevalence of hereditary diseases and calculating the proportion of hidden heterozygous mutation carriers.
• Dermatoglyphics method. Rapid assessment of patterns on palms and fingers. Specific changes (such as a single transverse palmar crease) can serve as markers for certain chromosomal syndromes, such as Down syndrome.

Cytogenetic and molecular technologies

Modern genetics relies on high-precision laboratory tests prescribed by a geneticist depending on the clinical presentation.

Cytogenetic method involves microscopic examination of the karyotype—the set of characteristics of the chromosome complement. The result is often presented as an idiogram, where chromosomes are arranged in order of decreasing size. The method is effective for detecting large chromosomal and genomic mutations.

For finer analysis, molecular-cytogenetic methods are used:

1. FISH method (Fluorescence in Situ Hybridization). The use of fluorescent labels allows for detailed study of individual chromosome regions and rapid analysis of a large array of cells.
2. Chromosomal microarray analysis (CMA). It has the highest resolution, detecting copy number variations, microdeletions, microduplications, and mosaic forms of aneuploidy.

Molecular-genetic methods work directly with the nucleotide sequence:

• PCR (polymerase chain reaction). Allows synthesizing millions of copies of the desired DNA fragment in vitro within a few hours, requiring a minimal amount of initial biological material.
• Sequencing. Determination of the exact nucleotide sequence, including next-generation sequencing (NGS) technologies. Used for diagnosing hemophilia, mitochondrial diseases, and comprehensive tumor profiling.
• Direct mutation detection. Used to confirm diagnoses such as cystic fibrosis, phenylketonuria, or Duchenne muscular dystrophy.

Prenatal and embryonic diagnostics

A comprehensive approach to assessing the health of an unborn child includes genetic, biochemical, and ultrasound examinations performed up to the 22nd week of pregnancy.

Non-invasive methods are safe for the fetus and mother. These include ultrasound (visualization of fetal structures), maternal serum biochemical screening, and analysis of circulating cell-free fetal DNA (available from the 10th week).

Invasive methods involve entering the uterine cavity to collect material:

• Chorionic villus sampling — obtaining chorionic villus cells.
• Amniocentesis — collection of amniotic fluid.
• Cordocentesis — puncture of the umbilical cord to obtain fetal blood.

As part of in vitro fertilization (IVF) programs, preimplantation genetic testing is available. Using micromanipulation, a cell is extracted from an embryo at the 8-blastomere stage for genetic testing. This allows transferring only healthy embryos to the uterus (e.g., Rh-negative in cases of a history of severe Rh-incompatibility).

Somatic cell nuclear transfer technology is also used in experimental embryology.

Mitochondrial DNA analysis

Mitochondrial DNA (mtDNA) is studied using the same basic technologies: sequencing, PCR, and restriction fragment length polymorphism (RFLP) analysis.

The main feature of mtDNA is its colossal copy number. Hundreds and thousands of such molecules are contained in a single cell. Due to this, mtDNA resists degradation much better than nuclear DNA. This property makes it an ideal object for paleogenetic research, the study of ancient human populations, and forensic medical examination of degraded remains.

Class and exam questions

What else to study on this topic

• Mathematical apparatus of the Hardy-Weinberg principle and calculation of heterozygous carrier state
• Somatic cell genetics methods: hybridization and cloning
• Biological modeling of genetic defects in animals
• Comparative embryological method and the rule of triple parallelism
• ICSI technology and in vitro embryo culture

Other topics in the "Additional Topics" section

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