Genetics - A. V. Sivolob 2008

Human Genetics
Humans as a Genetic Subject

Obviously, among All living organisms, humans are the most fascinating genetic subjects for us. At the same time, however, humans can be considered the worst possible subjects for experimental genetics: addressing research objectives involves complex methodological and ethical challenges that make humans problematic as an object of genetic research. Let us examine these difficulties and the ways to overcome them.

1. The impossibility of experimental cross-breeding under artificial conditions is completely understandable, primarily due to ethical considerations. One cannot use a pre-designed scheme to select parents with desired genotypes and obtain and analyze their offspring: humans marry freely, regardless of their partner's genotype and without any "research" agenda. Restricting this freedom (by banning undesirable marriages or promoting desirable ones) was the core idea of eugenics—a concept of biological human "improvement" based on the recognition of the "genetic foundations" of physical and mental inequality among races and social classes. Attempts at such "improvement of the human breed" were carried out under the auspices of 20th-century totalitarian regimes.

Thus, in genetic analysis concerning humans, the foundation of the hybridological method—experimental crossing—disappears. There are several ways to overcome this "drawback":

✵ From A large number of married couples, a geneticist can select those of interest and, by analyzing pedigrees, draw Conclusions about The Nature of trait inheritance.

✵ A researcher can analyze the Inheritance of Traits in laboratory mammals and make preliminary conclusions regarding the possibility of inheriting similar traits in humans.

✵ The somatic Cell Hybridization method allows, in some cases, for genetic analysis using human cell cultures. By analyzing hybrids formed As a result of the fusion of Cells from two different individuals, one can draw conclusions about the Interaction of Non-Allelic Genes (the mutation complementation test) and perform genetic mapping.

2. Late sexual maturity and long generation times—changing a single human generation takes 20-30 years, which hinders inheritance analysis. The main methodological approaches to solving this problem include studying large human populations, recording traits over significant periods of time (pedigree analysis), and using mammalian and human somatic cell genetics Methods.

3. A small number of offspring. Conducting statistical analysis of trait segregation requires a sufficiently large number of offspring from a single pair of parents (see Chapter 3). Humans belong to so-called "low-fecundity species"—it is rare for more than one child to be born at a time. Thus, analyzing trait segregation using a single family as an example is practically impossible.

To solve this problem, researchers search for large families or select an appropriate number of smaller families where the trait of interest is observed. There are extensive pedigrees where the manifestation of certain traits can be traced over several generations: in a large pedigree, the number of offspring will be sufficient for analysis.

4. The absence of pure lines and the impossibility of obtaining them is another objective reason that hinders the analysis of trait inheritance. When analyzing the inheritance of dominant traits, a geneticist cannot always precisely determine the parents' genotypes and is forced to rely on more or less probable assumptions.

5. A large number of Chromosomes (linkage groups). The human chromosomal set consists of 23 pairs of chromosomes and, accordingly, 24 linkage groups: 22 autosomes and two sex chromosomes, X and Y. The large number of chromosomes complicates their genetic and cytological mapping, especially using classical genetics methods. These problems are addressed by methods of somatic cell hybridization between humans and other mammalian species (especially rodents) and the Application of Molecular cytogenetics techniques, such as fluorescence in situ hybridization (FISH—identifying the binding site of a complementary fluorescently labeled DNA probe on a chromosome). However, given the large number of human genes (approximately 21,000), it should be noted that human chromosome mapping remains far from complete, even against the backdrop of the fully sequenced Human Genome.

6. The impossibility of establishing standard living conditions for different groups of individuals significantly complicates The Study of many human traits, especially those with polygenic inheritance. The environment has a tremendous impact on The Development of numerous traits, yet this impact cannot be manipulated at the researcher's will (Nutrition, microclimate, education, etc.). A major challenge for specialists in diagnosing Hereditary diseases is also the presence of phenocopies. This "drawback" of humans as objects of genetic research can be overcome by selecting groups from the vast diversity of human populations that share similar hereditary traits and environmental exposures. To study the relative IMPACT OF ENVIRONMENTAL conditions and heredity on trait development, twin studies and adoption studies are employed (see below).

To the described difficulties faced by Human Genetics, one must add A number of so-called organizational shortcomings that unfortunately persist in many countries: unsatisfactory record-keeping and mortality registration, as well as disease Diagnosis and statistics.

Despite the aforementioned problems, humans as a genetic object also possess a number of significant advantages: There is a vast body of data on normal and pathological human anatomy, physiology, and biochemistry; communication with the research subject is possible, which facilitates gathering information about relatives and helps investigate traits related to sensory perception, emotions, and intellect; and The Human Genome has been almost completely sequenced.