BIOLOGY Lecture Notes - Golden Pages 2003

8. FUNDAMENTALS OF MEDICAL GENETICS. METHODS FOR STUDYING HUMAN HEREDITY AND VARIATION

The primary objective of medical genetics is the identification and Prevention of Hereditary Disorders.

The Study of genetics most commonly employs the following Methods: genealogical, twin, population, biochemical, cytogenetic, and somatic Cell Hybridization.

The genealogical method is based on tracing a specific trait among relatives. Information gathering begins with the proband—the individual through whom the pedigree is initiated. The method involves collecting family history data regarding one or more traits to determine the hereditary Nature of the trait, whether dominant, recessive, or sex-linked.

Autosomal traits manifest equally in both sexes. They can be dominant (where a single Gene is sufficient for phenotypic expression) or recessive (requiring two genes for phenotypic expression).

Traits localized on the X chromosome can be either dominant or recessive. In dominant X-linked inheritance, the trait appears in both females and males when a single copy of the gene is present. In X-linked recessive inheritance, males are affected much more frequently, whereas in females, such a gene is phenotypically expressed only in the homozygous state.

The twin method is used to assess the relative Influence of the genotype and the environment on trait development. Because monozygotic (identical) twins share identical genotypes, any observed differences are driven by environmental factors. Dizygotic twins provide an opportunity to analyze the converse scenario: environmental conditions are shared, whereas their genotypes differ.

Biochemical Methods are utilized to diagnose Metabolic Disorders, which are molecular diseases resulting from mutant genes. These methods involve identifying specific Enzymes—Proteins—or their intermediate metabolic products.

The cytogenetic method is based on the microscopic examination of Chromosomes. A normal karyotype consists of 46 chromosomes: 22 pairs of autosomes and 2 sex chromosomes.

Quantitative morphometric analysis is employed for chromosome identification. This involves measuring chromosome length in microns and determining The ratio of the short arm length to the total chromosome length (the centromeric index).

In accordance with the Denver Classification, all human autosomes are divided into seven groups:

✵ Group 1-3: large chromosomes; chromosomes 1 and 3 are metacentric, and chromosome 2 is submetacentric;

✵ Group 4-5: large submetacentric chromosomes;

✵ Group 6-12: medium-sized submetacentric chromosomes;

✵ Group 13-15: medium acrocentric chromosomes;

✵ Group 16-18: small submetacentric chromosomes;

✵ Group 19-20: short metacentric chromosomes;

✵ Group 21-22: short acrocentric chromosomes.

Chromosome staining reveals the presence of alternating dark and light bands, or disks. The specific arrangement pattern of these bands is strictly characteristic of each individual chromosome.

The female karyotype contains two X chromosomes, one of which forms a sex Chromatin body (Barr body). Females with an XO karyotype (X monosomy — Turner syndrome) lack sex chromatin. In the case of XXX trisomy, females exhibit two sex chromatin bodies, whereas males with a 47 (XXY) karyotype possess one, just like a normal female. Sex chromatin analysis enables biological Sex Determination using Blood Cells, buccal epithelium, and other tissue samples.

Somatic cell hybridization methods. Somatic cells contain the full Complement of Genetic information. Human somatic cells can be obtained from various Organs and Tissues (Skin, Bone Marrow, blood, embryonic tissues, etc.). Research at THE CELLULAR LEVEL offers greater precision than whole-Organism studies. The application of somatic cell hybridization makes it possible to investigate the mechanisms of primary gene action and gene interactions. Somatic cell cultures are used to evaluate the mutagenic EFFECTS OF ENVIRONMENTAL factors, thereby expanding the potential for precise biochemical Diagnosis of Hereditary diseases in adults.

Pharmaceutical Aspects of Medical Genetics

Medical practice frequently encounters variations in individual sensitivity to pharmacological agents. Pharmacogenetics emerged at the intersection of pharmacology, biochemistry, and genetics to study The Role of genetic factors in individual Variability of drug response.

When identical doses of a drug are administered to different people, resulting blood concentrations will vary. This depends on The activity of several enzymes responsible for drug METABOLISM, absorption, and elimination from the body. Reduced enzymatic activity can lead to the accumulation of the drug, potentially triggering toxic effects. Conversely, administering the same dose to an individual with elevated enzymatic activity will result in rapid degradation of the pharmaceutical agent, rendering the therapeutic effect negligible. Thus, The Fate of drugs within the body is governed by enzymes, the activity of which is fundamentally determined by genetic factors.

An example of a pharmacogenetic reaction is The Use of suxamethonium chloride (ditilin) in anesthesiology, which blocks neuromuscular transmission. Its administration typically results in the relaxation of Respiratory Muscles for a few minutes followed by the resumption of breathing. However, in some individuals, The breakdown of ditilin is impaired due to a deficiency of the enzyme cholinesterase. In recessive homozygotes, this leads to prolonged paralysis, which can be fatal.

Another example is hereditary glucose-6-phosphate dehydrogenase (G6PD) deficiency. This enzyme is present in red blood cells, and the gene responsible for its synthesis is located on the X chromosome. A mutant recessive allele leads to reduced G6PD synthesis. Under normal conditions, this enzyme deficiency is asymptomatic; however, the administration of certain medications, such as antimalarial drugs (quinine), triggers hemolysis due to disrupted metabolic processes in The Cell membranes.