BIOLOGY Lecture Notes - Golden Pages 2003

6. FUNDAMENTALS OF HEREDITY

The Czech scientist Gregor Mendel (1822–1884), based on the results of his experiments on cross-breeding various pea varieties, formulated the regularities currently known as Mendel's Laws.

Mendel's first law is the law of uniformity of the first-generation hybrids, or the law of dominance: when crossing homozygous parental forms, all individuals in the first generation of offspring (F₁) are uniform in phenotype. Furthermore, the trait that manifested in F₁ was termed dominant, while the suppressed trait of the second parental form was termed recessive.

Mendel's second law (the law of segregation): after crossing F₁ descendants of two heterozygous parents in the F₂ generation, segregation of the offspring According to the analyzed trait (phenotype) was observed in a 3 : 1 ratio under complete dominance, and a 1 : 2 : 1 ratio under incomplete dominance.

Mendel's third law (the law of independent assortment): segregation for each pair of traits occurs independently of other pairs of traits. This law holds true only for genes located on different Chromosomes. Exact Mendelian segregation can be expected only when the analyzed progeny is sufficiently large.

Mendel's experiments served as the foundation for The Development of modern genetics—the science that studies the two Fundamental properties of organisms: heredity and Variability. He succeeded in identifying the Patterns of inheritance thanks to fundamentally new methodological approaches. First, Mendel made a fortunate choice of research object—the garden pea—working with which he obtained, over several generations, true-breeding (constant) forms suitable for crossing. Second, he analyzed the inheritance of individual pairs of traits in the offspring of crossed plants that differed by one, two, or three pairs of contrasting alternative traits. Third, he not only recorded the obtained results but also subjected them to mathematical Processing. These listed simple research techniques constituted a fundamentally new hybridological method for studying inheritance. The combination of Genetic Methods used to study inheritance is referred to as genetic analysis.

The Genotype as an Integrated System

Genotype is the set of genes (genome) that determine the development of all hereditary traits and properties of an Organism. When formulating genetic concepts regarding the relationship between a Gene and a trait, it was initially assumed that each trait corresponds to a specific hereditary factor responsible for its development. It was subsequently established that There is a vast number of properties and traits in living organisms determined by two, three, or even many pairs of genes, and conversely, a single gene often affects multiple traits. Moreover, the action of a gene can be modified by the proximity of other genes or by environmental conditions. Thus, ontogenesis is driven not so much by isolated genes, but rather by the entire genotype as an integrated system featuring complex connections and interactions among genes. This system constantly changes and evolves over time. As a result of gene, chromosomal, and genomic Mutations, new genes constantly emerge, leading to The formation of qualitatively new chromosomes and even entire genomes.