Mendelian Genetics: Inheritance and Genetic Variation

Posted on Dec 10, 2024 in Biology

Mendelian Genetics: Understanding Heredity

Genetics is the branch of biology that studies heredity and the mechanisms by which biological traits are transmitted from generation to generation.

  • Gene: A segment of DNA that contains the information for the synthesis of a protein. This term was first used by Johannsen.
  • Allele: Each of the different forms that a gene can have. Alleles arise from mutations.
  • Locus: The specific location of a gene on a chromosome. A chromosome contains many loci.
  • Genotype: The set of genes an individual inherits from their parents.
  • Genome: The complete set of genes present in a species.
  • Phenotype: The observable characteristics of an individual, resulting from the interaction of its genotype and the environment.

All cells of the same individual have the same genotype, whereas the phenotype can vary depending on the genotype and environment. In diploid individuals, each character is represented by two alleles, one on each homologous chromosome, inherited from the father and mother.

  • Homozygous individuals (purebred): Have two identical alleles for a specific trait.
  • Heterozygous individuals (hybrid): Have two different alleles for a specific trait.

Types of Inheritance

  • Dominant Inheritance: One allele (dominant) masks the expression of the other allele (recessive).
  • Intermediate Inheritance: The hybrid exhibits a phenotype intermediate between the two parental phenotypes. The alleles are called equipotent.
  • Codominance: Both alleles are fully expressed in the hybrid. This occurs in blood groups.

Mendel’s Experiments and Discoveries

Gregor Mendel, an Austrian monk, joined the Augustinian Order in 1843. He began his experiments on heredity in 1856, using the garden pea plant (Pisum sativum). Mendel conducted thousands of crosses in the monastery of Brünn (now Brno).

Mendel’s Successes

  1. Choice of Material: Mendel chose pea plants because:
    • Purebred varieties were available.
    • They were easy to grow and manipulate.
    • They had clearly defined traits with contrasting forms (e.g., smooth vs. wrinkled, green vs. yellow).
    • They naturally self-pollinated, preventing contamination from other plants.
    • Artificial fertilization was easily performed.
  2. Rigorous Experimental Methodology: Mendel applied a rigorous experimental methodology, which facilitated later statistical analysis. He crossed purebred plants differing in one trait (parental generation), obtained the first filial generation (F1), analyzed the results, then crossed two F1 individuals to obtain the second filial generation (F2), and analyzed those results. This allowed him to deduce the laws governing the transmission of traits and predict the outcome of new crosses.

In 1900, three botanists (Hugo de Vries, Carl Correns, and Erich von Tschermak) independently rediscovered Mendel’s work, realizing that their findings had already been described by Mendel.

Chromosome Theory of Inheritance

Mendel’s findings were published in 1866 but received little attention. In 1900, De Vries, Correns, and Tschermak independently reached the same conclusions as Mendel. When they were about to publish their work, they rediscovered Mendel’s publication and presented their work as a confirmation of Mendel’s findings. From that time on, Mendel received recognition for his work.

In 1902, Walter Sutton and Theodor Boveri independently observed a parallelism between the inheritance of hereditary factors and the behavior of chromosomes during meiosis and fertilization. They proposed that hereditary factors were located on chromosomes, establishing the chromosome theory of inheritance. This connected the understanding of inheritance with cytology.

In 1909, the term “gene” was introduced to refer to hereditary factors. Thomas Hunt Morgan later confirmed this theory through his work with the fruit fly (Drosophila melanogaster).

Variations in Mendelian Ratios

There are cases where Mendel’s laws are seemingly not followed:

  • Multiple Allelism: When a gene has more than two possible alleles. Example: Blood groups are determined by a gene with three alleles (IA, IB, i). IA and IB are codominant and dominant over i. These genes determine the presence or absence of agglutinogens (A and B) on the surface of red blood cells.
  • Lethal Genes: Genes that, when homozygous, cause the death of the individual. They can be dominant or recessive. Example: In mice, a pair of genes determines coat color. The allele for yellow coat (A) is dominant over the allele for black coat (a). However, the homozygous AA genotype is lethal, resulting in a 2:1 phenotypic ratio (yellow: black) instead of the expected 3:1 ratio.
  • Polygeny: When a trait is controlled by multiple genes whose expressions interact. Example: Additive polygeny, such as height and weight, where the phenotypic effect of each gene pair adds up. Skin color, as studied by Davenport, is influenced by multiple gene pairs, with darker skin having more dominant alleles. Today, it is believed that skin color is controlled by at least three gene pairs, while height is controlled by around ten gene pairs.