Principles of Genetics: From Mendel to Modern Concepts
Artificial Selection
Individuals with advantageous traits from both parents were selected. Descendants expressing desired characters more strongly and lacking undesirable ones were chosen. This process was repeated to obtain pure breeds.
Johan Gregor Mendel (1822-1884)
Mendel crossed different manifestations of the same character (antagonistic phenotypes) and obtained a uniform F1 generation.
Act of Uniformity
Obtained: When crossing two pure races, all offspring are equal.
Mendel found that biological information exists in duplicate:
- Hereditary Factor: Gene present.
- Each organism inherits two factors for each character, one from each parent.
Genotype: Set of genes of an organism.
Phenotype: All observable characteristics of an organism, depending on genotype and environment.
Homozygotes: Pure-bred individuals.
Heterozygous: Hybrids.
Segregation Law
The two hereditary factors for a character do not merge but remain distinct throughout the individual’s life. They separate and are distributed during gamete formation.
Law of Independence
Antagonistic hereditary factors remain independent through generations, grouping randomly in descendants.
Linked Genes
Genes on the same chromosome tend to be inherited together.
Morgan’s Experiments
Morgan demonstrated that genes are located on chromosomes in a linear arrangement. Homologous recombination occurs through chromatid interbreeding.
- Locus: The position a gene occupies on a chromosome.
- Alleles: Genes occupying the same locus.
- Linkage: Tendency of genes on the same chromosome to be inherited together. Deviation from independent segregation is observed in test cross proportions.
- Crossover: Exchange of genetic material during meiosis, leading to genetic recombination.
- Genetic Recombination: The greater the recombination frequency, the greater the distance between genes. This allows for chromosome mapping.
Sex Determination
Individuals capable of generating sex cells (gametes): mobile (male) and sessile (female).
- Primary Sexual Characters: Gonads and copulatory organs.
- Secondary Sexual Characters: Traits differentiating males and females, not essential for reproduction.
Diploid species have two types of chromosomes:
- Autosomes: Same in both males and females.
- Heterochromosomes (Sex Chromosomes): Determine the sex of the individual (X and Y).
- Homogametic Sex: XX.
- Heterogametic Sex: XY.
Mechanisms of sex inheritance:
- Male Heterogametic: XX female, XY male (humans); XX male, XO female (grasshoppers).
- Female Heterogametic: ZZ male, ZW female (birds).
Sex-Linked Characters
Appear predominantly in one sex. X and Y chromosomes have homologous and differential segments.
- Gynandromorphic Characters: Determined by the differential segment of the X chromosome.
- Holandric Characters: Determined by the Y chromosome.
- Hemizygosis: Males (XY) express both alleles of holandric and gynandromorphic characters.
- Homozygous: Females (XX) express recessive alleles only if present on both X chromosomes.
Examples of Sex-Linked Traits
- Daltonism: Inability to distinguish green from red, governed by recessive genes on the X chromosome (protanopia, deuteranopia, tritanopia). More common in men.
- Hemophilia: Inherited disease characterized by impaired blood clotting, mostly due to genes on the X chromosome. Almost exclusively in men.
Other Genetic Concepts
- Sex-Limited Characters: Manifest depending on the sex of the individual.
- Pleiotropy: A single gene pair influences multiple traits.
- Gene Interaction: Multiple genes interact to define a single character.
- Penetrance: Proportion of individuals expressing a specific trait.
- Gene: Unit of hereditary material, a nucleic acid fragment carrying information for a character.
- Character: Morphological or physiological characteristic of a species (e.g., eye color).
- Qualitative Character: Possessed or not possessed.
- Quantitative Character: Shows continuous variation in the population, influenced by polygenes.
- Haploid: Having one gene for each character.
- Diploid: Having two genes for each character.
- Homologous Chromosomes: Chromosomes with the same loci.
- Allelic Series: Multiple alleles for the same locus.
- Antagonistic Factors: Different alleles.
- Homologous Genes: Genes occupying the same locus on different homologous chromosomes.
- Genotype: Set of genes in an organism.
- Phenotype: Set of expressed characters, depending on genotype and environment.
- Homozygous (Pure Breed): Individual possessing identical alleles for a character.
- Heterozygous (Hybrid): Individual possessing different alleles for a character.
- Incomplete Dominance: One allele shows incomplete dominance, resulting in an intermediate phenotype in hybrids.
- Codominant Inheritance: Both alleles are equally expressed in hybrids (e.g., MN blood groups).
- Dihybrid: Individual heterozygous for two gene pairs.
- Polyhybrid: Individual heterozygous for multiple gene pairs.
- Lethal Alleles: Imperfect alleles causing death, usually recessive.
- Backcross or Testcross: Crossing an individual with a homozygous recessive individual.