Introduction to one gene inheritance - all types and sex-limited

Mendel's Laws of Inheritance

Introduction to Genetics

  • Foundational Origin: Genetics began in an abbey garden, revolving around heredity, which refers to how traits are passed down generations.

  • Gregor Mendel's Contributions:

    • Mendel conducted pioneering experiments on pea plants, laying the groundwork for genetic science.

Mendel's Experiments

  • Pea Plant Crosses:

    • Crossed pea plants to observe trait inheritance across generations.

    • Noted Parental Generation (P): 100% of plants had purple flowers.

    • Noted First Generation (F1): 75% purple flowers in offspring.

    • Second Generation (F2) demonstrated a 3:1 ratio of purple to white flowers.

Mendel's Laws of Segregation

  1. Alleles: Alternative gene versions (alleles) exist for traits.

    • Example: b = blue, B = brown.

  2. Inheritance of Alleles:

    • Each organism inherits two alleles for a character; one from each parent.

    • Homozygous: Two identical alleles (e.g., BB or bb).

    • Heterozygous: Two different alleles (e.g., Bb).

  3. Dominance: If alleles differ, the dominant allele determines the organism's appearance.

    • Example:

      • bb = Blue (recessive)

      • BB = Brown (dominant)

      • Bb = Brown (dominant).

  4. Law of Segregation: During gamete formation, allele pairs segregate so that each sperm or egg carries only one allele for each inherited character.

Experimentation Results

  • Mendel's True-Breeding Parents:

    • Purple flowers crossed with white (P: PP x pp).

    • All first-generation offspring were purple (F1: all Pp).

    • F1 plants fertilized to create the F2 generation, resulting in the phenotypic ratio of 3 purple:1 white.

Chromosomal Inheritance

  • Homologous Chromosomes: Each diploid cell carries pairs of homologous chromosomes, each bearing different alleles.

Genotype and Phenotype Correlation

  • Genotype: The genetic makeup of an organism (e.g., PP, Pp, pp).

  • Phenotype: Observable characteristics (e.g., purple or white flowers).

  • Determination of characteristics by phenotype is often not complete without genotypic context.

  • Example: Both Pp and PP exhibit the same purple phenotype, leading to the uncertainty of their genotype unless tested further.

Testcross Methodology

  • Testcross Definition: A cross between an unknown genotype and a homozygous recessive genotype to ascertain the unknown's allele composition.

    • Used to distinguish between homozygous and heterozygous dominant individuals.

Variations on Mendel's Laws

  1. Incomplete Dominance: Results in the blending of traits (e.g., red x white flowers yielding pink offspring).

  2. Multiple Alleles: Some genes have more than two alleles (e.g., ABO blood groups with IA, IB, and i alleles).

  3. Lethal Alleles: Certain combinations of alleles can be lethal (e.g., homozygous lethal genotypes).

  4. Penetrance and Expressivity: Refers to the likelihood of exhibiting a phenotype given a particular genotype and the variability in expressivity of phenotypes among individuals.

Sex-linked Traits

  • Sex Chromosomes: Determine gender in many species; males have XY, and females have XX in mammals.

  • Sex-linked Genes: Traits controlled by genes on sex chromosomes; X-linked genes often result in different phenotypes in males (more likely affected due to a single X chromosome).

Conclusion

  • Mendel's experiments paved a pathway for modern genetics, illuminating how traits are inherited through alleles and how dominance, segregation, and various genetic principles apply across different organisms. The understanding of genotype and phenotype helps in comprehending heredity, and variations exist due to numerous factors, including chromosome interactions and sex-linked inheritance patterns.