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Mendelian Genetics and Monogenic Inheritance — Quick Notes

  • Mendel's laws (Segregation, Independent Assortment) established particulate inheritance.

  • Key terms: gene, allele, genotype, phenotype, homozygous, heterozygous, dominant, recessive, true-breeding.

  • Monohybrid Crosses: F1 shows dominant trait; F2 ratios: phenotypic 3:1, genotypic 1:2:1 .

  • Punnett squares predict offspring genotypes and phenotypes.

  • Phenotype: the form taken by a character and occurs in contrasting fashion (variants)

  • Variant: a trait found in two or more versions within a single species

  • True Breeding (pure line): variety produces the same trait over several generations

Chromosomal Basis of Inheritance

  • Somatic cells are diploid (2n); undergo mitosis (produces two identical diploid cells).

  • Gametes are haploid (n); formed via meiosis (produces four haploid cells, generating genetic variation).

  • Meiosis I (reductional division): homologous chromosomes separate, recombination (crossing over) occurs via synaptonemal complex and chiasmata.

  • Meiosis II (equational division): sister chromatids separate.

    • Meiosis produces the gametes

      • Paired factors segregate from each other during gamete formation

  • Recombination and independent assortment drive genetic variation and Mendelian ratios.

DNA-Level Gene Discovery

  • Mutants help reveal gene function compared to wild-type.

  • Molecular tools: Restriction enzymes cut DNA; RFLP and Southern blot detect DNA sequence variations (e.g., in PKU).

Pedigree Analysis

  • Used to study human inheritance patterns where controlled crosses are not possible.

  • Modes of Inheritance:

    • Autosomal recessive: e.g., PKU; skips generations, parents often carriers.

    • Autosomal dominant: e.g., Huntington disease; appears in every generation, affected are usually heterozygous.

    • X-linked recessive: more affected males, no male-to-male transmission.

    • X-linked dominant: rare, affected fathers pass to all daughters.

    • Y-linked: only affects males, transmitted father to son (e.g., SRY gene).

Probability & Risk

  • Sum Rule: P(A or B) = P(A) + P(B) for mutually exclusive events.

  • Product Rule: P(A and B) = P(A) \times P(B) for independent events.

  • Dihybrid cross (phenotype): 9:3:3:1 for two independently assorting traits.

Historical Context

  • Mendel's work established particulate inheritance, replacing earlier theories like pangenesis, preformationism, and blending inheritance.

Stages of the Asexual Cell Cycle

  • Stages of the Celly Cycle

    • Mitosis

    • DNA Synthesis

    • Gap

      • G0

        • The cell has either postponed deciding to divide or made the decision never to divide again