Heredity and Evolution

Heredity Questions Darwin Couldn’t Answer

• Heredity Mechanism: Darwin pondered how heredity operates, contemporaneously while Gregor Mendel explored similar inquiries.

  • Jemules Theory:

    • Darwin proposed the existence of "jemules," tiny particles produced by body parts, believed to transfer traits during reproduction.

    • Limitations: Initial thoughts questioned if a trait could transfer fully (e.g., amputated parents leading to offspring without limbs).

    • Mixing: Jemules were thought to mix from parents leading to averages in traits (e.g., height).

    • Critique: Short + Tall parent theory failed, demonstrating extremes do not necessarily result in an average child.

• Mendel's Findings:

  • Mendel's work laid foundational insights into heredity, though he and Darwin never collaborated.

• Coin Flip Experiment:

  • Outcomes questioned fairness using Chi-Square tests (X²).

    • Observational data showed frequencies and required considerations on significance.

  • Chi-Square Analysis:

    • Defined as:

      • X² = Σ (O - E)² / E

      • Degrees of Freedom (DF) = outcomes - 1.

      • Goodness of fit assessed based on critical value and p-values to evaluate null hypothesis regarding observed vs. expected results.

Mendel and Punnett Squares

• Mendel’s Experiment:

  • Used pea plants with male and female reproductive organs.

    • Pollen, containing two sperm cells, was manipulated to study trait inheritance (e.g., using artificial pollination).

  • Dominant and Recessive Traits:

    • Identified genotypes and phenotypes through experimentation.

    • Punnett Squares:

      • Introduced by Reginald Crundall Punnett to mathematically predict offspring traits.

      • Dominant alleles represented with capital letters, recessive with lowercase (e.g., RR for red, rr for white).

• Genotypic and Phenotypic Ratios:

  • F1 generation showcases heterozygous traits; ratios of offspring determined from parental stock.

• Characteristics:

  • Zygotes defined as diploid cells from fusing gametes.

  • Types of dominance:

    • Homozygous Dominant (AA), Homozygous Recessive (aa), Heterozygous (Aa).

• Variability in Traits:

  • Recognizes lethal alleles and their consequences on f1 generation.

Mendel’s Rules

• Law of Dominance: One dominant allele masks the effect of a recessive allele.

• Law of Segregation: States two alleles for a trait separate during formation of gametes.

• Law of Independent Assortment: Traits are inherited independently if they are situated on different chromosomes.

• Violations of Mendel’s Claims:

  • Examples include incomplete dominance (traits blend) and codominance (both traits are expressed).

  • Blood Types: Exhibits co-dominance with A, B, AB, O classification.

• Polygenic Traits: Traits influenced by multiple alleles, e.g., height and skin color, often exhibiting environmental influences but not altering the genetic equation.

Meiosis and Gametes

• Meiosis:

  • Resulting from formation of gametes (sperms and eggs), contributing to genetic diversity, in contrast to mitosis which creates identical cells.

  • Chromosomal Count:

    • Humans have 46 chromosomes; gametes contain 23 chromosomes.

  • Reduction Division: Chromosomes split from diploid (46) to haploid (23).

• Stages of Meiosis:

  • Includes prophase, metaphase, anaphase, and telophase, with meiosis resulting in unique combinations of genetic material from independent assortment and crossing over events.

• Nondisjunction: A failure in chromosome separation resulting in genetic disorders.

Heredity in Evolution

• Punnett Squares in Populations:

  • A method to predict allele distribution in successive generations, allowing for estimation of dominant and recessive traits in the gene pool.

• Hardy-Weinberg Principle:

  • Establishes conditions for gene frequency stability and factors causing evolution if these criteria are violated (e.g., mutation, selection, migration).

• Understood Equations: For allele and genotype frequencies used to clarify population equilibrium.

• Genetic Drift:

  • Random shifts in allele frequency, leading to evolutionary changes over generations.

Meiosis Mistakes and Genetic Disorders

• Karyotyping: Images of chromosomes used to identify genetic abnormalities.

• Common Disorders:

  • Down Syndrome: Trisomy 21; Patau Syndrome: Trisomy 13; Edwards Syndrome: Trisomy 18.

• Risks: Correlation between maternal age and incidence of chromosomal disorders.

Pedigrees and Genetic Mapping

• Pedigree Analysis: Used to trace inheritance patterns using squares (males) and circles (females), distinguishing traits across generations.

• Reproductive Isolation: Framework for understanding speciation processes, whether pre-zygotic or post-zygotic, based on various biological barriers.

• Speciation Types:

  • Allopatric (geographical isolation) vs. Sympatric (reproductive isolation without geography).

Conclusion

• Speciation Mechanism:

  • Defined species based on interbreeding capabilities, emphasizing genetic variations’ roles in evolution and adaptation.

  • Importance of isolation in driving evolutionary divergence and facilitating the emergence of new species over time.