Genetics and Evolution Notes

Royal Intermarriage and Genetics

  • European royal families practiced intermarriage for political alliances and bloodline purity.
  • This led to restricted gene pools and increased similarity in genotypes.
  • Consanguinity: Offspring of related parents exhibiting greater genetic similarities.
  • Habsburg family: Known for inbreeding, resulting in the “Habsburg lip” (prognathism).
  • Prognathism: Mandibular misalignment, causing a prominent lower jaw.
  • Charles II of Spain: Severely affected by Habsburg lip, impairing his ability to chew.

Classical Genetics

  • Classical genetics concepts were developed in the mid-1800s.
  • Study in conjunction with molecular genetics.

Evolution

  • Changes in the gene pool over time.
  • Hardy-Weinberg principle: Quantifies genetics of non-evolving populations.

Fundamental Concepts of Genetics

  • Genes: DNA sequences coding for heritable traits.
  • Chromosomes: Organized genes and non-coding DNA for easy transfer during mitosis and meiosis.
  • Alleles: Alternative forms of a gene (e.g., I^A, I^B, and i for ABO blood antigens).
  • Genotype: Genetic combination an individual possesses.
  • Phenotype: Observable trait resulting from a genotype.
  • Homologous Chromosomes: Two copies of each chromosome, except for male sex chromosomes (X and Y).
  • Locus: Specific location of a gene on a chromosome.
  • Individuals inherit two alleles for each gene (except for sex-linked genes in males).

Allele Expression

  • Dominant Allele: Only one copy needed for phenotypic expression (represented by a capital letter).
  • Recessive Allele: Two copies needed for expression (represented by a lowercase letter).
  • Homozygous Genotype: Identical alleles for a gene.
  • Heterozygous Genotype: Different alleles for a gene.
  • Hemizygous Genotype: Only one allele present (e.g., X chromosome in males).

Patterns of Dominance

  • Complete Dominance: One dominant allele masks the recessive allele.
  • Codominance: More than one dominant allele exists, and both are expressed (e.g., I^A and I^B blood antigens).
  • Incomplete Dominance: Heterozygote expresses an intermediate phenotype (e.g., red flower x white flower = pink flower).

Penetrance and Expressivity

  • Penetrance: Proportion of individuals with an allele who express the phenotype.
  • Full Penetrance: 100% of individuals with the allele show symptoms (e.g., Huntington's disease with >40 sequence repeats).
  • High Penetrance: Most, but not all, individuals with the allele show symptoms.
  • Reduced/Low/Non-Penetrance: Fewer individuals show symptoms.
  • Expressivity: Varying phenotypes with identical genotypes.
  • Constant Expressivity: Same genotype results in the same phenotype.
  • Variable Expressivity: Same genotype results in different phenotypes.
  • Neurofibromatosis Type II: Autosomal dominant disease with variable expressivity (tumors, cataracts, skin lesions).

Mendelian Concepts

  • Gregor Mendel: Developed genetics principles in the 1860s using pea plants.

Mendel's First Law: Law of Segregation

  • Genes exist in alternative forms (alleles).
  • Organisms have two alleles for each gene, inherited from each parent.
  • Alleles segregate during meiosis, so gametes carry only one allele per trait.
  • If alleles differ, only one is fully expressed (dominant), the other is silent (recessive).
  • Codominance and incomplete dominance are exceptions.
  • Cellular Correlate: Separation of homologous chromosomes during anaphase I of meiosis.

Mendel's Second Law: Law of Independent Assortment

  • Inheritance of one gene doesn't affect inheritance of another gene.
  • Spermatogonia and oogonia undergo genome replication before meiosis I; sister chromatids are held together at the centromere.
  • Prophase I: Homologous chromosomes pair up to form tetrads; genetic material is swapped between chromatids, creating new allele combinations.
  • Complication: Linked genes (non-independent assortment).
  • Both segregation and independent assortment increase genetic diversity.
  • Genetic diversity improves species' ability to evolve and adapt.

DNA as Genetic Material

  • Mendel didn't know DNA was the genetic material; genes are made of DNA.
  • Early 1900s: Work was rediscovered.
  • Early to mid 1900s: Protein was believed to be inheritable material.

Griffith's Experiment (1920s)

  • Frederick Griffith: Studied Streptococcus pneumoniae.
  • Two strains: Virulent (smooth capsule) and non-virulent (rough capsule).
  • Virulent strain injected into mice caused death.
  • Heat-killed virulent strain didn't cause disease.
  • Non-virulent strain didn't cause disease.
  • Dead virulent + live non-virulent caused death; live bacteria with smooth capsules were found.
  • Transforming Principle: Live non-virulent bacteria acquired ability to form smooth capsules from dead virulent bacteria.

Avery, MacLeod, and McCarty Experiment

  • Identified the transforming principle.
  • Purified heat-killed virulent S. pneumoniae.
  • Separated the components into different extracts.
  • One extract transformed non-virulent S. pneumoniae, making them deadly.
  • Degrading DNA in the extract prevented transformation; degrading proteins didn't.
  • Conclusion: DNA is the transforming substance.

Hershey-Chase Experiment (1952)

  • Confirmed DNA independently carries genetic information.
  • Created bacteriophages with radiolabeled DNA (phosphorus) and protein (sulfur).
  • Allowed phages to infect non-labeled bacteria.
  • Centrifuged to separate material outside the cells from the bacterial cells.
  • No radiolabeled protein entered cells, but radiolabeled DNA did.
  • Viruses must enter cells to replicate.
  • Conclusion: DNA is the heritable genetic material.