Biology A-Level: Inherited Change & Selection and Evolution - Summary Notes

Key Definitions

  • Homologous Chromosomes: Pair of chromosomes in diploid cells with the same structure, genes (not necessarily alleles), and loci, pairing in meiosis to form a bivalent.
  • Autosomes: 22 matching chromosomes in humans.
  • Sex Chromosomes: Non-matching pair, X and Y.
  • Gene: DNA length coding for a protein or polypeptide.
  • Allele: A specific gene variety.
  • Locus: Gene's position on a chromosome.
  • Diploid Cell (2n): Possesses two complete chromosome sets.
  • Haploid Cell (n): Possesses one complete chromosome set.

Meiosis

  • Halves chromosome number to form haploid gametes; without it, chromosome number would double each generation.
  • Meiosis I is a reduction division.
  • Meiosis II functions like mitosis.
  • Events in meiosis produce genetic variation: independent assortment and crossing over.

Gametogenesis

  • Spermatogenesis: Formation of male gametes (sperm) in testes.
    • Diploid spermatogonia $\rightarrow$ primary spermatocytes $\rightarrow$ secondary spermatocytes $\rightarrow$ spermatids $\rightarrow$ spermatozoa.
  • Oogenesis: Formation of female gametes (ova) in ovaries.
    • Diploid oogonia $\rightarrow$ primary oocytes (arrested in prophase I) $\rightarrow$ secondary oocyte + polar body; if fertilized: $\rightarrow$ ovum.

Gametogenesis in Flowering Plants

  • Male gametes are nuclei in pollen grains (anthers).
  • Female gametes are nuclei in embryo sacs (ovules).
  • Pollen mother cells divide by meiosis $\rightarrow$ four haploid cells; nuclei divide by mitosis $\rightarrow$ two haploid nuclei (pollen grains).
  • Spore mother cell in ovule divides by meiosis $\rightarrow$ four haploid cells (one develops into embryo sac); haploid nucleus divides by mitosis three times $\rightarrow$ eight haploid nuclei (one becomes female gamete).
  • Plants use meiosis for pollen and embryo sac production; gametes form inside via mitosis.

Genetic Variation

  • Chiasmata: Links formed during prophase I, enabling crossing over.
  • Crossing Over: Chromatid pieces swap, creating new allele combinations.
  • Independent Assortment: Homologous chromosomes align independently during metaphase I, creating different gamete combinations.

Genotype and Phenotype

  • Genotype: Alleles possessed by an organism.
  • Homozygous: Two identical alleles (e.g., HbAHbAHb^AHb^A).
  • Heterozygous: Two different alleles (e.g., HbAHbSHb^AHb^S).
  • Phenotype: An organism's characteristics, influenced by genotype and environment.
  • HbSHbSHb^SHb^S: Sickle cell anemia.
  • HbAHbAHb^AHb^A: Normal.
  • HbAHbSHb^AHb^S: Sickle cell trait (carrier), malaria-resistant.

Monohybrid Crosses

  • Codominance: Both alleles affect phenotype (e.g., snapdragons).
  • Dominant Allele: Masks recessive allele in heterozygote.
  • Recessive Allele: Only expressed when homozygous.
  • F1 Generation: Offspring of homozygous dominant x homozygous recessive cross.
  • F2 Generation: Offspring of F1 x F1 cross.
  • Test Cross: Cross of dominant phenotype with homozygous recessive to determine genotype.

Multiple Alleles & Sex Linkage

  • Most genes have multiple alleles (e.g., blood groups).
  • Females are XX, males are XY.
  • Sex Linkage: Genes on sex chromosomes (e.g., hemophilia on X).
  • Probability calculations for sex-linked traits.

Dihybrid Crosses

  • Independent assortment produces four gamete types (e.g., AD, Ad, aD, ad).
  • Dihybrid cross ratios: 9:3:3:1 (heterozygous x heterozygous).

Gene Interaction

  • Different loci interact to affect a single trait (e.g., feather color in chickens).
  • Dominant allele I causes white feathers, regardless of C allele.
  • Ratio of a dihybrid cross between a heterozygous organism and a homozygous recessive organism where the alleles show complete dominance 1 : 1 : 1 : 1
  • 9 : 3 : 3 : 1 (typical of a dihybrid cross between two heterozygous organisms where the two alleles show complete dominance and where the genes are on different chromosomes)

Linkage

  • Genes on the same chromosome are inherited together.
  • Modified phenotypic ratios.

Recombination

  • Recombinant offspring result from crossing over.
  • Crossover value: % of recombinant offspring.

Chi-Squared Test

  • Compares observed and expected results.
  • Formula: χ2=∑(O−E)2E\chi^2 = \sum \frac{(O-E)^2}{E}
  • Degrees of freedom = number of data classes - 1.
  • If χ2\chi^2 value < critical value, differences are due to chance.
  • Null hypothesis: No significant difference between observed and expected values.

Gene Mutation

  • Change in DNA sequence, creating new allele.
  • Mutagen: Increases mutation rate.
  • Types: base substitution, addition, deletion.
  • Addition/deletion causes frameshifts.
  • Substitution may be silent.

Genetic Diseases

  • Sickle Cell Anemia:
    • Base substitution (Val for Glu).
    • Causes insoluble Hb, sickle-shaped RBCs, anemia.
  • Albinism:
    • Autosomal recessive or sex-linked.
    • Tyrosinase mutation affects melanin production.
  • Huntington’s Disease:
    • Dominant allele.
    • CAG triplet repeats lead to neurological disorder.
    • More repeats = earlier onset.

Genetic Variation Sources

  • Independent assortment.
  • Crossing over.
  • Random mating.
  • Random fertilization.
  • Mutation.

Phenotypic Variation

  • Influenced by environment.
  • Discontinuous (Qualitative): Distinct categories (e.g., blood groups).
  • Continuous (Quantitative): Range of values (e.g., height).
  • Polygenes: multiple genes contribute to a trait.

Population Genetics

  • Environmental factors limit population size.
  • Natural selection: best-adapted individuals survive and reproduce.
  • Fitness: ability to survive and transmit genes.
  • Selection pressure: environmental factor favoring certain traits.
    • Stabilizing, directional, disruptive selection.

Examples of Selection

  • Antibiotic resistance in bacteria.
  • Industrial melanism in moths.
  • Sickle cell anemia and malaria resistance.

Genetic Drift

  • Random change in allele frequency, especially in small populations.
  • Founder effect: small group establishes new population.

Hardy-Weinberg Principle

  • Calculates genotype frequencies in large, random mating populations. p2+2pq+q2=1p^2 + 2pq + q^2 = 1
  • Assumptions: no selection, no migration, random mating.

Selective Breeding

  • Artificial selection for desired traits.
  • Examples: dairy cattle (docility, growth, milk yield), crop improvement (disease resistance, short stems).

Speciation

  • Formation of new species.
  • Species: organisms that can interbreed to produce fertile offspring.
  • Reproductive isolation: prevents interbreeding.
    • Prezygotic: prevents zygote formation.
    • Postzygotic: occurs after zygote formation.

Modes of Speciation

  • Allopatric: Geographic separation leads to divergence.
  • Sympatric: Ecological or behavioral separation in same area.

Molecular Evidence

  • mtDNA: tracks maternal lineage, used to determine relatedness.
  • Protein sequences (cytochrome c): similarities indicate common ancestry.

Extinctions

  • Caused by climate change, competition, habitat loss, human activities.

Gene Regulation

  • Structural genes: code for proteins.
  • Regulatory genes: control gene expression.
  • Repressible enzymes: synthesis inhibited by repressor protein.
  • Inducible enzymes: synthesis induced by substrate.

Lac Operon

  • Prokaryotic gene regulation (E. coli).
  • Structural genes: lacZ ($\beta$-galactosidase), lacY (permease), lacA (transacetylase).
  • Regulatory gene codes for repressor protein.
  • Lactose binds repressor, allowing transcription.

Eukaryotic Gene Control

  • Transcription factors regulate gene expression.
  • Hormones act through transcription factors.
  • Gibberellin stimulates amylase mRNA transcription in seeds.