Principles of Inheritance I

Gregor Mendel

  • Austrian monk who used peas to study genetics systematically.
  • Studied traits such as seed coat color, seed color, pod color, flower color, seed shape, pod shape, stem height, and flower position.

Mendel's Experiments

  • P generation: smooth seeds crossed with wrinkled seeds.
  • F1 generation: all smooth seeds.
  • F1 cross: Monohybrid cross or F1 progeny cross.

Terminology

  • Character: heritable feature (e.g., flower color).
  • Trait: variant for a character (e.g., purple/white flowers).
  • Phenotype: organism's traits (e.g., the color of its flower).
  • Allele: alternative forms of a gene (e.g., A or a).
  • Genotype: combination of alleles present in an individual (e.g., AA, Aa, or aa).
  • Normal/Wild-type: Most common allele observed in the population.

Mendel’s Genetic Hypothesis

  • True-breeding AAAA crossed with aaaa.
  • F1 Progeny: AaAa.
  • Self Fertilization F1 Cross using a Punnett square.

Phenotype vs Genotype

  • Cross AAAA (Round) with aaaa (Wrinkled).

Mendel's Ratios

  • Phenotype ratio ~ 3:1 (dominant: recessive).
  • Genotype ratio ~ 1:2:1 (AA:Aa:aaAA : Aa : aa).
  • Genotype ratio AA:AaAA:Aa in dominant trait ~ 1:2.

Genetics

  • Dominant: trait expressed in a heterozygous individual.
  • Recessive: trait only expressed when the individual is homozygous for the recessive allele.

Multiple Traits

  • Independent Assortment: Mendel examined if the inheritance pattern applied to different traits independently.
  • Dihybrid F1 Progeny: Round/Yellow (WWGG) crossed with Wrinkled/green (wwgg) resulted in F1 progeny that were Round/Yellow (WwGg).

Mendel's Laws

  • First Law: The two members of a gene pair segregate from each other in meiosis; each gamete has an equal probability of obtaining either member of the gene pair.
  • Second Law: The law of independent assortment; unlinked or distantly linked segregating gene pairs assort independently at meiosis.

Genetic Linkage

  • When Independent Assortment isn’t there…
    • Two genes: flower color and shape of pollen grains.
    • Crossed pure lines P/P⋅L/L×p/p⋅l/lP/P · L/L × p/p · l/l.
    • F1 - P/p⋅L/lP/p · L/l All purple and long.

Genetic Mapping

  • Map based on A-B recombination.
  • Map based on A-C recombination.

The Heritable Material

  • Double-stranded helix.
  • Information storage.
  • Replicable.
  • Traits encoded by distinct packets of information.
  • Inherited from both parents.

DNA

  • DNA found in the nucleus.
  • Human cell has 3.8x1093.8 x 10^9 bp – fully extended DNA of human cell covers 6ft (2m).
  • Different orders of chromatin packing give rise to the highly condensed metaphase chromosome.

Chromosomes

  • Karyotype - chromosome complement human 46, XY or 46, XX.
  • 44 (22 pairs) Autosomes.
  • 2 (1 pair) Sex chromosomes.
  • Majority of human cells - diploid 2 copies of each autosome.
  • Gametes (spermatazoa/oocytes) - haploid 1 copy of each autosome.

Chromosome Theory of Heredity (1902)

  • Genes are parts of the visible structures called chromosomes.
  • Recognized independently by Sutton and Boveri.

The Gene Concept

  • Alternative versions of a gene account for variations in characters.
  • Alleles - 1 allele for each gene inherited from each parent.
  • If alleles the same – homozygous.
  • If alleles not the same - heterozygous.

Modes of Inheritance

  • Autosomal dominant.
  • Autosomal recessive.
  • Sex-linked.

Trisomy

  • 3 copies of a chromosome present.
  • Person has 47 chromosome not 46.
  • Examples: Down’s Syndrome (21), Edward Syndrome (18), Patau Syndrome (13), Turner’s Syndrome.
  • Trisomy can be detected by Fluorescent In Situ Hybridization.