Genetics 3.1

• Mendel used a Model Experiment approach to study patterns of inheritance
• Mendel facts

  • Born in 1822
  • Province of Austria
  • University of Vienna
  • Unaccredited teacher - monastery
  • Pea garden in bruno - Czechoslovakia
  • Results - 1865 - Reported to Bruno society for the natural sciences
  • Published in 1866 “Experiments on Plant hybrids”
• Nobody really cared about Mendel's work until after he died
• More Mendel facts

  • Abbott of the Monastery - 1868
  • Spent time in administrative work, drinking, parties, smoking cigars, etc.
  • Died in 1884
  • 1900 - Carl Correns (Germany), Hugo de Vries (Netherlands), Erich von Tschermak (Austria) rediscovered Mendel’s work.
  • Mendel was known as the Father of Genetics
• Mendel used a model experimental approach to study patterns of inheritance
• He chose the pea garden as his model system because of 5 reasons

  • Easy to grow
  • True-breeding strains
  • Controlled matings - self or cross-fertilization
  • Grows to maturity in one season
  • Has observable characteristics with 2 distinct forms
• Pea plants have 7 visible/contrasting traits

  • Seed shape
  • Seed color
  • Pod shape
  • Pod color
  • Flower color
  • Flower position
  • Stem height
• Since Mendel’s work was written in mathematical terms, it was hard for people to relate it to genetics or understand it.
• Mendel’s postulates were eventually accepted as the basis for Mendelian (transmission) genetics.
• The monohybrid cross reveals how one trait is transmitted from gen to gen.
• Monohybrid crosses involve a single pair of contrasting traits.
• In a MC, the parents are the P1 generation, and their offspring are the F1 generation.

  Offspring that arise from the self-fertilization of the F1 gen are the F2 gen.
• true-breeding plant: a plant that always produces offspring of the same phenotype when self-fertilized
• In the F1 gen of a MC, all of the plants have just 1 of the 2 contrasting traits.
• In the F2 gen, ¾ of the plants exhibit the same trait as the F1 gen, and ¼ exhibit the contrasting trait that disappeared in the F1 gen.

  To explain these results, Mendel proposed the existence of “particulate unit factors” for each trait.
• He suggested that these factors (genes) are passed unchanged from gen to gen, determining various traits expressed by each individual plant.
• Mendel’s MC weren’t sex dependent. It didn’t matter if  a tall male plant pollinated a dwarf female plant, or vice versa. The results would be the same. This is a reciprocal cross. 
• Mendel proposed 3 postulates of inheritance:

  1. Unit factors exist in pairs
  2. In the pair of unit factors for a single characteristic in an individual, one unit factor is dominant and the other is recessive
  3. The paired unit factors segregate (separate) independently during gamete formation
• Parents plant- P gen, their hybrid offspring - F1 gen, a cross of the F1 plants - F2
• For each characteristic, an organism inherits 2 alleles, one from each parents; the alleles can be the same or different
• A homozygous genotype has identical alleles

  A heterozygous genotype has 2 different alleles
• Mendel’s 3 postulates in more depth:

  1. Unit factors in pairs- Genetic character are controlled by unit factors existing in pairs in individual organism
  2. Dominance/Recessiveness - In the pair of unit factors for a single characteristic in an individual, one unit factor in dominant, the other is recessive
  3. Segregation- The paired unit factors segregate (separate) independently during gamete formation
• Genes are found in alternative versions called alleles. A genotype is the listing of alleles an individual carries for a specific gene.
• The genotype is the genetic makeup of an individual.

  The phenotype is the physical expression of the genetic makeup.
• In 1905, a guy who wasn’t connected with Mendel at all, R.C. Punnett developed the Punnett square. People began to use his square to represent a few combinations of genes and phenotypes
• True breeding – Homozygous genes. DD or dd. Organisms that are homozygous for genes.
• Testcross - A way to determine whether an individual displaying the dominant phenotype is homozygous or heterozygous for that trait.
• When a heterozygote is crossed with a recessive homozygote, you get a 2:2 ratio. 50% dominant 50% recessive
• Mendel’s dihybrid cross generated a unique F2 ratio. A dihybrid cross involves 2 pairs of contrasting traits. 9:3:3:1
• The product law can be used to predict the frequency with which 2 independent events will occur simultaneously
• Mendel’s 4th postulate: independent assortment- Traits assort independently during gamete formation. All possible combinations of gametes will form with equal frequency
• Trihybrid crosses involving 3 independent traits show that Mendel’s rules apply to any number of traits
• In a trihybrid cross, the F1 gen will have all dominant alleles for each gene
• The forked-line (branched diagram) method is easier to use than a Punnett square for analysis of inheritance of larger number of traits
• Mendel suggested that heredity resulted in discontinuous variation, as opposed to the existing continuous variation hypothesis of his time- in which offspring were thought to be a blend of the parental phenotypes
• Chromosomal theory of inheritance- Proposed that the separation of chromosomes during meiosis could be the basis for Mendel’s principles of segregation and independent assortment
• Independent assortment leads to extensive genetic variation
• A major consequence of independent assortment is the production of genetically dissimilar gametes

  Genetic variation results from independent assortment and is very important to the process of evolution
• Probability- The number of times a particular events occurs, divided by the total number of opportunities for the event to occur
• Sum rule- Combined probability of 2 (or more) mutually exclusive events occurring is the sum of their individual probabilities