Slide 1Slide 2Slide 3Mendelian GeneticsSlide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16MUTATIONSSlide 18Slide 19Slide 20Slide 21Slide 22Slide 23COMPLEX INHERITANCESlide 25Slide 26Slide 27Slide 28NON-MENDELIAN INHERITANCESlide 30Slide 31Slide 32Slide 33Biology 101Biology 101Fall, 2007Fall, 2007Week 4 – GeneticsInherited traitsWeek 4 – GeneticsInherited traitsGregor Mendel (1822-1884) is generally regarded as the father of genetics. Josef Kölreuter discovered in the 1760’s that offspring could have features of only one parent, or could be intermediate between both.GENETICS - before and after MendelKarl Friederich von Gaertner did >10,000 hybridization experiments in the 1820's. Some of these identified the traits in peas (purple flower color, pod color and seed shape) that were subsequently used by Mendel in his "laws" of inheritance.The textbook (Stern) accentuates the work of Barbara McClintock. This was certainly important: McClintock was awarded a Nobel Prize in 1983However, the transposable elements she discovered relate more directly to epigenetics than to genetics.Barbara McClintockMendelian GeneticsSeveral individuals had studied inheritance of traits, but Mendel applied what is now known as “the Scientific Method” to address what was a puzzling situation. In particular, he:Gregor Mendel was an Augustinian monk with training in agricultural science and mathematics.5. Kept accurate records of his experiments and results, enabling others to repeat them.4. Counted offspring from each cross and analyzed the results mathematically.3. Followed not only the offspring of the first cross, but also those of subsequent crosses.1. Tested a specific hypothesis and planned his experiments carefully, using clear examples.2. Obtained pure-breeding lines for starting his experiments.Mendel derived two basic “laws”:•INDEPENDENT ASSORTMENT The inheritance of a pair of factors for one trait is independent of that for other factors.•SEGREGATION Hereditary traits are determined by discrete factors (genes) that occur in pairs, one from each parent.(b) Genes are present in linear arrays on chromosomes. If two genes are close together on the same chromosome (i.e., linked), there is relatively little chance that they will assort independently.(a) The involvement of a gene pathway (polygenes) for a trait means that it may not be governed by a single, discrete, factor.In fact, various situations cause variations in the above events so that many scientists no longer consider the above situations “laws”. Importantly:Dominance: Sometimes one factor dominates the other factor. A dominant trait masks/suppresses the alternative (recessive) trait for a particular feature. Conversely, a recessive trait is masked or suppressed by the dominant trait for the feature in question.Segregation: Paired factors segregate during the formation of reproductive cells (meiosis I) so that each cell gets one of the factors.Gene is the modern term for one of Mendel's paired "factors".Independent assortment: When considering two or more pairs of traits, the factors for each pair of traits assort independently to the reproductive cells.A plant is heterozygous for a given gene if it has different alleles for that gene on the two homologous chromosomes bearing the gene.A plant is homozygous for a given gene if it has identical alleles for that gene on both homologous chromosomes bearing the gene.Alleles are genes at the same position (locus) on homologous chromosomes (i.e. chromosomes that carry the same genes and that pair up early in meiosis I). Homologous chromosomes in an individual may carry the same or different alleles at a given locus.Phenotype: the physical form or appearance of an organism. This may differ from the genotype because of dominance and other regulatory events that mask the full expression of the genotype.Genotype: the genetic constitution of an organism.The genotypic ratio for the same progeny is 1:2:1Thus: a phenotypic ratio of 3:1 is typical for the progeny of a monohybrid cross between two parents heterozygous for a dominant trait.Progeny ratios of phenotypes often reveal the genetic state of plants that were crossedPunnett square is a useful diagram for determining the predicted ratios of offspring resulting from a genetic cross. (see Stern, p. 246). – Make one yourself at: http://www.usoe.k12.ut.us/curr/science/sciber00/7th/genetics/sciber/punnett.htmW. Bateson (worked with Punnett)PPPPGametes are reproductive cells, e.g. egg cells & sperm.A phenotypic ratio of 9:3:3:1 is typical for the progeny of a dihybrid cross between parents that are both heterozygous for two dominant/recessive traits.MUTATIONSsingle base insertion error: CCTGAGG  GGACTTGG  CCTGAAGGsingle base deletion error: CCTGAGG  GGAC CC CCTG GGsingle base substitution error: CCTGAGG  GGACACC CCTGTGGMutations can arise in many ways. Most are point mutations, consisting of single base substitutions, insertions or deletions that occur as nucleotide “typographical errors” during DNA replication:In normal β‑globin (one of the two polypeptides in hemoglobin), the sixth amino acid is Glutamic acid, a charged amino acid residue. •Sickle cell anemia is a clear example of the effects that can arise from what appears to be a very minor change in DNA – a single base substitution error: mRNA sequences: CCUGAGG  CCUGUGG amino acids coded for: Glu  ValAs a result, β‑globin does not assume its correct tertiary structure, hemoglobin function is deficient, and the affected erythrocytes are deformed, with a “sickle” shape.As a result of the error, the codon for the sixth amino acid is changed so that Valine, a nonpolar amino acid, is incorporated into β‑globin instead of glutamic acid, during messenger translation.•Exposure to chemicals and radiation can give rise single base changes. Exposure to sunlight radiation can lead to methylation or other chemical modifications of DNA that prevent proper expression of genetic information, e.g., cytosine methylation: CCTGAGG CmCTGAGGLarge changes are typically lethal and are therefore not propagated/conserved, but many heritable defects result from such events.Since the process of selection of the fittest has optimized most systems, the vast majority of mutations are harmful.However, some will be beneficial, and the cell with the new genetic