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Mendel & the Gene Idea

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  1. Mendel & the Gene Idea Campbell and Reece Chapter 14

  2. Definitions • Character: observable, heritable feature that may vary among individuals in a population • Trait: 1 of 2 or more detectable variants in a genetic character • True-breeding: refers to organisms that produce offspring of the same variety over many generations of self-pollination • Hybridization: cross-breeding 2 true-breeding individuals

  3. Mendel’s Experiments • Advantages of using peas: • several characters with “either-or” traits • short generation time • large #s of offspring • each flower contained both male & female organs

  4. Mendel’s Experiments • started with true breeders • cross-pollinated 2 contrasting, true-breeding pea varieties (hybridization) • true-breeding parents = P generation • their hybrid offspring = F1 generation • F1 self-pollinated = F2 generation

  5. Mendel’s Experiment • did quantitative analysis of thousands of genetic crosses • deduced 2 principles of heredity: • Law of Segregation (monohybrids) • Law of Independent Assortment (dihybrids)

  6. Law of Segregation

  7. Alleles • alternative versions of a gene

  8. Mendel’s Model • Alternative versions of genes (alleles) account for variations in inherited characters. • For each character, an organism inherits 2 copies of a gene, one from each parent. • If the 2 alleles @ a locus differ, then the dominant allele determines the organism’s appearance & the recessive allele has no noticeable effect on the organism’s appearance

  9. Mendel’s Model continued 4. Law of Segregation: the 2 alleles for a heritable character separate during gamete formation & end up in different gametes. (correlates to 2 homologous chromosomes separating in Meiosis I)

  10. Punnett Squares • diagramatical device for predicting the allele composition of offspring from a cros between individuals froma known genetic makeup. • P signifies dominant • p signifies recessive

  11. More Genetic Vocabulary • Homozygous: having 2 identical alleles for a given gene • Heterozygous: having 2 different alleles for a given gene • Phenotype: the observable physical & physiological traits of an organism, determined by its genetic makeup • Genotype: the genetic makeup or set of alleles of an organism • Testcross: breeding an organism of unknown genotype with a homozygous recessive to determine the unknown genotype

  12. Phenotype versus Genotype

  13. Testcross

  14. Law of Independent Assortment • states basically that in a dihybrid cross each allele for the 2 characters being crossed has equal opportunity

  15. Laws of Probability • Probabilities of all possible outcomes for an event = 1 • outcome of any particular toss of a coin is unaffected by the results of any previous tosses

  16. The Multiplication Rule • used to determine the probability that 2 or more independent events will occur together in some specific combination • multiply the probability of 1 event by the probability of the 2nd event

  17. The Addition Rule • the probability that any 1 of 2 or more mutually exclusive events will occur is calculated by adding their individual probabilities (which we calculate using the multiplication rule)

  18. Time for Handouts

  19. Beyond Simple Mendelian Genetics

  20. Degrees of Dominance • Alleles can show different degrees of dominance or recessiveness in relation to each other • Mendel’s peas characters were examples of complete dominance (all or none) • Incomplete Dominance: neither allele is completely dominant or recessive

  21. Incomplete Dominance • Snapdragons

  22. Codominance • 2 alleles affect the phenotype in separate, distinguishable ways

  23. Type of Dominance?

  24. Relationship between Dominance & Phenotype • When a dominant allele coexists with a recessive allele in a heterozygote, they do not actually interact. • It’s in the pathway from genotype to phenotype that dominance & recessiveness come into play

  25. An Example • Mendel’s peas Round/wrinkled • R allele codes for an enzyme that helps convert an unbranched form of starch  branched form in the seed • r codes for a defective form of same enzyme leading to an accumulation of unbranched starch which leads to excess water entering seed by osmosis

  26. later, when seed dries it wrinkles • If R present, it makes enough enzyme to make enough branched starch to prevent wrinkling

  27. Degree of Dominance/Recessiveness • sometimes depends on how closely we look • example: Tay Sachs disease • homozygous recessive • Those with it cannot metabolize certain lipids in neurons  lipids accumulate  child suffers neurological events (seizures, blindness, degeneration of motor & mental performance)

  28. Tay Sachs continued • when study heterozygotes vs. homozygous dominant individuals: heterozygotes have an intermediate level of the activity of enzyme that metabolizes this lipid than do homozygous dominant individuals • on biochemical level acts like incomplete dominance since ½ the normal enzyme activity is sufficient to prevent lipid accumulation, heterozygotes have normal phenotype • on molecular level it is really an example of codominance

  29. Frequency of Dominant Allele • dominant allele not always more frequent allele in a population • example: polydactyly • extra fingers or toes • 1/400 babies born in USA • some caused by presence of a dominant allele

  30. Multiple Alleles • most genes exist in >2 allelic forms • example: ABO blood groups

  31. Pleiotrophy • most genes have multiple phenotypic effects

  32. Epistasis • Greek: standing apart • phenotypic expression of a gene at one locus alters that of a gene at 2nd locus • example: color of labs

  33. Polygenic Inheritance • Quantitative Characters: phenotypes vary in gradation along continuum in a population (height, skin color) • Polygenic Inheritance: an additive effect of 2 or more genes on a single phenotypic character, several genes  single phenotype (converse of pleiotrophy: 1 gene  several characters)

  34. Nature /Nuture • for humans: very old ? • generally, genotype is NOT associated with a rigidly defined phenotype • see range of phenotypic possibilities due to environmental influences • phenotypic range is called: norm of reaction for a genotype • generally, broadest for polygenic characters

  35. Multifactorial Characters • The environment contributes to the quantitative nature of polygenic characters which are referred to as multifactorial • influenced by genetics & environment (nutritional status, exposure to infectious disease, general well-being)

  36. Integrating a Mendelian View of Heredity & Variation • in place of looking at organisms as single gene  single phenotype • view organism as whole: emergent properties of all genes  all aspects of its phenotype • In most cases, a gene’s impact on phenotype is affected by genes & by the environment

  37. In light of all the possibilities of gene interaction it was extremely lucky that Mendel chose to study inheritance in the garden pea he chose.

  38. Pedigree • a diagram of a family tree with conventional symbols, showing the occurrence of heritable characters in parents & offspring over multiple generations

  39. Pedigree Symbols

  40. Dominant Trait

  41. Recessive Trait

  42. Behavior of Recessive Alleles • generally, the recessive homozygous either has a malfunctioning protein or no protein at all • heterozygous individuals produce enough of the normal protein to have “normal” phenotype & are called carriers