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The father of genetics: Gregor MendelA monk who was trained in science at the University of ViennaGenes, Alleles, and ChromosomesHomologous chromosomes carry the same kinds of genes for the same characteristicsGenes for the same characteristic are found at the same loci on both homologous chromosomesGenes for a characteristic found on homologous chromosomes may not be identicalAlternative versions of genes found at the same gene locus are called allelesAlleles are formed by mutationsIf a mutation occurs in gametes (sperm or egg) then the mutation can be passed on from parent to offspringAn organism’s alleles may be the same or differentIf both homologous chromosomes carry the same allele (gene form) at a given gene locus, the organism is homozygous or true-breeding at that locusIf two homologous chromosomes carry different alleles at a given locus, the organism is heterozygous at that locus, also called hybridThe Punnett Square Method: predicts offspring genotypes and phenotypes from combinations of parental gametes1. Assign letters to the different alleles of the characteristic under consideration (uppercase for dominant, lowercase or recessive)2. Determine the possible gametes from both parents3. Write the gametes from each parent, along each side of a 2x2 grid (Punnett square)4. Fill in the genotypes of each pair of combined gametes in the grid5. Determine the genotypic ratio based on the outcome of the Punnett square6. Based on which traits are dominant, and which are recessive, calculate the predicted phenotypic ratioHow were the Principles of Inheritance Discovered?Gregor MendelChose edible pea plant for his experimentsAllowed him to see patterns in the way plant characteristics were inheritedWhy pea plants?Pea flowers have stamens (male structures that produce pollen), pollen contains the sperm (male gametes), sperm are gametes and pollen is the vehiclePea flowers have carpels (female structures housing the ovaries), which produce the eggs (female gametes)Pea flower petals enclose both male and female flower parts and prevent entry of pollen from another pea plantBecause of their structure, pea flowers naturally self-fertilizePollen from the stamen transfers to the carpel of the same plant, where the sperm then fertilizes the plant’s eggsMendel was able to mate two different plants by cross-fertilizationFemale parts (carpels) were dusted with pollen from other selected plantsHow are Single Traits Inherited?Pea plants that are homozygous for a particular characteristic always produce the same phenotypeIf a plant is homozygous for purple flowers, it will always produce offspring with purple flowersPlants that are homozygous for a characteristic are described as true-breedingA genetic cross is the mating of pollen and eggs (from same or different parents)The parents used in a cross are part of the parental generation (known as P)The offspring of the P generation are members of the first filial generation (F1)Offspring of the F1 generation are members of the F2 generationMendel’s flower color experiments:Crossed a true-breeding (homozygous) purple flower plant which a true-breeding (homozygous) white flower plant (the P generation)F1: all purple flowered plantsMendel allowed the F1 generation to self-fertilizeF2: ¾ purple and ¼ white, 3:1 ratioThe results showed that the white trait didn’t disappear in F1, but was hiddenMendel allowed F2 to self-fertilizeF3: all white flowered F2 produced white flowered offspring (proved to be true-breeding or homozygous)F3: purple flowered F2 plants produced 3 offspring1 out of 3 were true-breeding for purpleThe others were heterozygous, producing both white and purple offspringMendel determined that out of 4 offspring in F2, 1 was homozygous for purple, 1 was homozygous for white, and the other two were heterozygousGenotype: the particular combination of the two alleles carried by an individualPhenotype: the physical expression of the genotype (ex: purple or white flowers)Test cross: used to deduce whether an organism with a dominant phenotype is homozygous or heterozygous1. Cross the unknown dominant-phenotype organism (P_) with a homozygous recessive organism (pp)2. If the dominant-phenotype organism is homozygous dominant (PP), only dominant-phenotype offspring will be produced (Pp)3. If the dominant-phenotype organism is heterozygous (Pp), approximately half the offspring will be of recessive phenotype (pp)How is sex determined in the offspring?Mammals have a set of sex chromosomes that dictate genderFemales: two X chromosomesMales: an X and a Y chromosomeThe rest of the (non-sex) chromosomes occur in identical pairs and are called autosomesFor organisms with males=XY and females=XX, the sperm sex chromosome determines the sex of the offspringDuring sperm formation, each receives the X or Y chromosome, along with a copy of all autosomesThe female has only X, so the unfertilized egg will always have just an X chromosomeEgg + sperm with a Y = male; Egg + X-bearing sperm, a female is producedMany traits don’t follow simple Mendelian rules of inheritanceNot all traits are completely controlled by a single geneA trait may not be completely dominant to anotherIncomplete dominance: the pattern of inheritance when the heterozygous phenotype is intermediate between the two homozygous phenotypesHuman hair texture is influenced by a gene with two incompletely dominant alleles, C1 and C2Two copies of the C1 allele gives curly hairTwo copies of the C2 allele gives straight hairHeterozygotes (C1C2 genotype) have wavy hairIf two wavy haired people mate, their children could have any of the three hair types: C1C1=curly, C1C2=wavy, straight=C2C2A single gene may have multiple allelesAn individual may have at most two different gene allelesA species may have multiple alleles for a given characteristicHowever, each individual still carries two alleles for this characteristicHuman blood typesHuman blood group genes produce blood types A, B, AB, and O (three alleles: A, B, and o)Alleles A and B code for enzymes that add different sugar molecules to the ends of glycoproteins that protrude from red blood cellsAllele o codes for a nonfunctional enzyme that doesn’t add any sugar moleculesBlood types A, B, AB, and O arise as a result of the actions of these allelesAlleles A and B are both dominant to o (but are not dominant to each other, co-dominance)AA or Ao genotypes  type ABB or Bo genotypes  type Boo genotypes  type OAB individuals


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LSU BIOL 1201 - Mendel and the Gene Idea

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