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  • *Mendel and Heredity

  • *Gregor Mendel (1822-1884)Responsible for the Laws governing Inheritance of Traits

  • *Gregor Johann MendelAustrian monkStudied the inheritance of traits in pea plantsDeveloped the laws of inheritanceMendel's work was not recognized until the turn of the 20th century

  • *Gregor Johann MendelBetween 1856 and 1863, Mendel cultivated and tested some 28,000 pea plantsHe found that the plants' offspring retained traits of the parentsCalled the Father of Genetics"

  • *Mendel stated that physical traits are inherited as particlesMendel did not know that the particles were actually Chromosomes & DNAMendel & Particulate Inheritance

  • *Genetic TerminologyGenetics - study of heredityHeredity - passing of traits from parent to offspring Trait - any characteristic that can be passed from parent to offspring

  • *Types of Genetic CrossesMonohybrid cross - cross involving a single trait ex. flower color Dihybrid cross - cross involving two traits ex. flower color & plant height

  • *Punnett SquareUsed to help solve genetics problems

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  • *Designer GenesAlleles - two forms of a gene (dominant & recessive) Dominant - stronger of two genes expressed in the hybrid; represented by a capital letter (R) Recessive - gene that shows up less often in a cross; represented by a lowercase letter (r)

  • *Genes and Physical TraitsGenotype - gene combination for a trait (e.g. RR, Rr, rr) Phenotype - the physical feature resulting from a genotype (e.g. red, white)

  • *Genotype & Phenotype in FlowersGenotype of alleles: R = red flower r = yellow flowerAll genes occur in pairs, so 2 alleles affect a characteristicPossible combinations are:

    GenotypesRR RrrrPhenotypesRED RED YELLOW

  • *GenotypesHomozygous genotype - gene combination involving 2 dominant or 2 recessive genes (ex RR or rr); also called pure Heterozygous genotype - gene combination of one dominant & one recessive allele(ex Rr); also called hybrid

  • *Genes and Environment Determine Characteristics

  • *Mendels Pea Plant Experiments

  • *Reproduction in Flowering PlantsPollen contains spermProduced by the stamenOvary contains eggsFound inside the flowerPollen carries sperm to the eggs for fertilizationSelf-fertilization can occur in the same flowerCross-fertilization can occur between flowers

  • *Mendels Experimental MethodsMendel hand-pollinated flowers using a paintbrushHe could snip the stamens to prevent self-pollinationHe traced traits through the several generations

  • *How Mendel BeganMendel produced pure strains by allowing the plants to self-pollinate for several generations

  • *Seven Pea Plant TraitsSeed shape --- Round (R) or Wrinkled (r)Seed Color ---- Yellow (Y) or Green (y)Pod Shape --- Smooth (S) or wrinkled (s)Pod Color --- Green (G) or Yellow (g)Flower position---Axial (A) or Terminal (a)Plant Height --- Tall (T) or Short (t)Flower color --- Purple (P) or white (p)

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  • *Mendels Experimental Results

  • *What Do the Peas Look Like?

  • *GenerationsParental P1 Generation = the parental generation in a breeding experiment.F1 generation = the first-generation offspring in a breeding experiment. (1st filial generation)From breeding individuals from the P1 generationF2 generation = the second-generation offspring in a breeding experiment. (2nd filial generation) From breeding individuals from the F1 generation

  • *Following the GenerationsCross 2 Pure Plants TT x ttResults in all Hybrids TtCross 2 Hybrids get 3 Tall & 1 Short TT, Tt, tt

  • *Monohybrid Crosses

  • *Trait: Seed ShapeAlleles: R Roundr WrinkledCross: Round seeds x Wrinkled seedsRR x rrP1 Monohybrid CrossRRrrRrRrRrRrGenotype(s): RrPhenotype(s): RoundGenotypic Ratio: All alike RrPhenotypic Ratio: All alike Rr

  • *P1 Monohybrid Cross ReviewHomozygous dominant x Homozygous recessiveOffspring all Heterozygous (hybrids)Offspring called F1 generationGenotypic & Phenotypic ratio is ALL ALIKE

  • *Trait: Seed ShapeAlleles: R Roundr WrinkledCross: Round seeds x Round seedsRr x RrF1 Monohybrid CrossRrrRRRrrRrRrGenotype: RR, Rr, rrPhenotype: Round & wrinkledG.Ratio:1 RR:2 Rr:1 rrP.Ratio: 3:1

  • *F1 Monohybrid Cross ReviewHeterozygous x heterozygousOffspring: 25% Homozygous dominant RR 50% Heterozygous Rr 25% Homozygous Recessive rrOffspring called F2 generationGenotypic ratio is 1:2:1Phenotypic Ratio is 3:1

  • *And Now the Test CrossMendel then crossed a pure & a hybrid from his F2 generationThis is known as an F2 or test crossThere are two possible testcrosses: Homozygous dominant x Hybrid Homozygous recessive x Hybrid

  • *Trait: Seed ShapeAlleles: R Roundr WrinkledCross: Round seeds x Round seedsRR x RrF2 Monohybrid Cross (1st)RRrRRRRrRRRrGenotype: RR, RrPhenotype: RoundGenotypic Ratio: 1:1Phenotypic Ratio: All alike

  • *Trait: Seed ShapeAlleles: R Roundr WrinkledCross: Wrinkled seeds x Round seedsrr x RrF2 Monohybrid Cross (2nd)rrrRRrrrRrrrGenotype: Rr, rrPhenotype: Round & WrinkledG. Ratio: 1:1P.Ratio: 1:1

  • *F2 Monohybrid Cross ReviewHomozygous x heterozygous(hybrid)Offspring: 50% Homozygous RR or rr 50% Heterozygous RrPhenotypic Ratio is 1:1Called Test Cross because the offspring have SAME genotype as parents

  • *Mendels Laws

  • *Law of Dominance In a cross of parents that are pure for contrasting traits, only one form of the trait will appear in the next generation.All the offspring will be heterozygous and express only the dominant trait.RR x rr yields all Rr (round seeds)

  • *Law of Dominance

  • *Law of SegregationDuring the formation of gametes (eggs or sperm), the two alleles responsible for a trait separate from each other.Alleles for a trait are then "recombined" at fertilization, producing the genotype for the traits of the offspring.

  • *Applying the Law of Segregation

  • *Law of Independent Assortment Alleles for different traits are distributed to sex cells (& offspring) independently of one another. This law can be illustrated using dihybrid crosses.

  • *Dihybrid CrossA breeding experiment that tracks the inheritance of two traits.Mendels Law of Independent Assortmenta. Each pair of alleles segregates independently during gamete formationb. Formula: 2n (n = # of heterozygotes)

  • *Question:How many gametes will be produced for the following allele arrangements?

    2n (n = # of heterozygotes)




  • *Answer:1. RrYy: 2n = 22 = 4 gametesRY Ry rY ry

    2. AaBbCCDd: 2n = 23 = 8 gametesABCD ABCd AbCD AbCdaBCD aBCd abCD abCD

    3. MmNnOoPPQQRrssTtQq: 2n = 26 = 64 gametes

  • *Dihybrid CrossTraits: Seed shape & Seed colorAlleles: R round r wrinkled Y yellow y green RrYy x RrYyRY Ry rY ryRY Ry rY ryAll possible gamete combinations

  • *Dihybrid Cross

  • *Dihybrid CrossRound/Yellow: 9 Round/green: 3 wrinkled/Yellow: 3 wrinkled/green: 19:3:3:1

  • *Summary of Mendels laws

  • *Incomplete DominanceandCodominance

  • *Incomplete DominanceF1 hybrids have an appearance somewhat in between the phenotypes of the two parental varieties.Example: snapdragons (flower)red (RR) x white (rr)RR = red flowerrr = white flowerAll Rr = pink(heterozygous pink)RrRrRrRr

  • *Incomplete Dominance

  • *CodominanceTwo alleles are expressed (multiple alleles) in heterozygous individuals.Example: blood type

    1.type A= IAIA or IAi2.type B= IBIB or IBi3.type AB= IAIB4.type O= ii

  • *Codominance ProblemExample:homozygous male Type B (IBIB)x heterozygous female Type A (IAi)

  • *Another Codominance ProblemExample: male Type O (ii) x female type AB (IAIB)

  • *Sex-linked TraitsTraits (genes) located on the sex chromosomesSex chromosomes are X and YXX genotype for femalesXY genotype for malesMany sex-linked traits carried on X chromosome

  • *Sex-linked TraitsSex ChromosomesExample: Eye color in fruit flies

  • *Sex-linked Trait ProblemExample: Eye color in fruit flies (red-eyed male) x (white-eyed female) XRY x XrXrRemember: the Y chromosome in males does not carry traits.RR = red eyedRr = red eyedrr = white eyedXY = maleXX = femaleXR XrXr YXR XrXr Y50% red eyed female50% white eyed male

  • *PedigreePedigree - a family history that shows how a trait is inherited over several generations

  • *Pedigree - Female Carriers

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