MENDELLIAN GENETICS

MENDELLIAN GENETICSBY GROUP 5 MEMBERS:CAPATAYANDELESTETANGENTEVAGILIDADCATURAASGregor Johann 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"

Site of Gregor Mendels experimental garden in the Czech Republic

Mendel stated that physical traits are inherited as particlesMendel did not know that the particles were actually Chromosomes & DNA

Particulate Inheritance

Genetic TerminologyTrait - any characteristic that can be passed from parent to offspring Heredity - passing of traits from parent to offspring Genetics - study of heredity

Types of Genetic CrossesMonohybrid cross - cross involving a single traite.g. flower color Dihybrid cross - cross involving two traits e.g. flower color & plant heightPunnett SquareUsed to help solve genetics problems

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)

Genotype & Phenotype in FlowersGenotype of alleles:R = red flowerr = 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 (e.g. RR or rr); also called pure Heterozygous genotype - gene combination of one dominant & one recessive allele (e.g. Rr); also called hybridMendels Pea Plant Experiments

Reproduction in Flowering PlantsPollen contains spermProduced by the stamenOvary contains eggsFound inside the flower

Pollen carries sperm to the eggs for fertilizationSelf-fertilization can occur in the same flowerCross-fertilization can occur between flowers

How Mendel BeganMendel produced pure strains by allowing the plants to self-pollinate for several generationsEight 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)Seed Coat Color ---Gray (G) or White (g)Flower position---Axial (A) or Terminal (a)Plant Height --- Tall (T) or Short (t)Flower color --- Purple (P) or white (p)

Mendels Experimental ResultsThe theoretical or expected ratio of plants producing round or wrinkled seeds is 3 round :1 wrinkledMendels observed ratio was 2.96:1The discrepancy is due to statistical errorThe larger the sample the more nearly the results approximate to the theoretical ratioGeneration GapParental 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 generationFollowing the Generations

Cross 2 Pure PlantsTT x ttResults in all HybridsTtCross 2 Hybridsget3 Tall & 1 ShortTT, Tt, ttP1 Monohybrid CrossTrait: Seed ShapeAlleles: R Roundr WrinkledCross: Round seeds x Wrinkled seedsRR x rrRRrrRrRrRrRrGenotype: RrPhenotype: RoundGenotypicRatio: All alikePhenotypicRatio: All alikeP1 Monohybrid Cross ReviewHomozygous dominant x Homozygous recessiveOffspring all Heterozygous (hybrids)Offspring called F1 generationGenotypic & Phenotypic ratio is ALL ALIKEF1 Monohybrid CrossTrait: Seed ShapeAlleles: R Roundr WrinkledCross: Round seeds x Round seedsRr x RrRrrRRRrrRrRrGenotype: RR, Rr, rrPhenotype: Round & wrinkledG.Ratio: 1:2:1P.Ratio: 3:1F1 Monohybrid Cross ReviewHeterozygous x heterozygousOffspring:25% Homozygous dominant RR50% Heterozygous Rr25% Homozygous Recessive rrOffspring called F2 generationGenotypic ratio is 1:2:1Phenotypic Ratio is 3:1What Do the Peas Look Like?

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 HybridHomozygous recessive x HybridF2 Monohybrid Cross (1st)Trait: Seed ShapeAlleles: R Roundr WrinkledCross: Round seeds x Round seedsRR x RrRRrRRRRrRRRrGenotype: RR, RrPhenotype: RoundGenotypicRatio: 1:1PhenotypicRatio: All alikeF2 Monohybrid Cross (2nd)Trait: Seed ShapeAlleles: R Roundr WrinkledCross: Wrinkled seeds x Round seedsrr x RrrrrRRrrrRrrrGenotype: Rr, rrPhenotype: Round & WrinkledG. Ratio: 1:1P.Ratio: 1:1F2 Monohybrid Cross ReviewHomozygous x heterozygous(hybrid)Offspring:50% Homozygous RR or rr50% Heterozygous RrPhenotypic Ratio is 1:1Called Test Cross because the offspring have SAME genotype as parentsMendels LawsResults of Monohybrid CrossesInheritable factors or genes are responsible for all heritable characteristics Phenotype is based on Genotype Each trait is based on two genes, one from the mother and the other from the father True-breeding individuals are homozygous ( both alleles) are the sameLaw 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)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 combinationsDihybrid CrossRRYYRRYyRrYYRrYyRRYyRRyyRrYyRryyRrYYRrYyrrYYrrYyRrYyRryyrrYyrryyRound/Yellow: 9

Round/green: 3

wrinkled/Yellow: 3

wrinkled/green: 19:3:3:1 phenotypic ratioRYRyrYryRYRyrYryDihybrid Cross

Round/Yellow: 9Round/green: 3wrinkled/Yellow: 3wrinkled/green: 19:3:3:1Test CrossA mating between an individual of unknown genotype and a homozygous recessive individual.Example: bbC__ x bbcc

BB = brown eyesBb = brown eyesbb = blue eyes

CC = curly hairCc = curly haircc = straight hairbCb___bcSummary of Mendels lawsLAWPARENT CROSSOFFSPRINGDOMINANCE

TT x tt tall x short100% Tt tallSEGREGATION

Tt x Tt tall x tall

75% tall 25% short

INDEPENDENT ASSORTMENT

RrGg x RrGg round & green x round & green9/16 round seeds & green pods 3/16 round seeds & yellow pods 3/16 wrinkled seeds & green pods 1/16 wrinkled seeds & yellow podsIncomplete 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 flowerRRr

r

Incomplete DominanceRrRrRrRrR

RrAll Rr = pink(heterozygous pink)produces theF1 generationrIncomplete 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= iiCodominance ProblemExample:homozygous male Type B (IBIB)x heterozygous female Type A (IAi)IAIBIBiIAIBIBi1/2 = IAIB1/2 = IBi IBIAiIBSex-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 chromosomeSex-linked TraitsSex ChromosomesXX chromosome - femaleXy chromosome - malefruit flyeye colorExample: Eye color in fruit flies