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Mendelian Mendelian GeneticsGenetics

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Gregor Gregor MendelMendel

(1822-1884)(1822-1884)

Responsible Responsible for the Laws for the Laws governing governing

Inheritance Inheritance of Traitsof Traits

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Gregor Johann MendelGregor Johann MendelAustrian monkAustrian monkStudied the Studied the inheritanceinheritance of of traits in traits in pea plantspea plantsDeveloped the Developed the laws of inheritancelaws of inheritanceMendel's work Mendel's work was not recognized was not recognized until the turn of until the turn of thethe 20th century 20th century

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Gregor Johann MendelGregor Johann MendelBetween Between 1856 1856 and 1863,and 1863, Mendel Mendel cultivated and cultivated and tested some tested some 28,000 pea plants28,000 pea plantsHe found that He found that the plants' the plants' offspring retained offspring retained traits of the traits of the parentsparentsCalled theCalled the “Father of “Father of Genetics"Genetics"

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Site of Site of Gregor Gregor Mendel’s Mendel’s experimentexperimental garden al garden in the in the Czech Czech RepublicRepublic

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Mendel stated Mendel stated that physical traits that physical traits are inherited as are inherited as “particles”“particles”Mendel did not Mendel did not know that the know that the “particles” were “particles” were actually actually Chromosomes & Chromosomes & DNADNA

Particulate InheritanceParticulate Inheritance

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Genetic TerminologyGenetic Terminology

TraitTrait - any characteristic - any characteristic that can be passed from that can be passed from parent to offspring parent to offspring HeredityHeredity - passing of - passing of traits from parent to traits from parent to offspring offspring GeneticsGenetics - study of - study of heredity heredity

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Designer Designer “Genes”“Genes”

AllelesAlleles – one of the t – one of the two forms of wo forms of a a gene gene (dominant & recessive)(dominant & recessive) DominantDominant - - stronger of two stronger of two genes expressed in the hybrid; genes expressed in the hybrid; represented byrepresented by aa capital letter capital letter (R)(R) RecessiveRecessive - - gene that shows up gene that shows up less often in a cross; represented less often in a cross; represented by aby a lowercase letter (r)lowercase letter (r)

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Genetic TerminologyGenetic Terminology

Monohybrid - Monohybrid - cross involving a cross involving a single trait e.g. flower colorsingle trait e.g. flower colorDihybrid – Dihybrid – cross involving two cross involving two traits e.g. flower color & plant traits e.g. flower color & plant heightheightTrue Breeding – True Breeding – all offspring will have the same characteristic generation after generation

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GenotypesGenotypesHomozygousHomozygous genotype - gene genotype - gene

combination involving 2 combination involving 2 dominant or 2 recessive genes dominant or 2 recessive genes (e.g. RR or rr);(e.g. RR or rr); also called also called pure pure  HeterozygousHeterozygous genotype - gene genotype - gene combination of one dominant & combination of one dominant & one recessive allele    (one recessive allele    (e.g. Rr);e.g. Rr); also calledalso called hybridhybrid

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More TerminologyMore Terminology

GenotypeGenotype - - gene gene combination for a traitcombination for a trait (e.g. RR, Rr, rr) (e.g. RR, Rr, rr) PhenotypePhenotype - - the physical the physical feature resulting from a feature resulting from a genotypegenotype (e.g. red, white) (e.g. red, white)

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Genotype & Phenotype in Genotype & Phenotype in FlowersFlowers

Genotype of alleles:Genotype of alleles:RR = red flower= red flowerrr = yellow flower= yellow flower

All genes occur in pairs, so All genes occur in pairs, so 22 allelesalleles affect a characteristic affect a characteristic

Possible combinations are:Possible combinations are:

GenotypesGenotypes RRRR RRrr rrrr

PhenotypesPhenotypesRED RED RED RED YELLOWYELLOW

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GenotypesGenotypesParental Generation (P1) - Parental Generation (P1) - the

parental generation in a breeding experimentFirst Filial generation (F1) – First Filial generation (F1) – First generation created by parentsSecond Filial generation (F2) - Second Filial generation (F2) - the the second-generation offspring in a second-generation offspring in a breeding experiment. breeding experiment.

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Mendel’s Pea Mendel’s Pea Plant Plant

ExperimentsExperiments

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Reproduction in Flowering Reproduction in Flowering PlantsPlants

Pollen contains spermPollen contains spermProduced by the stamenProduced by the stamen

Ovary contains eggsOvary contains eggsFound inside the flowerFound inside the flower

Pollen carries sperm to Pollen carries sperm to the eggs for fertilizationthe eggs for fertilization

Self-fertilizationSelf-fertilization can can occur in the same occur in the same flowerflower

Cross-fertilizationCross-fertilization can can occur between occur between flowersflowers

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Mendel’s Experimental Mendel’s Experimental ResultsResults

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Did the observed ratio match the theoretical ratio?Did the observed ratio match the theoretical ratio?

The theoretical or expected ratio of The theoretical or expected ratio of plants producing round or wrinkled plants producing round or wrinkled seeds is seeds is 3 round :1 wrinkled3 round :1 wrinkled

Mendel’s observed ratio was 2.96:1Mendel’s observed ratio was 2.96:1

The discrepancy is due to The discrepancy is due to statistical statistical errorerror

The The larger the samplelarger the sample the more the more nearly the results approximate to nearly the results approximate to the theoretical ratiothe theoretical ratio

Looking closer at Mendel’s Looking closer at Mendel’s workwork

true-breedingpurple-flower peas

true-breeding white-flower peasX

100%100%

purple-flower peasF1generation(hybrids)

25%white-flower peas

F2generation

75%purple-flower peas

3:1

P

self-pollinate

Test crossTest cross• Cross-breed the dominant phenotype

— unknown genotype — with a homozygous recessive (pp) to determine the identity of the unknown allele

x

ppis itPP or Pp?

x

PP pp

Test crossTest cross

p p

P

P

Pp

Pp Pp

Pp

p p

P

p

Pp

pp pp

Pp

x

Pp pp

100%50%:50%

1:1

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Mendel’s LawsMendel’s Laws

Mendel’s laws of heredity Mendel’s laws of heredity (#1)(#1)

• Law of segregation – when gametes are produced

during meiosis, homologous chromosomes separate from each other

– each allele for a trait is packaged into a separate gamete

PP

P

P

pp

p

p

Pp

P

p

Law of SegregationLaw of Segregation• What meiotic event

creates the law of segregation?

Meiosis 1

Monohybrid crossMonohybrid cross• Some of Mendel’s

experiments followed the inheritance of single characters – flower color– seed color – monohybrid crosses

• Other of Mendel’s experiments followed the inheritance of 2 different characters – seed color and

seed shape– dihybrid crosses

Dihybrid crossDihybrid crosstrue-breeding

yellow, round peastrue-breeding

green, wrinkled peasx

YYRR yyrrP

YyRr

100%F1generation(hybrids)

yellow, round peas

self-pollinate

F2generation

9/16yellowround peas

9:3:3:13/16

greenround peas

3/16yellow

wrinkledpeas

1/16green

wrinkledpeas

Y = yellowR = round

y = greenr = wrinkled

Dihybrid crossDihybrid crossYyRr YyRr

YR Yr yR yr

YR

Yr

yR

yr

YYRR YYRr YyRR YyRr

YYRr YYrr YyRr Yyrr

YyRR YyRr yyRR yyRr

YyRr Yyrr yyRr yyrr

x9/16

yellowround

3/16greenround

3/16yellow

wrinkled

1/16green

wrinkled

Mendel’s laws of heredity Mendel’s laws of heredity (#2)(#2)

• Law of independent assortment– each pair of alleles segregates into

gametes independently • 4 classes of gametes are produced

in equal amounts– YR, Yr, yR, yr

• only true for genes on separate chromosomes

YyRr

Yr Yr yR yR YR YR yr yr

Law of Independent Law of Independent AssortmentAssortment

• What meiotic event creates the law of independent assortment?

Meiosis 1

The chromosomal basis of Mendel’s laws…

Trace the genetic events through meiosis, gamete formation & fertilization to offspring

Review: Mendel’s laws of Review: Mendel’s laws of heredity heredity

• Law of segregation– monohybrid cross

• single trait

– each allele segregates into separate gametes• established by Meiosis 1

• Law of independent assortment– dihybrid (or more) cross

• 2 or more traits

– each pair of alleles for genes on separate chromosomes segregates into gametes independently• established by Meiosis 1

Genetics & ProbabilityGenetics & Probability• Mendel’s laws:

– segregation – independent assortment

reflect same laws of probability that apply to tossing coins or rolling dice

Probability & geneticsProbability & genetics• Calculating probability of

making a specific gamete is just like calculating the probability in flipping a coin– probability of tossing

heads? 50%– probability making a P

gamete…PP

P

P

Pp

P

p

50%

100%

Calculating probabilityCalculating probabilityPp x Pp

P pmale / sperm

P

p

fem

ale

/ eg

gs PP

Pp pp

Pp

sperm egg

1/2 1/2

offspring

=x 1/4P P PP

1/2 1/2 =x 1/4P p Pp

1/2 1/2 =x 1/4p p pp

1/2 1/2 =x 1/4

1/2

p P

• Chance that 2 or more independent events will occur together– probability that 2 coins tossed at the

same time will land heads up

– probability of Pp x Pp pp

Rule of multiplicationRule of multiplication

1/2 x 1/2 = 1/4

1/2 x 1/2 = 1/4

Calculating dihybrid Calculating dihybrid probabilityprobability

• Rule of multiplication also applies to dihybrid crosses– heterozygous parents — YyRr– probability of producing yyrr?

• probability of producing y gamete = 1/2• probability of producing r gamete = 1/2• probability of producing yr gamete =

1/2 x 1/2 = 1/4• probability of producing a yyrr offspring =

1/4 x 1/4 = 1/16

NOW TO PRACTICE!

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