1 mendelian genetics 2 gregor mendel (1822-1884) responsible for the laws governing inheritance of...
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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
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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
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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?
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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
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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
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Law of SegregationLaw of Segregation• What meiotic event
creates the law of segregation?
Meiosis 1
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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
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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
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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
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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
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Law of Independent Law of Independent AssortmentAssortment
• What meiotic event creates the law of independent assortment?
Meiosis 1
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The chromosomal basis of Mendel’s laws…
Trace the genetic events through meiosis, gamete formation & fertilization to offspring
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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
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Genetics & ProbabilityGenetics & Probability• Mendel’s laws:
– segregation – independent assortment
reflect same laws of probability that apply to tossing coins or rolling dice
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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%
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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
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• 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
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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
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NOW TO PRACTICE!
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