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Mendelian Mendelian InheritanceInheritance 1
OutlineOutline
Blending InheritanceBlending Inheritance
Monohybrid CrossMonohybrid Cross
Law of SegregationLaw of Segregation
Modern GeneticsModern Genetics
Genotype vs. PhenotypeGenotype vs. Phenotype
Punnett SquarePunnett Square
Dihybrid CrossDihybrid Cross
Law of Independent AssortmentLaw of Independent Assortment
Human Genetic DisordersHuman Genetic Disorders
Mendelian Mendelian InheritanceInheritance
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3Gregor Mendel
Mendelian Mendelian InheritanceInheritance 4
Gregor MendelGregor Mendel
Austrian monkAustrian monkStudied science and mathematics at Studied science and mathematics at University of ViennaUniversity of Vienna
Conducted breeding experiments with the Conducted breeding experiments with the garden pea garden pea Pisum sativumPisum sativum
Carefully gathered and documented Carefully gathered and documented mathematical data from his experimentsmathematical data from his experiments
Formulated fundamental laws of heredity in Formulated fundamental laws of heredity in early 1860searly 1860sHad no knowledge of cells or chromosomesHad no knowledge of cells or chromosomesDid not have a microscopeDid not have a microscope
5Fruit and Flower of the
Garden Pea
6Garden Pea TraitsStudied by
Mendel
Mendelian Mendelian InheritanceInheritance 7
Blending InheritanceBlending Inheritance
Theories of inheritance in Mendel’s time:Theories of inheritance in Mendel’s time:
Based on blendingBased on blending
Parents of contrasting appearance produce Parents of contrasting appearance produce offspring of intermediate appearanceoffspring of intermediate appearance
Mendel’s findings were in contrast with thisMendel’s findings were in contrast with this
He formulated the particulate theory of He formulated the particulate theory of inheritanceinheritance
Inheritance involves reshuffling of genes from Inheritance involves reshuffling of genes from generation to generationgeneration to generation
Mendelian Mendelian InheritanceInheritance 8
One-Trait InheritanceOne-Trait Inheritance
Mendel performed cross-breeding Mendel performed cross-breeding experimentsexperiments
Used “true-breeding” (homozygous) plantsUsed “true-breeding” (homozygous) plants
Chose varieties that differed in only one trait Chose varieties that differed in only one trait (monohybrid cross)(monohybrid cross)
Performed reciprocal crossesPerformed reciprocal crosses
Parental generation = PParental generation = P
First filial generation offspring = FFirst filial generation offspring = F11
Second filial generation offspring = FSecond filial generation offspring = F22
Formulated the Law of SegregationFormulated the Law of Segregation
9Mendel’s Monohybrid Crosses:An Example
Mendelian Mendelian InheritanceInheritance 10
Law of SegregationLaw of Segregation
Each individual has a pair of factors (alleles) Each individual has a pair of factors (alleles) for each traitfor each trait
The factors (alleles) segregate (separate) The factors (alleles) segregate (separate) during gamete (sperm & egg) formationduring gamete (sperm & egg) formation
Each gamete contains only one factor (allele) Each gamete contains only one factor (allele) from each pairfrom each pair
Fertilization gives the offspring two factors Fertilization gives the offspring two factors for each traitfor each trait
Mendelian Mendelian InheritanceInheritance 11
Modern Genetics ViewModern Genetics View
Each trait in a pea plant is controlled by two Each trait in a pea plant is controlled by two alleles (alternate forms of a gene)alleles (alternate forms of a gene)
Dominant allele (capital letter) masks the Dominant allele (capital letter) masks the expression of the recessive allele (lower-expression of the recessive allele (lower-case)case)
Alleles occur on a homologous pair of Alleles occur on a homologous pair of chromosomes at a particular gene locuschromosomes at a particular gene locus
Homozygous = identical allelesHomozygous = identical alleles
Heterozygous = different allelesHeterozygous = different alleles
12Homologous Chromosomes
Mendelian Mendelian InheritanceInheritance 13
Genotype Versus PhenotypeGenotype Versus Phenotype
Genotype Genotype
Refers to the two alleles an individual has for Refers to the two alleles an individual has for a specific traita specific trait
If identical, genotype is homozygousIf identical, genotype is homozygous
If different, genotype is heterozygousIf different, genotype is heterozygous
Phenotype Phenotype
Refers to the physical appearance of the Refers to the physical appearance of the individualindividual
Mendelian Mendelian InheritanceInheritance 14
Punnett SquarePunnett Square
Table listing all possible genotypes resulting Table listing all possible genotypes resulting from a crossfrom a cross
All possible sperm genotypes are lined up on All possible sperm genotypes are lined up on one sideone side
All possible egg genotypes are lined up on the All possible egg genotypes are lined up on the other sideother side
Every possible zygote genotypes are placed Every possible zygote genotypes are placed within the squareswithin the squares
15Punnett Square ShowingEarlobe Inheritance
Patterns
Mendelian Mendelian InheritanceInheritance 16
Monohybrid TestcrossMonohybrid Testcross
Individuals with recessive phenotype always Individuals with recessive phenotype always have the homozygous recessive genotypehave the homozygous recessive genotype
However, Individuals with dominant However, Individuals with dominant phenotype have indeterminate genotypephenotype have indeterminate genotype
May be homozygous dominant, orMay be homozygous dominant, or
HeterozygousHeterozygous
Test cross determines genotype of individual Test cross determines genotype of individual having dominant phenotypehaving dominant phenotype
17One-Trait Test CrossUnknown is Homozygous
Dominant
Mendelian Mendelian InheritanceInheritance 18
Two-Trait InheritanceTwo-Trait Inheritance
Dihybrid cross uses true-breeding plants Dihybrid cross uses true-breeding plants differing in two traitsdiffering in two traits
Observed phenotypes among FObserved phenotypes among F22 plants plants
Formulated Law of Independent AssortmentFormulated Law of Independent Assortment
The pair of factors for one trait segregate The pair of factors for one trait segregate independently of the factors for other traitsindependently of the factors for other traits
All possible combinations of factors can occur All possible combinations of factors can occur in the gametesin the gametes
19Two-Trait (Dihybrid) Cross
Meiosis & Sexual Meiosis & Sexual ReproductionReproduction
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Meiosis & Sexual Meiosis & Sexual ReproductionReproduction 21
OutlineOutlineReduction in Chromosome NumberReduction in Chromosome Number
Meiosis OverviewMeiosis Overview Homologous PairsHomologous Pairs
Genetic VariationGenetic Variation Crossing-OverCrossing-Over Independent AssortmentIndependent Assortment FertilizationFertilization
Phases of MeiosisPhases of Meiosis Meiosis IMeiosis I Meiosis IIMeiosis II
Meiosis Compared to MitosisMeiosis Compared to Mitosis
Human Life CycleHuman Life Cycle
Meiosis & Sexual Meiosis & Sexual ReproductionReproduction 22Meiosis:Meiosis:
Halves the Chromosome NumberHalves the Chromosome Number
Special type of cell divisionSpecial type of cell division
Used only for sexual reproductionUsed only for sexual reproduction
Halves the chromosome number prior to Halves the chromosome number prior to fertilizationfertilization
Parents diploidParents diploid
Meiosis produces haploid gametesMeiosis produces haploid gametes
Gametes fuse in fertilization to form diploid Gametes fuse in fertilization to form diploid zygotezygote
Becomes the next diploid generationBecomes the next diploid generation
Meiosis & Sexual Meiosis & Sexual ReproductionReproduction 23Homologous Pairs ofHomologous Pairs of
ChromosomesChromosomes
In diploid body cells chromosomes occur in pairsIn diploid body cells chromosomes occur in pairs
Humans have 23 different types of chromosomesHumans have 23 different types of chromosomes
Diploid cells have two of each typeDiploid cells have two of each type
Chromosomes of the same type are said to be Chromosomes of the same type are said to be homologoushomologous They have the same lengthThey have the same length Their centromeres are positioned in the same placeTheir centromeres are positioned in the same place One came from the father (the paternal homolog) the One came from the father (the paternal homolog) the
other from the mother (the maternal homolog)other from the mother (the maternal homolog) When stained, they show similar banding patternsWhen stained, they show similar banding patterns Because they have genes controlling the same traits Because they have genes controlling the same traits
at the same positionsat the same positions
24Homologous Chromosomes
Meiosis & Sexual Meiosis & Sexual ReproductionReproduction 25Homologous Pairs ofHomologous Pairs of
ChromosomesChromosomes
Homologous chromosomes have genes controlling Homologous chromosomes have genes controlling the same trait at the same positionthe same trait at the same position Each gene occurs in duplicateEach gene occurs in duplicate A maternal copy from the motherA maternal copy from the mother A paternal copy from the fatherA paternal copy from the father
Many genes exist in several variant forms in a large Many genes exist in several variant forms in a large populationpopulation
Homologous copies of a gene may encode identical Homologous copies of a gene may encode identical or differing genetic informationor differing genetic information
The variants that exist for a gene are called allelesThe variants that exist for a gene are called allelesAn individual may have:An individual may have:
Identical alleles for a specific gene on both homologs Identical alleles for a specific gene on both homologs (homozygous for the trait), or(homozygous for the trait), or
A maternal allele that differs from the corresponding A maternal allele that differs from the corresponding paternal allele (heterozygous for the trait)paternal allele (heterozygous for the trait)
26Overview of Meiosis
Meiosis & Sexual Meiosis & Sexual ReproductionReproduction 27Phases of Meiosis I:Phases of Meiosis I:
Prophase I & Metaphase IProphase I & Metaphase I
Meiosis I (reductional division):Meiosis I (reductional division):Prophase IProphase I
Each chromosome internally duplicated Each chromosome internally duplicated (consists of two identical sister chromatids)(consists of two identical sister chromatids)
Homologous chromosomes pair up – synapsisHomologous chromosomes pair up – synapsis
Physically align themselves against each other Physically align themselves against each other end to endend to end
End view would show four chromatids – TetradEnd view would show four chromatids – TetradMetaphase IMetaphase I
Homologous pairs arranged onto the Homologous pairs arranged onto the metaphase platemetaphase plate
Meiosis & Sexual Meiosis & Sexual ReproductionReproduction 28Phases of Meiosis I:Phases of Meiosis I:
Anaphase I & Telophase IAnaphase I & Telophase I
Meiosis I (cont.):Meiosis I (cont.):Anaphase IAnaphase I
Synapsis breaks upSynapsis breaks up Homologous chromosomes separate from one Homologous chromosomes separate from one anotheranother
Homologues move towards opposite polesHomologues move towards opposite poles Each is still an internally duplicate chromosome Each is still an internally duplicate chromosome with two chromatidswith two chromatids
Telophase ITelophase I Daughter cells have one internally duplicate Daughter cells have one internally duplicate chromosome from each homologous pairchromosome from each homologous pair
One (internally duplicate) chromosome of each One (internally duplicate) chromosome of each type (1n, haploid)type (1n, haploid)
Meiosis & Sexual Meiosis & Sexual ReproductionReproduction 29Phases of Meiosis I:Phases of Meiosis I:
Cytokinesis I & InterkinesisCytokinesis I & Interkinesis
Meiosis I (cont.):Meiosis I (cont.):Cytokinesis ICytokinesis I
Two daughter cellsTwo daughter cells Both with one internally duplicate chromosome Both with one internally duplicate chromosome of each typeof each type
HaploidHaploid Meiosis I is reductional (halves chromosome Meiosis I is reductional (halves chromosome number)number)
InterkinesisInterkinesisSimilar to mitotic interphaseSimilar to mitotic interphaseUsually shorterUsually shorterNo replication of DNANo replication of DNA
Meiosis & Sexual Meiosis & Sexual ReproductionReproduction 30Genetic Variation:Genetic Variation:
Crossing OverCrossing Over
Meiosis brings about genetic variation in two key Meiosis brings about genetic variation in two key ways:ways: Crossing-over between homologous chromosomes, Crossing-over between homologous chromosomes,
andand Independent assortment of homologous chromosomesIndependent assortment of homologous chromosomes
1. Crossing Over:1. Crossing Over: Exchange of genetic material between nonsister Exchange of genetic material between nonsister
chromatids during meiosis Ichromatids during meiosis I At synapsis, a nucleoprotein lattice (called the At synapsis, a nucleoprotein lattice (called the
synaptonemal complex) appears between homologuessynaptonemal complex) appears between homologues Holds homologues togetherHolds homologues together Aligns DNA of nonsister chromatids Aligns DNA of nonsister chromatids Allows crossing-over to occurAllows crossing-over to occur
Then homologues separate and are distributed to Then homologues separate and are distributed to different daughter cellsdifferent daughter cells
31Crossing Over
Meiosis & Sexual Meiosis & Sexual ReproductionReproduction 32Genetic Variation:Genetic Variation:
Independent AssortmentIndependent Assortment
2. Independent assortment:2. Independent assortment:When homologues align at the When homologues align at the metaphase plate:metaphase plate:
They separate in a random mannerThey separate in a random manner
The maternal or paternal homologue may The maternal or paternal homologue may be oriented toward either pole of mother be oriented toward either pole of mother cellcell
Causes random mixing of blocks of Causes random mixing of blocks of alleles into gametesalleles into gametes
33Independent Assortment
34Recombination
Meiosis & Sexual Meiosis & Sexual ReproductionReproduction 35Genetic Variation:Genetic Variation:
FertilizationFertilization
When gametes fuse at fertilization:When gametes fuse at fertilization:
Chromosomes donated by the parents are Chromosomes donated by the parents are combinedcombined
In humans, (2^In humans, (2^2323))2 2 = 70,368,744,000,000 = 70,368,744,000,000 chromosomally different zygotes are possiblechromosomally different zygotes are possible
If crossing-over occurs only onceIf crossing-over occurs only once
(4^23)(4^23)22, or 4,951,760,200,000,000,000,000,000,000 , or 4,951,760,200,000,000,000,000,000,000 genetically different zygotes are possiblegenetically different zygotes are possible
Meiosis & Sexual Meiosis & Sexual ReproductionReproduction 36Genetic Variation:Genetic Variation:
SignificanceSignificance
Asexual reproduction produces genetically Asexual reproduction produces genetically identical clonesidentical clones
Sexual reproduction cause novel genetic Sexual reproduction cause novel genetic recombinationsrecombinations
Asexual reproduction is advantageous when Asexual reproduction is advantageous when environment is stableenvironment is stable
However, if environment changes, genetic However, if environment changes, genetic variability introduced by sexual reproduction variability introduced by sexual reproduction may be advantageousmay be advantageous
Meiosis & Sexual Meiosis & Sexual ReproductionReproduction 37Phases of Meiosis II:Phases of Meiosis II:
Similar to MitosisSimilar to Mitosis
Metaphase IIMetaphase II OverviewOverview
UnremarkableUnremarkable Virtually indistinguishable from mitosis of two haploid Virtually indistinguishable from mitosis of two haploid
cellscells Prophase II – Chromosomes condenseProphase II – Chromosomes condense Metaphase II – chromosomes align at metaphase Metaphase II – chromosomes align at metaphase
plateplate Anaphase IIAnaphase II
Centromere dissolvesCentromere dissolves Sister chromatids separate and become daughter Sister chromatids separate and become daughter
chromosomeschromosomes Telophase II and cytokinesis IITelophase II and cytokinesis II
Four haploid cellsFour haploid cells All genetically uniqueAll genetically unique
38Meiosis I & II in Plant Cells
Meiosis & Sexual Meiosis & Sexual ReproductionReproduction 39
Meiosis versus MitosisMeiosis versus Mitosis
MeiosisMeiosis Requires Requires twotwo nuclear nuclear
divisionsdivisions Chromosomes Chromosomes synapsesynapse and and
cross overcross over Centromeres Centromeres survivesurvive
Anaphase IAnaphase I HalvesHalves chromosome chromosome
numbernumber Produces Produces fourfour daughter daughter
nucleinuclei Produces daughter cells Produces daughter cells
genetically genetically differentdifferent from from parent and each otherparent and each other
Used only for Used only for sexualsexual reproductionreproduction
MitosisMitosis Requires Requires oneone nuclear nuclear
divisiondivision Chromosomes Chromosomes do notdo not
synapsesynapse nor nor cross overcross over Centromeres Centromeres dissolvedissolve in in
mitotic anaphasemitotic anaphase PreservesPreserves chromosome chromosome
numbernumber Produces Produces twotwo daughter daughter
nucleinuclei Produces daughter cells Produces daughter cells
genetically genetically identicalidentical to to parent and to each otherparent and to each other
Used for Used for asexualasexual reproduction and reproduction and growthgrowth
40Meiosis Compared to Mitosis
41Meiosis I Compared to Mitosis
42Meiosis II Compared to Mitosis
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http://www.youtube.com/watch?v=Ba9LXKH2ztU&list=PLC02FBC20B99B6B4D
Degrees of Dominance
• Complete dominanceoccurswhenphenotypesoftheheterozygoteanddominanthomozygoteareidentical
• Inincomplete dominance,thephenotypeofF1hybridsissomewherebetweenthephenotypesofthetwoparentalvarieties
• Incodominance,twodominantallelesaffectthephenotypeinseparate,distinguishableways
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Complete dominance
46Complete dominance
Mendelian Mendelian InheritanceInheritance 47
Incomplete DominanceIncomplete Dominance
Heterozygote has phenotype intermediate Heterozygote has phenotype intermediate between that of either homozygotebetween that of either homozygote
Homozygous red has red phenotypeHomozygous red has red phenotype
Homozygous white has white phenotypeHomozygous white has white phenotype
Heterozygote has pink (intermediate) Heterozygote has pink (intermediate) phenotypephenotype
Phenotype reveals genotype without test Phenotype reveals genotype without test crosscross
Fig.14-10-1
Red
P Generation
Gametes
WhiteCRCR CWCW
CR CW
Fig.14-10-2
Red
P Generation
Gametes
WhiteCRCR CWCW
CR CW
F1 GenerationPinkCRCW
CR CWGametes 1/21/2
Fig.14-10-3
Red
P Generation
Gametes
WhiteCRCR CWCW
CR CW
F1 GenerationPinkCRCW
CR CWGametes 1/21/2
F2 Generation
Sperm
Eggs
CR
CR
CW
CW
CRCR CRCW
CRCW CWCW
1/21/2
1/2
1/2
Mendelian Mendelian InheritanceInheritance 51
Multiple Alleles and CodominanceMultiple Alleles and Codominance
Some traits controlled by multiple allelesSome traits controlled by multiple alleles
The gene exists in several allelic forms (but each individual The gene exists in several allelic forms (but each individual only has two)only has two)
ABO blood typesABO blood types
The alleles:The alleles: IIAA = A antigen on red cells = A antigen on red cells IIBB = B antigen on red cells = B antigen on red cells II = Neither A nor B antigens = Neither A nor B antigens
Fig.14-11
IA
IB
i
A
B
none(a) The three alleles for the ABO blood groups and their associated carbohydrates
Allele Carbohydrate
GenotypeRed blood cell
appearancePhenotype
(blood group)
IAIA or IA i A
BIBIB or IB i
IAIB AB
ii O
(b) Blood group genotypes and phenotypes
53Inheritance of Blood Type
Mendelian Mendelian InheritanceInheritance 54
Polygenic InheritancePolygenic Inheritance
Occurs when a trait is governed by two or Occurs when a trait is governed by two or more genes having different allelesmore genes having different alleles
Each dominant allele has a quantitative effect Each dominant allele has a quantitative effect on the phenotypeon the phenotype
These effects are additiveThese effects are additive
Result in continuous variation of phenotypesResult in continuous variation of phenotypes
55Height in Human Beings
56Frequency Distributions in
Polygenic Inheritance
Fig.14-UN2
Degree of dominance
Complete dominanceof one allele
Incomplete dominanceof either allele
Codominance
Description
Heterozygous phenotypesame as that of homo-zygous dominant
Heterozygous phenotypeintermediate betweenthe two homozygousphenotypes
Heterozygotes: Bothphenotypes expressed
Multiple alleles In the whole population,some genes have morethan two alleles
CRCR CRCW CWCW
IAIB
IA , IB , i
ABO blood group alleles
PP Pp
Example
Mendelian Mendelian InheritanceInheritance 58
Red-Green Colorblindness Red-Green Colorblindness ChartChart
Mendelian Mendelian InheritanceInheritance
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Human Genetic DisordersHuman Genetic Disorders