Mendelian Inheritance 1Outline Blending Inheritance Monohybrid Cross Law of Segregation Modern...

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


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


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


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


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


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


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


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


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


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


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)


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


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


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


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

43

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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


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


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


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


Red-Green Colorblindness Red-Green Colorblindness ChartChart

Mendelian Mendelian InheritanceInheritance

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Human Genetic DisordersHuman Genetic Disorders

Mendelian Inheritance 1Outline Blending Inheritance Monohybrid Cross Law of Segregation Modern...

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