Biology 2250Biology 2250Principles of GeneticsPrinciples of Genetics

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Mendelian GeneticsMendelian Genetics

Topics:Topics:

- - Transmission of DNA during cell divisionTransmission of DNA during cell division

Mitosis and MeiosisMitosis and Meiosis

- Segregation - Segregation

- Sex linkage- Sex linkage

- Inheritance and probability - Inheritance and probability

- Independent Assortment - Independent Assortment

- Mendelian genetics in humans- Mendelian genetics in humans

- Linkage- Linkage

- Gene mapping- Gene mapping

- Tetrad Analysis (mapping in fungi)- Tetrad Analysis (mapping in fungi)

- Extensions to Mendelian Genetics- Extensions to Mendelian Genetics

- Gene mutation- Gene mutation

- Chromosome mutation- Chromosome mutation

- Quantitative and population genetics- Quantitative and population genetics

Basic Concepts of GeneticsBasic Concepts of Genetics

Cell/nuclear divisionCell/nuclear division

Mitosis (somatic tissue): Mitosis (somatic tissue):

identical cellsidentical cells

Meiosis (germ tissue):Meiosis (germ tissue):

gametes (variation)gametes (variation)Behaviour of chromosomes can explain Behaviour of chromosomes can explain the behaviour of genes (segregation and the behaviour of genes (segregation and independent assortment)independent assortment)

Chromosome and DNA Chromosome and DNA ReplicationReplication

1.1. DNA replication results inDNA replication results in

chromosome replicationchromosome replication

2. Nuclear and cell division2. Nuclear and cell division

Meiosis

2n2n nn

2n2n 2n2n

Mitosis

MitosisMitosis

1. 1. one parent cell--------> 2 identicalone parent cell--------> 2 identical

daughter cellsdaughter cells

2. same in all organisms2. same in all organisms

3. simple:3. simple:

(a) each chromosome(a) each chromosome

doubles (identical)doubles (identical)

(b) identical halves separate(b) identical halves separate

MeiosisMeiosis(overview)(overview)

diploid (2N) ---------------> haploid (N) gametesdiploid (2N) ---------------> haploid (N) gametes

CChromosome replication hromosome replication once once --------> 2 nuclear --------> 2 nuclear

divisions (meiosis I, divisions (meiosis I, II)II)

one nucleus -------------------> 4 nucleione nucleus -------------------> 4 nuclei

MitosisMitosis

Meiosis I (reductional division)Meiosis I (reductional division)

Pairing of homologous Pairing of homologous chromosomeschromosomes

Continued Continued

Meiosis II (equational division)Meiosis II (equational division)

4 products 4 products MEIOSIS ANIMATION MEIOSIS ANIMATION (Textbook website)(Textbook website)

MitosisMitosis

Meiosis I (Meiosis I (reductionalreductional division)division)

Pairing of homologous Pairing of homologous chromosomeschromosomes

Meiosis II (Meiosis II (equationalequational division)division)

4 products 4 products

Mitosis MeiosisMitosis Meiosis

somatic cells cells of sexual cyclesomatic cells cells of sexual cycle

one doubling one doublingone doubling one doubling

1 division 1 division 2 cells 2 divisions 2 cells 2 divisions 4 4 cellscells

same amount of DNA ½ amount of DNAsame amount of DNA ½ amount of DNA

Comparison (Fig. 4-24)Comparison (Fig. 4-24)

Mitosis MeiosisMitosis Meiosis

# chrs doesn’t change #chrs. halved# chrs doesn’t change #chrs. halved

No pairing of chr. Synapsis of homologsNo pairing of chr. Synapsis of homologs

Centromeres divide at Centromeres divide at NotNot at anaphase I, at anaphase I,

anaphase but at anaphase IIanaphase but at anaphase II

Conservative VariationConservative Variation

Comparison (continued)Comparison (continued)

Genetic TerminologyGenetic Terminology

Genes: hereditary elementsGenes: hereditary elements

Alleles: forms of a gene: Alleles: forms of a gene: A , a A , a

b, bb, b++

Genotypes Gene pairsGenotypes Gene pairs

Heterozygote: Heterozygote: Aa bbAa bb++

Homozygotes: Homozygotes: AA aa bb bAA aa bb b+ + bb++

Genes on ChromosomesGenes on ChromosomesExpect behaviour of genes to correlate with Expect behaviour of genes to correlate with the behaviour of chromosomes:the behaviour of chromosomes:

genes chromosomesgenes chromosomes

Diploid (2n) Diploid (2n) AA, Aa, aa pairsAA, Aa, aa pairs

Haploid (n) Haploid (n) A one setA one set

AA

aa

BB

bb

AA

aa

bb

BB

Meiosis IMeiosis I

OROR

GenesGenes

Correlation of genes andChromosomes duringmeiosis

AA

aa

Meiosis IIMeiosis II

AA

AA

aa

aa

AA

AA

aa

aa

1/21/2

1/21/2

Mendelian GeneticsMendelian Genetics

GenesGenes - cannot be observed directly - cannot be observed directly

PhenotypesPhenotypes - observed directly - observed directly

** ** inheritance of phenotypes used to infer theinheritance of phenotypes used to infer the

inheritance of genes inheritance of genes

Mendelian GeneticsMendelian Genetics

Requirements:Requirements:

1. Attributes of the phenotype that vary among1. Attributes of the phenotype that vary among

individualsindividuals

2. Phenotypic variation caused by genetic2. Phenotypic variation caused by genetic

differencesdifferences

Mendel’s ExperimentsMendel’s ExperimentsSeven Pea varieties “Seven Pea varieties “TrueTrue Breeding Lines” Breeding Lines”

Character Character PhenotypesPhenotypes

1. seed shape round, wrinkled1. seed shape round, wrinkled

2. seed colour yellow, green2. seed colour yellow, green

3. flowers (pods) axial, terminal3. flowers (pods) axial, terminal

4. pods full, constricted4. pods full, constricted

5. pods yellow, green5. pods yellow, green

6. flowers violet, white6. flowers violet, white

7. stem tall, dwarf7. stem tall, dwarf

Advantages:Advantages:

• easy to groweasy to grow

• matures in a seasonmatures in a season

• self-fertilizingself-fertilizing

• easy to cross fertilize easy to cross fertilize

Cross Pollination

Mendel’sMendel’s Experimental Approach Experimental Approach

• suitable experimental organismsuitable experimental organism

• examine few traits in each experimentexamine few traits in each experiment

• accurate quantitative recordsaccurate quantitative records

• analyzed data------> formulated hypothesesanalyzed data------> formulated hypotheses

Genetical Analysis (pea shape)Genetical Analysis (pea shape)

PParentalarental round X wrinkled round X wrinkled crosscross

FF11 (filial)(filial) round round

selfself

FF22 3/4 round 1/4 wrinkled (3:1) 3/4 round 1/4 wrinkled (3:1)

Hypothesis to explain resultsHypothesis to explain results

1. Hereditary determinants (genes)1. Hereditary determinants (genes)

2. Each adult plant has a gene pair (alleles)2. Each adult plant has a gene pair (alleles)

FF11 plants: one allele dominant phenotype plants: one allele dominant phenotype

one allele recessive phenotypeone allele recessive phenotype

Hypothesis (continued)Hypothesis (continued)3. Alleles of a gene pair segregate equally3. Alleles of a gene pair segregate equally

into the gametesinto the gametes

4. Each gamete has only one allele of a gene4. Each gamete has only one allele of a gene

pairpair

5. Gametes combine at random to form zygote5. Gametes combine at random to form zygote

HypothesisHypothesis

P P AAAA X X aaaa

A a gametesA a gametes

FF11 AaAa

Self Self AaAa X X AaAa

FF22

FF22

Self FSelf F11 AaAa X X AaAa

equalequal

segregationsegregation

1/2 1/2 AA 1/2 1/2 aa

1/2 1/2 AA 1/4 1/4 AAAA 1/4 1/4 AaAa

1/2 1/2 aa 1/4 1/4 AaAa 1/4 1/4 aaaa

FF22 Genotypes 1/4 Genotypes 1/4 AAAA 2/4 2/4AaAa 1/4 1/4 aaaa (1:2:1) (1:2:1)FF22 Phenotypes 3/4 Phenotypes 3/4 A- A- 1/4 1/4 aaaa (3:1) (3:1)

Test of Equal Segregation Test of Equal Segregation Hypothesis Hypothesis (Test Cross)(Test Cross)

round wrinkledround wrinkled

R R rr X X r rr r

rr

1/2 1/2 R R 1/2 1/2 R rR r round round

1/2 1/2 r r 1/2 1/2 r rr r wrinkled wrinkled

1:1 round:wrinkled1:1 round:wrinkled

Mendel’s First LawMendel’s First Law

Equal segregation of two alleles of a gene pairEqual segregation of two alleles of a gene pair

during gamete formationduring gamete formation

Genetic TerminologyGenetic Terminology

Genes:Genes: hereditary elements hereditary elements

Alleles:Alleles: forms of a gene: forms of a gene: A aA a

Heterozygote:Heterozygote: AaAa

Homozygotes:Homozygotes: AA aaAA aa

GenotypesGenotypes

PhenotypesPhenotypes

DominanceDominance AAAA, , AaAa same phenotype same phenotype

different genotypesdifferent genotypes

SummarySummary

Experimental:Experimental:

1. Two pure breeding lines1. Two pure breeding lines

2. Cross --------> F2. Cross --------> F11 hybrid hybrid

3. Self F3. Self F1 1 ------> F------> F22

SummarySummary

Results:Results:

1. F1. F11 one phenotype one phenotype

2. F2. F2 2 3:1 ratio of 2 phenotypes3:1 ratio of 2 phenotypes

SummarySummary

Inference:Inference:

1. Single major gene1. Single major gene

2. dominant phenotype2. dominant phenotype

3. equal segregation3. equal segregation

4. existence of genes inferred4. existence of genes inferred

observed

Expected 3 : 1Expected 3 : 1DominantDominantPhenotypePhenotype

AA x aa ------AA x aa ------ Aa ---- Aa ---- ¾ AA, Aa ¼ aa ¾ AA, Aa ¼ aa

P FP F1 1 FF22

Mendelian genetics Mendelian genetics applies to all organismsapplies to all organisms

Correlation of genes and Correlation of genes and chromosomeschromosomes

Fig. 5-2Fig. 5-2

BbBb

BbBb

Virtual FlyVirtual Fly

F 1F 1

ParentsParents

XX

F 2F 2

3 3 wildtypewildtype

....

1 1 vestigialvestigial

ParentalParental AA AA x x aa aa

F1 HybridF1 Hybrid Aa Aa

AaAa x x AaAa

F2 F2 AA Aa aaAA Aa aa

1 : 2 : 11 : 2 : 1

(¼ ½ ¼ )(¼ ½ ¼ )

A aA a

A AA AaA AA Aa

a Aa aaa Aa aa

F2 ratioF2 ratio

3 : 13 : 1

Principle of SegregationPrinciple of SegregationImplicationsImplications

1.1. Answer questions on inheritanceAnswer questions on inheritance

2.2. Explore other questionsExplore other questions

Examples:Examples:

Shell Colour Variation in the molluscs:Shell Colour Variation in the molluscs:

Scallop: HermaphoditesScallop: Hermaphodites self-fertilization self-fertilization

Parent OffspringParent Offspring

dark 11 0dark 11 0

orange 12 45orange 12 45

dark orangedark orange

1.1.

ExamplesExamples

2. 2. PaternityPaternity

FamiliesFamilies

mother offspring father ?mother offspring father ?

AAAA all all AB BBAB BB

AAAA 26 26AAAA, 24 , 24 AB ABAB AB

AAAA 48 48 AAAA, 2 , 2 AB AA + BB ? AB AA + BB ?

Examples:Examples:

3. 3. Detecting Hybrids:Detecting Hybrids:

Species 1 Species 2Species 1 Species 2

AA x BBAA x BB

Hybrids ABHybrids AB

HybridHybrid tross.tross. edulisedulis

MytilusMytilus

AB AA BB BBAB AA BB BB

Sex-linked InheritanceSex-linked Inheritance

Correlation between inheritanceCorrelation between inheritance

of genes and sexof genes and sex

Sex LinkageSex Linkagereciprocal crossesreciprocal crosses

XX

RoundRound wrinkledwrinkled

XX

wrinkled Roundwrinkled Round

Drosophila melanogasterDrosophila melanogaster(T. H. Morgan)(T. H. Morgan)

XX

Red eyeRed eye(wild)(wild)

White eyeWhite eye(mutant)(mutant)

Red Eye White eyeRed Eye White eyeDrosophilaDrosophila

Cross ACross A

red female red female X X white malewhite male

FF11 all red all red

FF22 red : white red : white

3 : 13 : 1

white all malewhite all male

red 2 : 1 female : malered 2 : 1 female : male

Cross BCross B

white female white female X X red malered male

FF11 females males females males

FF22 females males females males

1 : 1 : 1 : 11 : 1 : 1 : 1

ConclusionsConclusions

1. Sex and eye-colour gene associated1. Sex and eye-colour gene associated

2. Criss-cross inheritance2. Criss-cross inheritance

- daughters inherit father’s phenotype- daughters inherit father’s phenotype

- sons inherit mother’s phenotype- sons inherit mother’s phenotype

InterpretationInterpretation

Sex chromosomes (XSex chromosomes (X,, Y) Y)

Females:Females: 3 pairs of autosomes3 pairs of autosomes

1 pair of sex chromosomes (XX)1 pair of sex chromosomes (XX)

Males:Males: 3 pairs of autosomes3 pairs of autosomes

1 heteromorphic pair (XY)1 heteromorphic pair (XY)

MaleMale

FemaleFemale

autosomes

ChromosomesChromosomes

ExplanationExplanation

Eye colour gene on X chromosomeEye colour gene on X chromosome

Wildtype (red) dominant to whiteWildtype (red) dominant to white

sex chromosomes eye-colour genotypesex chromosomes eye-colour genotype

Females X X WW Ww wwFemales X X WW Ww ww

Males X Y W wMales X Y W wwhitewhiteRedRed

Alternative NotationAlternative Notation

eye-colour genotypeeye-colour genotype

Females wFemales w++ ww++ ww++ ww ww ww

MalesMales w w++ Y wY Y wY

Eye colourEye colour

Cross A Cross BCross A Cross B

FEMALEFEMALE MALEMALE

Fig. 5-8Fig. 5-8

Cross A Cross BCross A Cross B

F 1F 1

F 2F 2

XX XX

Cross BCross B

wwww XX w w++YY

FF11 ww ww++ wY wY

FF22 w Y w Y

w ww wYw ww wY

ww+ + wwww++ w w++YY

1 : 1 : 1 : 11 : 1 : 1 : 1

ConclusionConclusion

1. 1. Eye colour gene associated with sex Eye colour gene associated with sex chromosome (X)chromosome (X)

2. 2. Eye colour not related to sexual function. Eye colour not related to sexual function. Genes on sex chromosomes not related to Genes on sex chromosomes not related to sexual functionsexual function

Sex DeterminationSex Determination

XX XY XXY XOXX XY XXY XO

DrosophilaDrosophila **

Human Human ** **

** Sterile Sterile

klinefelterklinefelter turnerturnerHomogameticHomogametic

HeterogameticHeterogametic

Sex DeterminationSex Determination

Humans:Humans: Y chr. -------> maleness Y chr. -------> maleness

DrosophilaDrosophila:: sex -----> X/A ratio sex -----> X/A ratio

2X/2A = 1.0 ------> 2X/2A = 1.0 ------>

X/2A = 0.5 ------>X/2A = 0.5 ------>

GynandromorphsGynandromorphs

XXw,mw,m

XXw,mw,m X X+,++,+

Nondisjunction duringNondisjunction duringEarly development Early development (mitotic cell division)(mitotic cell division)

Normal Normal mitosismitosis

XXw,mw,m X X+,++,+ XXw,mw,m X X+,++,+

Wild femaleWild female

XXw,mw,m X X+,++,+

XXw,mw,m X X+,++,+

XXw,mw,m XXw,mw,m X X+,++,+

GynandromorphsGynandromorphs

XXw,mw,m XXw,mw,m X X+,++,+ Nondisjunction duringNondisjunction duringEarly developmentEarly development

Other sex determining systemsOther sex determining systems

Birds & MothsBirds & Moths

homogametic heterogametichomogametic heterogametic

ZZ ZWZZ ZW

Sex Linkage (X Y)Sex Linkage (X Y)

- Y chromosome lacks homology- Y chromosome lacks homology

with Xwith X

- alleles on X expressed in males- alleles on X expressed in males

- - hemizygoushemizygous: a gene present in: a gene present in

one copyone copy X Xww Y Y

SSummaryummary

Traits controlled by a single gene:Traits controlled by a single gene:

- - precise Mendelian ratios: 3:1, 1:1, 1:2:1precise Mendelian ratios: 3:1, 1:1, 1:2:1

- ratios due to chromosome segregation- ratios due to chromosome segregation

during meiosisduring meiosis

- sex linked inheritance- sex linked inheritance sex chromosomes sex chromosomes