Chapter 5Chapter 5

Chromosome Mapping Chromosome Mapping in Eukaryotesin Eukaryotes

How can scientists determine where genes are How can scientists determine where genes are

located on the chromosome?located on the chromosome?

5.15.1 Genes Linked on the Same Chromosome Genes Linked on the Same Chromosome Segregate TogetherSegregate Together

5.25.2 Crossing Over Serves as the Basis of Crossing Over Serves as the Basis of Determining the Distance between Genes Determining the Distance between Genes during Chromosome Mappingduring Chromosome Mapping

5.35.3 Determining the Gene Sequence during Determining the Gene Sequence during Mapping Relies on the Analysis of Multiple Mapping Relies on the Analysis of Multiple CrossoversCrossovers

Chapter 5Chapter 5Chromosome MappingChromosome Mapping

5.65.6 DrosophilaDrosophila Genes Have Been Extensively Genes Have Been Extensively MappedMapped

5.75.7 Crossing Over Involves a Physical Exchange Crossing Over Involves a Physical Exchange between Chromatidsbetween Chromatids

5.85.8 Recombination Occurs between Mitotic Recombination Occurs between Mitotic ChromosomesChromosomes

5.95.9 Exchanges Also Occur between Sister Exchanges Also Occur between Sister ChromatidsChromatids

Chapter 5Chapter 5Chromosome MappingChromosome Mapping

Morgan & Morgan & DrosophilaDrosophila

Thomas Hunt MorganThomas Hunt Morgan - first to associate a - first to associate a specific gene with specific chromosome in specific gene with specific chromosome in early 20th centuryearly 20th century

Drosophila melanogasterDrosophila melanogaster, a fruit fly species , a fruit fly species that eats fungi on fruitthat eats fungi on fruit Prolific breeders - generation time of 2 weeksProlific breeders - generation time of 2 weeks

Linked GenesLinked Genes

Linked genes Linked genes - g- genes located on the same enes located on the same chromosomechromosome

Tend to be inherited together Tend to be inherited together Results of crosses with linked genes deviate Results of crosses with linked genes deviate

from those expected according to from those expected according to independent assortmentindependent assortment

Recall: Independent AssortmentRecall: Independent Assortment

Morgan & LinkageMorgan & Linkage

Inheritance of characters for body color and Inheritance of characters for body color and wing size in wing size in DrosophilaDrosophila Wild-type body color is gray (Wild-type body color is gray (bb++) and the mutant ) and the mutant

black (black (bb)) Wild-type wing size is normal (Wild-type wing size is normal (vgvg++) and mutant ) and mutant

vestigial (vestigial (vgvg))

Morgan crossed FMorgan crossed F11 heterozygous females heterozygous females

((bb++bvgbvg++vgvg) with homozygous recessive males ) with homozygous recessive males ((bbvgvgbbvgvg))

Should produce 4 phenotypes in a 1:1:1:1 Should produce 4 phenotypes in a 1:1:1:1 ratioratio

However, large number of wild-type (gray-However, large number of wild-type (gray-normal) and double-mutant (black-vestigial) normal) and double-mutant (black-vestigial) flies among the offspringflies among the offspring Correspond with those of the parentsCorrespond with those of the parents

Reasoned that body color & wing shape are Reasoned that body color & wing shape are usu. inherited together b/c their genes are on usu. inherited together b/c their genes are on same chromosomesame chromosome

Other two phenotypes (gray-vestigial and Other two phenotypes (gray-vestigial and black-normal) were fewer than expected from black-normal) were fewer than expected from independent assortmentindependent assortment

New phenotypic variations must be the result New phenotypic variations must be the result of crossing over (of crossing over (recombinationrecombination))

Complete LinkageComplete Linkage

Complete linkageComplete linkage - only parental - only parental (noncrossover) gametes are produced. (noncrossover) gametes are produced.

If crossing over between two linked genes If crossing over between two linked genes occurs between two nonsister chromatids, occurs between two nonsister chromatids, both parental and recombinant (crossover) both parental and recombinant (crossover) gametes are producedgametes are produced

Linkage GroupsLinkage Groups

Genes on the same chromosome are part of Genes on the same chromosome are part of a linkage group. a linkage group.

The number of linkage groups should The number of linkage groups should correspond to the haploid number of correspond to the haploid number of chromosomes.chromosomes.

RecombinationRecombination

Random orientation of homologous Random orientation of homologous chromosomes at metaphase 1 allows for chromosomes at metaphase 1 allows for cross overcross over

Linked genes tend to move together through Linked genes tend to move together through meiosis and fertilizationmeiosis and fertilization

Under normal Mendelian genetic rules, do not Under normal Mendelian genetic rules, do not expect linked genes to recombine into expect linked genes to recombine into assortments of alleles not found in parentsassortments of alleles not found in parents

Morgan’s ResultsMorgan’s Results

If wing shape & body color genes were If wing shape & body color genes were linked, we would expect Flinked, we would expect F11 offspring to offspring to produce only 2 types of gametes, produce only 2 types of gametes, bb+ + vgvg++ and and bvgbvg

Results of Morgan’s testcross didn’t conform Results of Morgan’s testcross didn’t conform to either independent assortment or complete to either independent assortment or complete linkagelinkage Independent assort: should produce a 1:1:1:1 Independent assort: should produce a 1:1:1:1

ratioratio Completely linked: should see a 1:1:0:0 ratio (only Completely linked: should see a 1:1:0:0 ratio (only

parental phenotypes)parental phenotypes)

Most of offspring had parental phenotypes, Most of offspring had parental phenotypes, suggesting linkage suggesting linkage

However, 17% of the flies were However, 17% of the flies were recombinants, suggesting incomplete linkagerecombinants, suggesting incomplete linkage

Morgan’s ProposalMorgan’s Proposal

Some mechanism occasionally exchanged Some mechanism occasionally exchanged segments b/t homologous chromsegments b/t homologous chrom Switched alleles b/t homologous chromSwitched alleles b/t homologous chrom

Crossing over results in more types of Crossing over results in more types of gametes than by Mendelian rules alonegametes than by Mendelian rules alone

Occasional production of recombinant Occasional production of recombinant gametes during prophase I accounts for gametes during prophase I accounts for occurrence of recombinant phenotypes in occurrence of recombinant phenotypes in Morgan’s testcrossMorgan’s testcross

The percentage of offspring resulting from The percentage of offspring resulting from recombinant gametes depends on the recombinant gametes depends on the distance between the two genes on the distance between the two genes on the chromosome.chromosome.

Review: RecombinationReview: Recombination

SynapsedSynapsed chromosomes in chromosomes in meiosis wrap around each other meiosis wrap around each other to create to create chiasmatachiasmata that are that are points of genetic exchange.points of genetic exchange.

Rate of RecombinationRate of Recombination

Two genes located relatively close to each Two genes located relatively close to each other along a chromosome are less likely to other along a chromosome are less likely to have a chiasma form between them, and it is have a chiasma form between them, and it is less likely that crossing over will occur.less likely that crossing over will occur.

SO…SO… Frequency of recombination reflects distance Frequency of recombination reflects distance

b/t genesb/t genes Farther apart 2 genes are, the higher the Farther apart 2 genes are, the higher the

recombination frequencyrecombination frequency

Farther apart 2 genes are, higher Farther apart 2 genes are, higher recombination frequencyrecombination frequency

We can use the recombination frequency to We can use the recombination frequency to calculate the RELATIVE distance of 2 genescalculate the RELATIVE distance of 2 genes

Calculating Gene DistancesCalculating Gene Distances

Interlocus distanceInterlocus distance - degree of - degree of crossing over between any two loci on a crossing over between any two loci on a single chromosome is proportional to the single chromosome is proportional to the distance between themdistance between them

Calculating Gene DistancesCalculating Gene Distances

One map unit = 1% recombination between One map unit = 1% recombination between two genes on a chromosome. two genes on a chromosome.

Map units are often called centimorgans (cM) Map units are often called centimorgans (cM) and are and are relativerelative distances, not exact ones. distances, not exact ones.

Alfred SturtevantAlfred Sturtevant

Recombination frequencies from Recombination frequencies from DrosophilaDrosophila crosses to map relative position of genes crosses to map relative position of genes along chromosomes - along chromosomes - linkage maplinkage map

Pupil of T.H. MorganPupil of T.H. Morgan

Sturtevant’s FindingsSturtevant’s Findings

Relative position of three fruit fly genes, body Relative position of three fruit fly genes, body color (color (bb), wing size (), wing size (vgvg), and eye color (), and eye color (cncn)) Recombination frequency b/t Recombination frequency b/t cncn and and bb is 9%. is 9%. Recombination frequency b/t Recombination frequency b/t cncn and and vgvg is 9.5%. is 9.5%. Recombination frequency b/t Recombination frequency b/t bb and and vgvg is 17%. is 17%. Only possible arrangement of 3 genes?Only possible arrangement of 3 genes? Eye color (Eye color (cncn) between the other 2) between the other 2

Recombination frequencies in mapping Recombination frequencies in mapping example not quite additive: 9% (example not quite additive: 9% (bb--cncn) + 9.5% ) + 9.5% ((cncn--vgvg) > 17% () > 17% (bb--vgvg))

Single CrossoversSingle Crossovers

A single A single crossover crossover (SCO) alters (SCO) alters linkage between linkage between two genes two genes ONLY if the ONLY if the crossover crossover occurs between occurs between those two genesthose two genes

Determining the gene sequence during Determining the gene sequence during mapping relies on the analysis of multiple mapping relies on the analysis of multiple crossoverscrossovers

However, multiple crossovers MAY cancel However, multiple crossovers MAY cancel each other outeach other out reduces observed number of recombinant reduces observed number of recombinant

offspringoffspring Genes father apart more likely to experience Genes father apart more likely to experience

multiple crossing over eventsmultiple crossing over events

Double Crossovers Double Crossovers

Double crossovers (DCOs) can be used to Double crossovers (DCOs) can be used to determine the order of three linked genesdetermine the order of three linked genes

To study double exchanges, three pairs of To study double exchanges, three pairs of genes must be investigated, each genes must be investigated, each heterozygous for two allelesheterozygous for two alleles

The expected frequency of double-crossover The expected frequency of double-crossover gametes is much lower than that of either gametes is much lower than that of either single-crossover gamete class.single-crossover gamete class.

As the distance between two genes As the distance between two genes increases, mapping experiments become increases, mapping experiments become more inaccuratemore inaccurate

When two genes are close together, the When two genes are close together, the accuracy of mapping is high. As the distance accuracy of mapping is high. As the distance between them increases, the accuracy of between them increases, the accuracy of mapping decreases.mapping decreases.

Chromosome BandingChromosome Banding

Chromosome Chromosome banding techniques banding techniques allow identification allow identification of the exact origin of of the exact origin of the crossoverthe crossover

In this way, it is In this way, it is possible to compile possible to compile gene maps of gene maps of human human chromosomeschromosomes