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Copyright © 2009 Pearson Education, Inc.

Chapter 5 Lecture

Concepts of Genetics Tenth Edition

Chromosome Mapping in Eukaryotes

Linked Genes

Mendel’s experiments

Linked Genes

Gene location

Genes on separate chromosomes

Genes on the same chromosome

Gamete types

Equal numbers of all possible allele combinations

More parental combinations than recombinant combinations

Independent Assortment vs. Gene Linkage

Example from Drosophila Red eyes, x Pink eyes Beige body Ebony body RRBB rrbb F1: Red eyes, Beige body

RrBb


Testcross: cross to individual of known genotype

F1:Red eyes X Pink eyes Beige body Ebony body RrBb rrbb


F2 phenotype Number of Offspring

Expected for Unlinked Genes

Red eyes Beige body

398 250

Pink eyes Ebony body

382 250

Red eyes Ebony body

108 250

Pink eyes Beige body

112 250


F1:Red eyes X Pink eyes Beige body Ebony body RrBb rrbb

RrBb Red Beige

Rrbb Red Ebony

rrBb Pink Beige

rrbb Pink Ebony

rb

RB Rb rB rb


Coupling or Cis Configuration

If genes are linked:

Red eyes, x Pink eyes Beige body Ebony body

R B R B

r b r b

F1: Red eyes, Beige body R B

r b

F1: Red eyes, Beige body


R B r b

R B

r b r B

X

R b

Four types of gametes are produced Parental Recombinant


F1:Red eyes X Pink eyes Beige body Ebony body R B r b r b r b

R B

r b R b r B r b

R B r b R b r B

r b r b r b r b


F2 phenotype Number of Offspring

Chromosome arrangement

Red eyes Beige body

398 RB//rb Parental

Pink eyes Ebony body

382 rb//rb Parental

Red eyes Ebony body

108 Rb//rb Recombinant

Pink eyes Beige body

112 rB//rb Recombinant

Genetic Map Units

1% recombination = 1 map unit = 1 centimorgan

108 + 112 x 100 = 22% 1000 These genes are located 22 map units

apart on the same chromosome.


5.1 Genes Linked on the Same Chromosome Segregate Together


Genes on the same chromosome demonstrate linkage in genetic crosses. In the absence of recombination all genes on a chromosome would show complete linkage, or non-independent assortment. linkage is 100%, recombination rate is 0%.


5.2 Crossing Over Serves as the Basis of Determining the Distance between Genes in Chromosome Mapping


Morgan and Crossing Over

Alfred Sturtevant demonstrated that the frequency of recombination could be used to order genes along a chromosome

Alfred Henry Sturtevant 1891-1970


Sturtevant noticed that the recombination rate between loci on the X chromosome varied. He produced the first genetic map where the distance between two loci = % recombination is expressed in terms of map units or centimorgans (cM). 1% recombination = 1 m.u. = 1 cM

Yellow white Miniature


Single Crossovers


5.3 Determining the Gene Sequence in Mapping Requires the Analysis of Multiple Crossovers

Ordering three genes (three-point mapping) must take into account that multiple crossovers can occur per chromosome

Double crossover has a lower rate than single crossover in general.

Single crossover between A and B: 10% Single crossover between B and C: 20% Double crossover: 10% x 20%=2% if the two events are independent.


A three-point mapping cross involving the yellow, white, and echinus genes in Drosophila melanogaster

Calculate distance between loci. Map distance between two genes = % of crossovers involving the two genes

Between y and w: 1.5%+0.06%=1.56% Between y and ec: 4%+0.06%=4.06%

F2 pheno=gamete genotype Obs. flies % w+ y+ ec+ 4685 94.4% w y ec 4759 w+ y+ ec 207 4.0% w y ec+ 193 w+ y ec 70 1.5% w y+ ec+ 80 w+ y ec+ 3 0.06% w y+ ec 3


A Mapping Problem in Maize


5.4 As the Distance Between Two Genes Increases, the Results of Mapping Experiments Become Less Accurate


When the distance between two genes is large, the % recombinants does not directly reflect map distance. That is, mapping becomes inaccurate because 2, 3, and more exchanges go undetected.


5.5 Drosophila Genes Have Been Extensively Mapped

Actually, the Drosophila genome has been sequenced


5.6 Lod Score Analysis and Somatic Cell Hybridization Were Historically Important in Creating Human Chromosome Maps


5.7 Chromosome Mapping is Now Possible Using DNA Markers and Annotated Computer Databases


5.8 Crossing Over Involves a Physical Exchange between Chromatids


Recombination Occurs between Mitotic Chromosomes


5.9 Exchanges Also Occur between Sister Chromatids

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