GENE LINKAGE,

CROSSING OVER

& CHROMOSOME MAPPING

Molecular Basis for Mendel’s Postulates

Unit factors in pairs

A pair of genes controls a trait

Dominance / Recessiveness

For different alleles, one will be expressed over

the other

Segregation

Chromosomes separate during meiosis

(each gamete receives 1 copy of a gene)

Independent Assortment

Genes for different traits are assorted

independently of each other during gamete

formation

Linkage

Genes contained on the same chromosome

are “linked”

Do not assort independently

Form linkage groups

Complete linkage = traits always associated together

Why would linked genes NOT assort

together?

Crossing Over

A random exchange of DNA between two

non-sister chromatids of homologous

chromosomes

Occurs during…?

Results in recombination of genetic material

Prevalence of recombination is dependent

on the distance between linked genes

Assortment, Linkage & Crossing Over

Fig. 7-1

Assortment, Linkage & Crossing Over

Fig. 7-1

Assortment, Linkage & Crossing Over

Fig. 8-1

Which of Mendel’s postulates is

violated when genes are linked?

A) Unit factors exist in pairs

B) Law of dominance

C) Law of segregation

D) Law of independent assortment

Single Cross Over (SCO) Events

Occurrence “outside”

linked genes…

Not phenotypically

detectable

Occurrence between

linked genes…

Parental gametes and

recombined gametes

Max. 50% gametes

show recombination

Fig. 8-5

Multiple Cross Over Events

2 or more crossovers between non-sister

chromatids

Ex., DCO’s (double crossovers)

Need 3 linked gene pairs to observe

Fig. 8-7

Which type of crossover event is most

common between two linked genes?

A) Single crossover

B) Double crossover

C) Triple crossover

Chromosome Mapping

Can use the knowledge of gene linkage and

crossover to…

Determine the order of genes on the

chromosome

Determine how far apart genes are on the

chromosome

Construct a map of the chromosome

Estimating Gene Distance

Based on observed crossover frequencies

between two genes

> frequency = > distance

Morgan & Sturtevant

X-linked mutations in Drosophila

Yellow body (y)

White eye (w)

Miniature wings (m)

Yellow, White, Miniature Experiments

Fig. 7-3

Yellow, White, Miniature Experiments

Fig. 7-3

Yellow, White, Miniature Experiments

Recombination frequencies

yellow, white 0.5%

white, miniature 34.5%

yellow, miniature 35.4%

Chromosome map

Distance in map units (mu)

Fig. 7-4

Practice – 2-point mapping

Three-Point Mapping

Three requirements

Organism producing crossover gametes must be

heterozygous for all traits considered

Phenotype must reflect genotype of parental

gametes (cross hetero x homozygous recessive)

Need enough progeny to recover representative

samples of all crossover classes

Practice – 3-point mapping

Phenotype Number

Ly Sb br 404

+ + + 422

Ly + + 18

+ Sb br 16

Ly + br 75

+ Sb + 59

Ly Sb + 4

+ + br 2

Total 1000

Practice – 3-point mapping

• What is the arrangement of alleles

on heterozygous parent?

– Look for most frequent phenotypes

– Not necessarily the right order, but the

right grouping of alleles

Practice – 3-point mapping

• Find correct sequence

– Look for which grouping yields observed

double crossover phenotypes

– Consider one set at a time until you find

the right one

– E.g.,

Practice – 3-point mapping

• Find correct sequence

– Previous wasn’t right. Which of these is?

Practice – 3-point mapping

• Now, find distances between loci

Practice – 3-point mapping

• Put it all together into a map

– Remember, the original problem gave you

assigned locations for two of the loci

already

Linkage & Mapping in Haploid Organisms

Haploid organisms as a genetic model:

Easy & cheap to maintain

Phenotypic expression is simplified

Mating of two strains produces a diploid

zygote, which subsequently undergoes

meiosis to produce tetrads

Tetrad analysis yields information about

frequency of crossing over

Linkage & Mapping in Haploid Organisms

Mapping the centromere

Where is a gene, relative to the centromere of its

chromosome?

Where is a second gene, relative to the

centromere of the same chromosome?

Together, you can deduce the relative location of

the genes with regards to each other

d = (1/2) * crossing over frequency

i.e., d = (1/2) * 30/150 = 0.1 map units

Modern Chromosome Mapping

DNA sequencing

Can produce a physical map of a chromosome

Base pair distance

DNA markers

Restriction fragment length polymorphisms

(RFLPs)

Microsatellites

Single-nucleotide polymorphisms

Sister Chromatid Exchanges

• Exchange of genetic

material between sister

chromatids during

mitosis

• Use BrdU uptake as a

DNA synthesis marker

– Study cells after two

rounds of mitosis to

observe SCE

– *BrdU = bromodeoxyuridine,

a thymidine analog