Linkage & crossing over

Linkage and Crossing Over AGB121 Dept. Of Animal Genetics & Breeding Veterinary College, Hebbal, Bangalore

Terminologies to Know • • • • • •

Linkage Crossing over Chiasma Recombination Gene mapping Interference

Dr R Jayashree, Asst. Professor(AGB)

Linkage • 2 genes on separate chromosomes assort independently at meiosis.

• 2 genes far apart on the same chromosome can also assort independently at meiosis.

• 2 genes close together on the same chromosome pair do not assort independently at meiosis.

• A recombination frequency << 50% between 2 genes shows that they are linked.

Dr R Jayashree, Asst. Professor(AGB)

Gene linkage was explained through fruit flies â&#x20AC;˘ Morgan found that linked traits are on the same chromosome. â&#x20AC;˘ Chromosomes, not genes, assort independently during meiosis.

Mutant

Wild type

Dr R Jayashree, Asst. Professor(AGB)

Testcross to show that two genes are linked

When there is crossover Dr R Jayashree, Asst. Professor(AGB)

Complete Linkage: P1 Cross bwhv  bw hv P1 :    bwhv bw hv Brown eyes –bw Red eyes –bw+ Thin veins- hv+ Heavy veins-hv

In complete linkage only parental gametes form

bwhv  F1:  bw hv

Dr R Jayashree, Asst. Professor(AGB)

Dr R Jayashree, Asst. Professor(AGB)

Crossing-over

Crossing-over is the reciprocal exchange of homologous chromatid segments of non-sister chromatid, involving the breaking and rejoining of DNA. Dr R Jayashree, Asst. Professor(AGB)

Mechanism of Crossing-Over

Dr R Jayashree, Asst. Professor(AGB)

The relationship between crossing-over and map distance Dr R Jayashree, Asst. Professor(AGB)

Chiasma

A chiasma (plural chiasmata) represents the physical manifestation of a chromosomal crossover event Dr R Jayashree, Asst. Professor(AGB)

Recombination

Recombinant gametes

Crossing-over is the event leading to genetic recombination between linked genes in both prokaryotes and eukaryotes.

Dr R Jayashree, Asst. Professor(AGB)

No LinkageIndependent Assortment

Linkage without Recombination

Linkage with Recombination

Creates only parental (noncrossover) gametes.

Dr R Jayashree, Asst. Professor(AGB)

creates parental gametes and recombinant (crossover) gametes.

Single Crossovers: Non-crossover (Parental) and Crossover (Recombinant) Gametes

Dr R Jayashree, Asst. Professor(AGB)

Progeny of single and double crossovers when two genes are linked

Dr R Jayashree, Asst. Professor(AGB)

Double Crossovers

Dr R Jayashree, Asst. Professor(AGB)

Types of Double Exchanges: Not All are Detectable

Dr R Jayashree, Asst. Professor(AGB)

Consequences of a double crossover in a triple heterozygote for three linked genes

Dr R Jayashree, Asst. Professor(AGB)

Rearrangement of the three genes to p j r

Dr R Jayashree, Asst. Professor(AGB)

Crosses of Two X-linked genes

Expect only parental types if no crossing over occurs Dr R Jayashree, Asst. Professor(AGB)

Dr R Jayashree, Asst. Professor(AGB)

Linkage Maps Linkage maps/ Chromosome maps show the relative locations of genes on a chromosome determined from recombination rate

• Interlocus distance is proportional to the degree of crossing over between.

– Little or no crossing over in close genes.

– Frequent, even multiple crossovers between distant genes Dr R Jayashree, Asst. Professor(AGB)

Morgan and Sturtevant (1913) used recombination frequencies to make a genetic map.

 A 1% crossover rate is a genetic distance of 1 map unit (mu).  A map unit is also called a centimorgan (cM).

 1% crossing over = 1 map unit (in humans 1mu = 1 million bp)

Dr R Jayashree, Asst. Professor(AGB)

Linkage maps estimate distances between genes â&#x20AC;˘ The closer together two genes are, the more likely they will be inherited together. â&#x20AC;˘ Cross-over frequencies are related to distances between genes.The higher the frequency, further the genes are apart

Dr R Jayashree, Asst. Professor(AGB)

Dr R Jayashree, Asst. Professor(AGB)

Three-point mapping, showing the testcross used and the resultant progeny

Dr R Jayashree, Asst. Professor(AGB)

Rewritten form of the testcross and testcross progeny based on the actual gene order p j r

c/o between p and j = 52+46+4+2/500 = 104/500 = 20.8%

c/o between j and r = 22+22+4+2/500 = 50/500 = 10% Dr R Jayashree, Asst. Professor(AGB)

Interference • Crossovers in adjacent chromosome regions are usually not independent. This interaction is called interference.

• A crossover in one region usually decreases the probability of a crossover in an adjacent region.

 observed # of double recombinan ts  Interference(I)  1    expected # of double recombinan ts   Dr R Jayashree, Asst. Professor(AGB)

Genetic Map of Drosophila melanogaster

Dr R Jayashree, Asst. Professor(AGB)

Morganâ&#x20AC;&#x2122;s experimental crosses of white-eye and miniature-wing variants of Drosophila melanogaster

Dr R Jayashree, Asst. Professor(AGB)

In an individual heterozygous at two loci, there are two arrangements of alleles:

1. The cis (coupling) arrangement has both wild-type alleles on one homologous chromosome, and both mutants on the other (e.g., w+ m+ and ww+ m). m+ w

m

2. The trans arrangement has one mutant and one wild type on each homolog (e.g., w+ m and w m+). w+

m

w

m+

Dr R Jayashree, Asst. Professor(AGB)

• Cross-over frequencies can be converted into map units.

• Ex: A 5% cross-over frequency equals 5 map units.

–gene A and gene B cross over 6.0 percent of the time

– gene B and gene C cross over 12.5 percent of the time

– gene A and gene C cross over 18.5 percent of the time