Lecture 6: Lecture 6: Feb. 13, 2006Feb. 13, 2006

Linkage and Linkage and crossoverscrossovers

Genetic mappingGenetic mappingThree-point crossThree-point cross

Two genes on nonhomologous chromosomes Two genes on nonhomologous chromosomes (unlinked genes) yield 4 kinds of gametes in equal (unlinked genes) yield 4 kinds of gametes in equal

proportionsproportions

Two genes on the same Two genes on the same chromosome (linked genes) can chromosome (linked genes) can

exist in two possible configurations exist in two possible configurations

Recombinant gametes are created by Recombinant gametes are created by recombination (crossing over) between recombination (crossing over) between

homologous chromosomeshomologous chromosomes

The genetic distance between 2 genes The genetic distance between 2 genes is expressed in map units (% is expressed in map units (%

recombination)recombination)

Crossing over must occur between 2 Crossing over must occur between 2 genes genes

to produce recombinant gametesto produce recombinant gametes

Here the crossing over did not occur between the 2 genes. As a result, all four gametes are nonrecombinant (parental combinations)

Two recombinations between a pair of Two recombinations between a pair of genes genes

result in 4 nonrecombinant gametesresult in 4 nonrecombinant gametes

Additivity of map distances can be Additivity of map distances can be used used

for creating genetic mapsfor creating genetic maps

For the 3 genes rb, y and cv with the map distances between rb-y and rb-cv shown in (A) above, there are 2 possible genetic maps.

The distance y-cv of 1.3 map units yields map (B) and y-cv distance of 13.7 map units yields map (C).

A genetic map of chromosome 10 of A genetic map of chromosome 10 of corncorn

Use of cytologically marked chromosomes shows Use of cytologically marked chromosomes shows that crossing over involves breakage that crossing over involves breakage

and reunion of chromosomesand reunion of chromosomes

Unusual inheritance of X-linked genes in crosses Unusual inheritance of X-linked genes in crosses involving female Drosophila with attached X-involving female Drosophila with attached X-

chromosomeschromosomes

The gametes generated The gametes generated by the 3 kinds of double by the 3 kinds of double

crossoverscrossovers

Consequences of a 2-strand Consequences of a 2-strand double crossover in a cross double crossover in a cross

involving 3 genesinvolving 3 genes

A mapping function corrects for the loss of A mapping function corrects for the loss of detectable recombinants due to multiple detectable recombinants due to multiple

crossoverscrossovers