CH 4 MAPPING EUKARYOTE CHROMOSOMES BY RECOMBINATION
Oct 3/2011 – Pg 129-134, 137-153, 163-165
- Why is a map of gene positions on the chromosomes so important?
1. Gene position is important when building complex genotypes required from experimental
purposes or for commercial applications.
2. Knowing the position occupied by a gene provides a way of zeroing in on its structure and
3. Genes present and arrangement on chromosomes are often slightly different in related
o Ex. Chromosome 2 is split into 2 shorter chromosomes in great apes compared
o Helps us deduce the evolutionary genetic mechanisms that caused these
genomes to diverge.
Chromosome maps - Chromosome maps tells us the arrangement of genes on chromosomes with gene positions
Loci (loci) and the distances between the loci based on some kind of scale.
- There are 2 types of maps currently used by geneticists:
1. Recombination-based maps: map the loci of genes identified by mutant phenotypes
showing single gene inheritance
2. Physical maps show the genes as segments arranged along the long DNA molecule that
constitutes a chromosome.
4.1 Diagnostics of Linkage
Recombination maps - Recombination maps of chromosomes are assembled 2 or 3 genes at a time using linkage
Linked genes - When 2 genes are linked, the loci of those genes are on the same chromosome and the alleles
on any one homolog are physically joined by the DNA between them.
frequency to recognize
linkage - Bateson and Punnett found a standard self of dihybrid F1, the F2 didn’t show 9:3:3:1 ratio.
- Certain combinations of alleles showed up more often than expected, as though they were
physically attached in some way. nd
- Morgan also found a deviation from Mendel’s 2 Law by looking at genes that affected eye
color (pr, purple, and pr+,red) and other gene affected wing length (vg, vestigial, and vg+,
o Pr/pr .vg/vg X pr+/pr+.vg+/vg+ pr+/pr.vg+/vg F1 dihybrid
o Testcross: pr+/pr,vg+vg X pr/pr.vg/vg (tester male)
o Morgan found 1339 pr+.vg+, 1195 pr.vg, 151 pr+.vg, and 154 pr.vg+
o These deviate from Mendelian prediction of 1:1:1:1
o The first 2 allele combinations are in the great majority, clearly indicating that they
are associated or linked.
- The recombinants are approximately equal in frequency or 10.7% frequency overall.
- In the dihybrid, the alleleic conformation must have been:
- Shows the tendency of linked alleles to be inherited as a package
- In another testcross Morgan made, he found the F1 dihybrid must have been:
- Dihybrid testcrosses follow the general pattern:
1. 2 equally frequent nonrecombinant classes totalling in excess of 50%
2. 2 equally frequent recombinant classes totalling less than 50% produced by 2 genes close
together on a chromosome – linked. How Crossovers produce - When homologous chromosomes pair at meiosis, the chromosomes occasionally break and
recombinants for linked exchange parts in a process called crossing over forming 2 new combinations called crossover
Crossing over - At meiosis, when duplicated homologous chromosomes pair with each other (2 dyads unite as a
bivalent), a cross-shaped structure called a chiasma forms in between the 2 nonsister
Linkage Symbolism and
- Linked genes may be present in one of two basic conformations.
Cis conformation 1. Cis conformation: when alleles are present on the same homolog
Trans conformation 2. Trans conformation: when alleles are on different homologs.
- Note the following conventions:
- Alleles on the same homolog have no punctuation between them
- A slash symbolically separates the two homologs
- Alleles are always written in the same order on each homolog
- Genes known to be on different chromosomes (unlinked genes) are separated by a semicolon
- Genes of unknown linkage are separated by a dot.
4.2 Mapping By
Recombinant Frequency - The further apart the genes are, the more likely that a crossover will take place and the higher
the proportion of recombinant products will be.
- The proportion of recombinants is a clue to the distance separating two gene loci on a
- A single crossover generates two reciprocal recombinant products which explains why the
reciprocal recombinant classes are generally approximately equal in frequency and the two
parental nonrecombinant types also must be equal in frequency.
- Sturtevant suggested that we can use the percentage of recombinants as a quantitative index of
the linear distance between two genes on a genetic map , or linkage map.
- In other meiotic divisions, there are no crossovers between genes, no recombinants result from
- The greater the chance of crossovers in the region between the genes, the greater the proportion
of recombinants that would be produced.
- The frequency of recombinants can be used to determine the map distance between the genes.
Genetic Map Unit - Genetic map unit (m.u.) is the distance between genes for which one product of meiosis in
100 is recombinant.
Recombinant Frequency o Ex. A recombinant frequency (RF) of 10.7% obtained by Morgan is defined as 10.7
mu. 1 map unit = recombinant frequency of 1%.
- His analysis strongly suggested that genes are arranged in some linear order, making map
o Ex. If the genetic distance between pr and vg loci in Drosophila is approximately 11
map units, we know that there will be 11% recombinants in the progeny from a
testcross of a female dihybrid heterozygote in cis conformation.
o These recombinants will consist of 2 reciprocal recombinants of equal frequency:
5.5% of each recombinant.
o 89% will be nonrecombianant with 44.5% each frequency.
Three Point Testcross
- Crossing a trihybrid with a triply recessive tester.
- Goal is to deduce whether 3 genes are linked and deduce their order and map distances between
o Ex. The mutant alleles are v (vermillion), cv (crossveinless), and ct (cut wing edges)
o Perform the following crosses: v+/v+.cv/cv.ct/ct x v/v. cv+/cv+. Ct+/ct+ o We obtain v/v+.cv/cv+.ct/ct and cross with a tester male
o From any trihybrid, only 8 gamete genotypes are possible and we can determine
which genotypes are recombinant.
o If the RF values are less than 50%, then the loci are linked.
- Linkage maps merely map the loci in releation to one another, with the use of standard map
units. We don’t know where the loci are on a chromosome or which specific chromosome