Transcript Gene Linkage

Linkage and Gene
Mapping
• Locus = physical location of a gene on a chromosome
• Homologous pairs of chromosomes often contain
alternative forms of a given gene = alleles
• Different alleles of the same gene segregate at meiosis I
• Alleles of different genes assort independently in gametes
• Genes on the same chromosome exhibit linkage: inherited
together
• Linked genes are not always inherited together
Mendel’s Laws:
Chromosomes
• Gene mapping methods use recombination
frequencies between alleles in order to determine the
relative distances between them
• Recombination frequencies between genes are inversely
proportional to their distance apart
• Distance measurement: 1 map unit = 1 percent
recombination (true for short distances)
Gene Mapping
• Genes with recombination frequencies less than 50 percent are on the
same chromosome = linked)
• Linkage group = all known genes on a chromosome
• Two genes that undergo independent assortment have recombination
frequency of 50 percent and are located on nonhomologous
chromosomes or far apart on the same chromosome = unlinked
• The closer together two genes are, the more likely they will be
inherited together.
• Cross-over frequencies are related to distances between
genes.
• The higher the frequency, the further the genes are apart
• Linkage maps show the relative locations of genes.
• Recombination frequency is specific for a particular pair
of genes
• Recombination frequency increases with increasing
distances between genes
• No matter how far apart two genes may be, the maximum
frequency of recombination between any two genes is 50
percent.
Recombination
• Recombination results
from crossing-over
between linked alleles.
• Recombination changes
the allelic arrangement
on homologous
chromosomes
Gene Mapping
• The map distance (cM) between two genes equals one
half the average number of crossovers in that region per
meiotic cell
• The recombination frequency between two genes
indicates how much recombination is actually observed in
a particular experiment; it is a measure of recombination
• Over an interval so short that multiple crossovers are
precluded (~ 10 percent recombination or less), the map
distance equals the recombination frequency because all
crossovers result in recombinant gametes.
• Genetic map = linkage map = chromosome map
Gene Mapping
• 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
• Draw a linkage map based on the following cross
over percentages:
• A – B = 8%
• B – C = 10%
• A – C = 2%
Example