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6.2 Mapping Pollen Hoarding
h ere are three steps in making a QTL map. First, you make a genetic
cross. Second, you construct a linkage map, showing how all your gene-
tic markers are linked. h ird, you do a QTL map on top of it. To make
the map, you must make crosses where you have parents that dif er
with respect to the markers they have at any given point in the genome,
and you know the marker that each parent has at each marker locus.
6.2.1 h e Cross
We instrumentally inseminated a low-strain queen with the semen of a
high-strain drone. From this cross we raised a new queen that was a
hybrid of the two strains. h e queen inherited one set of chromosomes
from her low-strain mother and one set of chromosomes from her
high-strain father. From her we raised more than 100 males. Males are
haploid—they have no father. h erefore, each male had a unique genome
that he inherited from his mother, a result of recombination during mei-
osis (Section 3.3 and Figure 3.4). We knew the origins (high or low
strain) of all markers because we saved the drone father and included
him in the marker analysis. We used 96 drones to construct the genetic
linkage map. We chose 96 because that is the number of wells on the
thermocycler we used. A thermocycler is a device that makes strands of
DNA in a tube that i ts into the well. DNA is made by a process called
the polymerase chain reaction. h e DNA produced contains the mark-
ers we use for constructing the maps. Note that today, mapping is done
by direct sequencing of DNA, but the principles are the same.
h e linkage map was derived from a total of 365 RAPD markers
(Figure 6.1) assembled into 26 linkage groups of at least three markers.
h e overall size of the map was 3,450 centimorgans (cM; see Section
3.3). h is is a huge recombinational map when it is compared with
other animal species. h e enormous size of the map is the reason we
had 26 linkage groups when there are only 16 chromosomes for the
honey bee. h e higher the rate of recombination, the more markers you
need for your map to become marker saturated and coalesce into the
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