Download Three-factor crosses

BIO421
Advanced Genetics
Lecture 5
Three-Factor Mapping
I. Three-factor crosses to order loci
A. Sometimes it is difficult to determine the order of nearby loci
1. The order can be determined by using a 3-factor cross (see Brenner 74, Table 8)
B. Procedure
1. Cross a double mutant (one locus is one of the problem genes, the other is a known
locus) with a single mutant (the other problem gene)
2. Select F2 that are recombinant for the chromosome from the double mutant
3. Assess the F3 phenotypes to determine gene arrangement
4. We will do several of these to give you the idea.
II. Three-factor crosses to determine position
A. Choosing mapping loci - they must flank your gene
B. The cross creates a triple heterozygote with your gene in the middle
C. The F1 produces 2 non-recombinant chromosomes and 4 recombinant chromosomes
D. In the F2, individuals that carry recombinant chromosomes are isolated
1. These individuals show the phenotype of only one of the mapping loci that flank your
gene
2. The F2 are allowed to self-fertilize (or subjected to a test-cross) to determine if they
are heterozygous for your new gene
E. The map location of your gene is determined by the number of crossover events between
the left marker and your gene and between your gene and the right marker.