Advanced Genetics
Two-Factor Mapping
Three-Factor Mapping
Lecture 4
I. New Business: Two-factor crosses to locate the gene to a chromosome
1. Mapping loci: what makes for a good mapping locus?
2. Two-factor cross:
a. Mapping to a chromosome
1) Assess the frequency of double homozygote F2
b. Mapping within a chromosome
1) Selecting double homozygote F2s is not good for this analysis: Why?
2) Select F2 that are homozygotes for the mapping locus and heterozygotes for
your gene
3) Calculation of the frequencies of the F2 genotypes and chromosomes
4) Calculation of the map distances
II. 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.
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