Genetics 301: Linkage in eukaryotes (chapter 6)

Lecture Topics/Genetics Home Page
I. Introduction
II. Mapping the genome in organisms that are diploid
A. Two-point crosses
1. Example: fruit fly eye color and wing shape
a. purple eye fruit fly x vestigial winged fruit fly
b. testcross the dihybrid formed
c. analyze the results
2. How can we explain the results?
3. Estimating the distance between loci on chromosomes
B. Three-point crosses
1. Example: fruit fly eye color, body color and wing shape (see handout)
2. Estimating the distances between the three loci (and determining the relative order of genes)
3. Accounting for discrepancies in map distances
C. Wrap-up comments about two and three point crosses
1. In a single cross, we cannot measure genes that are located more than 50 map units apart. Why not?
2. In three point crosses, we can determine if crossing over events are independent of each other using the coefficient of coincidence equation
III. Haploid mapping (tetrad analysis)
A. Linear tetrad analysis: mapping the distance between a locus and a centromere
1. MI and MII pattern asci (ascospore arrangement differs if crossing over occurs between locus and centromere)
2. Example: Neurospora spp. mating type alleles (A and a)
3. Determine map distance between locus and centromere
4. Maximum map distance that can be calculated (as a result of tetrad analysis) is 33 map units. Why?
B. Unordered tetrad analysis
1. Example: yeast with genotypes ab and a+b+
a. Parental ditypes
b. Tetratypes
c. Nonparental ditypes
2. Map distance between 2 loci? (examine the % recombinant chromatids)
3. Correction factor for map distance (takes into account double crossing over events)
4. This type of analysis (unordered tetrad analysis) can be used to map 2 loci in ordered tetrads, also