Mapping chromosome centromeres from tetratype frequencies

Summary

• 1. If data on tetratype frequencies refer to unlinked pairs of loci, even though the loci are not necessarily all unlinked, then centromeres can be mapped if the data include an odd-numbered closed set (or cycle) of frequencies (e.g. frequencies known fora andb,b andc, c anda, wherea,b andc are mutant genes). Formulae are given for calculating second-division segregation frequencies and their standard errors from such odd-numbered cycles.

• 2. If unlinked tetratype data include only an even-numbered cycle (e.g. data fora andb,b andc,c andd,d anda, whered is a fourth gene), then second-division segregation frequencies cannot be calculated, but a relationship exists between the tetratype frequencies. It can be used to test the homogeneity of such data.

• 3. If one of the tetratype frequencies refers to a linked pair of loci, centromeres can be mapped provided the frequencies form a cycle of at least three values, a.nd provided at least one of the linked genes is situated near to the centromere of its chromosome. Formulae are given for calculating second-division segregation frequencies from cycles of three and four loci, when one pair of loci show linkage.