The quantitative analysis of chromosome pairing and chiasma formation based on the relative frequencies of MI configurations. V. Interchange trisomics

Abstract

Information obtained previously and presently on chromosome pairing and chiasma formation in trisomics and in interchange heterozygotes has been applied in newly constructed models for calculating expected MI configuration frequencies in interchange trisomics. Good fit betwen calculated and observed frequencies in some and poor fit in other cases confirmed the expectation of genetic variation in the crossing-over potentials of some or all chromosome regions. If conclusions in respect of chromosome pairing pattern are to be based on relative frequencies of MI configurations, valid values for crossing-over potentials are required. These can only be obtained from genetically comparable material. A few more disturbing factors are recognised. Environmental effects are one of these factors but may have a relatively simple character. Good agreement between expected and observed frequencies of configurations was taken to indicate the validity of the assumption that homologous chromosome end segments have equal probability of being involved in pairing, irrespective of the length of the segment. This conclusion was confirmed by the segregation of chromosomal types in the progenies of interchange trisomics: the excess chromosome was combined as frequently with the interchange set and with the normal set respectively, as expected on basis of the same models, assuming 60–80% viability of trisomes compared to diploids.