Detection of linkage and crossing over
Linkage is detected by comparing the relative frequencies of different types of tetrads formed with those expected from independent segregation. If the two genes are linked, parental ditype tetrads will be the most frequent, non parental ditypes will be the least frequent, while tetratype tetrads will be of intermediate frequency. In contrast, independent segregation produces equal frequencies of parental and non parental ditype tetrads, and relatively low frequencies of tetratype tetrads.
Single crossing over between two linked genes, A and B, involving two non sister chromatids leads to the production of tetratype tetrads, 2ab: 2aB: 2Ab: 2AB. Non parental ditype tetrads, 4aB: 4Ab, are formed in case of four strand double crossing over where both the non-sister chromatids are involved. If it is a two strand double crossing over where both the non-sister chromatids are involved. If it is a two strand double crossing over, then 4ab: 4AB parental ditype tetrads are found. A three strand double crossing over will give tetratype tetrads just like single crossing over, i.e., 2ab:2aB:2AB:2Ab. The difference between the two types of crossing over has to be made on the basis of order of the spore inside the ascus.
If the crossing over takes place between gene A and its centromere in Neurospora then the gene would show a second division segregation and 2A:2a:2A:2a spore arrangement. It may also give rise to 2a:2A:2a:2A, or 2A:4a:2A or 2a:4A:2a spore arrangements depending upon the orientation of the crossover and noncrossover chromatids during meiotic division.
If there is a double crossing over, one between gene A and its centromere and another between the two genes A and B involving the same two non sister chromatids, then a spore arrangement of 2AB:2aB:2Ab:2ab is obtained. A double crossing over involving the four chromatids, then non parental ditype of 4Ab:aB spore arrangement is obtained. Other crossing over types are possible which can be identified from arrangement of spores inside the asci.
Information on mitotic recombination are often used for chromosome mapping in some fungi, viz., Aspergillus nidulans. In fungi vegetative hyphae of different genotypes called heterocaryons. Two haploid nuclei may also fuse to yield a diploid hyphae and a diploid hyphae may also undergo haploidization without meiosis. In the process crossing over may occur in the diploid state.