A simplified schematic of a single Holliday junction is show

A simplified schematic of a single Holliday junction is shown here. (It has been \"uncrossed\" to show all of the single-stranded regions.) One chromatid is shown in red, and the homologous chromatid is shown in blue. The red chromatid carries a dominant allele labeled A and a recessive allele labeled b, whereas the blue chromatid carries a recessive allele labeled a and a dominant allele labeled B. Potential cut sites for resolution of the Holliday junction are indicated by the numbered arrows. a) Which pair of cuts needs to occur to generate recombinant chromatids? b) What would be the genotypes of the two recombinant chromosomes?

Solution

a) Cuts should occur at positions 2 and 4 to get recombinants. A Holliday model can be resolved in two different ways. The first way is cut on both crossed strands and it results in non recombinants. Here, strands of different color that are joined together are crossed strands. So, cuts in these strands (i.e., at positions 1 and 3) gives non recominants. On the other hand, cuts on noncrossed strand results in recombinants. here non crossed strands are those in entirely single color (either blue alone or red alone).

b) As I mentioned above, cuts at positions 2 and 4 results in recombinants. So, the genotypes of recombinants will be \"AB\" for one chromosome and \"ab\" for the other. please note that genotypes of parental chromosome are \"Ab\" and \"aB\"