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The blue-white screen refers to a procedure of screening that allows for the brisk and convenient determination of recombinant bacteria in vector-based molecular cloning experiments. In the process, the DNA of interest is ligated into a vector. The blue colonies constitute of self-relegated plasmids that are devoid of DNA inserts obstructing the lac Z gene.
The white colonies contain bacteria that conduct plasmids, which possess the inserts of DNA fragments that obstruct the lac Z gene. Therefore, the researcher needs to sequence the plasmids of the white colonies prior to knowing which allele it constitutes.
By looking at a gene set up or a punnet square,a gene that's dominant would be represented by a capital letter and recessive traits are represented by lowercase letters for example brown eyes are considered a dominant trait which can be represented as BB for homozygous, and Bb for heterozygous. Since blue eyes are recessive they would be bb.