Q78 Which one of the following conjugations will result in formation of merodiploid?
Answer: Option (B) Hfr donor × F⁻ recipient
Hfr donor × F⁻ recipient conjugation transfers a portion of the bacterial chromosome, creating a merozygote (partial diploid). This differs from other pairings, which either transfer only the F plasmid or form stable partial diploids via F’ plasmids.
Option Analysis
Option (A): F⁺ donor × F⁻ recipient
F⁺ cells contain a free F plasmid, which transfers completely to F⁻ recipients during conjugation, converting them to F⁺ donors. No chromosomal DNA transfers, so no merozygote forms; recombination is rare.
Option (B): Hfr donor × F⁻ recipient
Hfr cells have the F factor integrated into the chromosome. Conjugation initiates rolling-circle replication, transferring chromosomal DNA starting from the oriT site. The bridge typically breaks before complete transfer, leaving the F⁻ recipient with partial donor chromosome (exogenote) alongside its own (endogenote), forming a merozygote.
Option (C): F⁺ donor × F⁻ recipient
Identical to option (A); only the autonomous F plasmid transfers, yielding F⁺ recipients without chromosomal partial duplication.
Option (D): F⁺ donor × Hfr recipient
Hfr cells act as donors due to their integrated F, but as recipients here, they already possess F functionality. F⁺ transfers the plasmid inefficiently or not at all; no merozygote results as both have chromosomal DNA.
Bacterial conjugation merozygote formation occurs specifically in Hfr donor × F⁻ recipient matings, a key concept in microbial genetics for CSIR NET exams. This process enables partial diploid analysis, distinguishing it from F⁺ or F’ transfers.
Conjugation Types Overview
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F⁺ × F⁻: Transfers F plasmid only, creating new donors.
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Hfr × F⁻: Chromosomal transfer yields merozygote via incomplete DNA injection.
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F’ × F⁻: Forms stable partial diploid (merozygote) with chromosomal genes on plasmid.
Merozygote Significance
Merozygotes allow dominance testing and gene mapping without full diploids. Hfr conjugation rarely completes (~1/10,000), ensuring transient partial diploids for recombination studies.
