Why are the recombinants produced from an Hfr x F⁻ cross rarely, if ever, F⁺?

Verified step by step guidance

1. Understand the nature of Hfr and F⁻ strains: An Hfr (High frequency recombination) strain has the F factor integrated into its bacterial chromosome, while an F⁻ strain lacks the F factor entirely.
2. Recall the process of conjugation between Hfr and F⁻: During conjugation, the Hfr cell attempts to transfer part of its chromosome, starting from the integrated F factor origin, into the F⁻ recipient.
3. Recognize that the F factor is part of the bacterial chromosome in Hfr cells, so the transfer begins with the F factor sequence but usually does not complete the entire F factor transfer before the mating bridge breaks.
4. Since the F factor is transferred last in the sequence, the F⁻ recipient rarely receives the complete F factor, meaning it usually remains F⁻ even after recombination.
5. Therefore, recombinants produced from an Hfr x F⁻ cross are typically chromosomal recombinants (with new genetic traits) but remain F⁻ because the full F factor is not transferred and integrated.

Key Concepts

Hfr and F⁻ Bacterial Mating

Hfr (High frequency recombination) strains have the F factor integrated into their chromosome, allowing them to transfer chromosomal genes to F⁻ cells during conjugation. F⁻ cells lack the F factor and can receive genetic material but typically do not become F⁺ after mating with Hfr.

Mechanism of F Factor Transfer in Hfr x F⁻ Cross

During conjugation, the Hfr cell initiates transfer from the integrated F factor origin, transferring chromosomal genes first. The entire F factor is rarely transferred because it is located at the end of the transfer sequence, so the recipient usually remains F⁻.

Recombinant Formation and F Factor Status

Recombinants arise when chromosomal genes from the Hfr donor integrate into the F⁻ recipient's genome. Since the full F factor is seldom transferred, recombinants typically gain new chromosomal traits but do not acquire the F plasmid, thus remaining F⁻ rather than becoming F⁺.