Final answer:
It is false that all recombinant bacteria will be a+b+ after a cross between an Hfr strain a+b+ and an F- strain a-b-. Conjugation and gene transfer are not always complete, leading to various recombinant outcomes, not just a+b+.
Step-by-step explanation:
The statement in the question is False. In a bacterial cross where the donor is an Hfr (High-frequency recombination) strain with alleles a+ and b+, and the recipient is an F- strain with alleles a- and b-, it is not guaranteed that all recombinant bacteria will be a+b+. During conjugation, the F plasmid is transferred from the Hfr cell to the F- cell along with some chromosomal DNA. However, the full transfer of the F plasmid and any chromosomal genes trailing it depends on the duration of mating. If the conjugation is interrupted before the entire set of genes has been transferred, the recipient may not receive the complete a+ or b+ alleles, and therefore, the recombinant bacteria can have various combinations of a and b alleles depending on which genes were successfully transferred and integrated.
Conjugation experiments using different Hfr strains have shown that the order of gene transfer can be used to map bacterial genes on the chromosome. The time taken to transfer genes during conjugation can reflect their size and position on the DNA. The result of this process is a new combination of genetic material, while the possibility of interruption before the complete transfer of desired genes can lead to different recombinant outcomes, rather than all recombinant bacteria being uniformly a+b+.