Cotransductants are the strains transduced to both a+ and b+ alleles. Co-transduction occurs when two genes are very near to each other. They should be close enough to enter into the phage particle. In the figure 5-29, the b+ allele could also be transduced instead of a+, but they will not be co-transduced into the recipient bacterium, since the two alleles are not closely related. Therefore, either a+ allele will be transduced or b+ allele will be transduced, but not both at a time.