The expression of the lac structural genes is when lactose is absent from the culture medium and is when lactose is added, because lactose binds to the lac and inactivates it.

The expression of the lac structural genes is LOW when lactose is absent from the culture medium and is HIGH when lactose is added, because lactose binds to the lac REPRESSOR and inactivates it.

Explanation:

Lac operon is expressed only when lactose sugar is available to the bacteria. When lactose is not present, the repressor protein binds to the operator sequence and does not allow RNA polymerase to transcribe the genes. Therefore, the expression of the lac operon is prevented in the absence of lactose. The presence of lactose allows its expression. Under these conditions, allolactose binds to the repressor protein rendering it not able to bind to the operator sequence.

the diploid number of chromosomes is sometimes represented by the symbol 2n. for the fruit fly, the diploid number is 8, which can be written as 2n = 8, where n represents twice the number of chromosomes in a sperm or egg cell.