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A functional unit of heredity is a gene. A gene is a nucleotide sequence in the genome that codes for a functional molecule. Various species have different numbers of genes.

The segmentation genes in Drosophila determine the correct sequence, number, and polarity of the embryo's body segments. The knirps gene acts as a gap gene during embryogenesis, controlling the expression of segmentation genes. The knirps gene regulates segmentation in the embryo's abdominal region.

The knirps gene is replaced with lacZ, as stated in the statement. It's a reporter gene, and the embryo would form with -galactosidase expressed. The pattern of Drosophila can be investigated by staining -galactosidase. The knirps mRNA is expressed in the posterior side of the Drosophila embryo at the middle. As a result of the action of -galactosidase on X-gal, the band would look blue-stained following incubation in the X-gal solution.