16. Segmentation genes in Drossophila are divided into three groups (gap, pair rule and segment polarity) based on their mutant phenotype.
Below are some of the major genes expressed in a sequential manner (with respect to the groups) affecting segmentation pattern.
(A) hairy → paired → tailless → patched
(B) hunchback → even-skipped → fushi tarazu → wingless
(C) odd-skipped → giant → paired → wingless
(D) tailless → hairy → fushi tarazu → gooseberry
Which of the above sequence(s) of genes expressed from early to late embryo is/are correct?
(1) (D) only                                                   (2) (A) and (B)
(3) (C) and (B)                                               (4) (B) and (D)

In Drosophila embryonic development, the establishment of the segmented body plan is orchestrated by a well-studied hierarchy of gene groups: gap genes, pair-rule genes, and segment polarity genes. These genes are expressed in a sequential and spatial manner, each setting the stage for the next phase of segmentation.

Gap genes are the earliest expressed in broad overlapping domains defining large embryonic regions. Examples include hunchback and tailless. These establish foundational territories along the anterior-posterior axis.

Following this, pair-rule genes like even-skipped, fushi tarazu, hairy, and paired produce striped patterns that sharpen the boundaries of segments, effectively dividing the embryo into alternating parasegments.

Finally, segment polarity genes such as wingless, patched, and gooseberry refine the anterior-posterior polarity within each segment, solidifying segmental identities.

Among the gene sequences proposed:

• (B) hunchback → even-skipped → fushi tarazu → wingless

• (D) tailless → hairy → fushi tarazu → gooseberry

accurately represent the early to late expression pattern across these groups, respecting the established developmental framework.

Thus, the correct option identifying sequential expression is:

(4) (B) and (D)

Answer: (4) (B) and (D)