(c) the genetic control of the development of body plans in different…

The idea

Body plans are the arrangement of body regions and structures along an organism's axes. Their development is controlled by genes that regulate where and when other genes are expressed. Homeobox (Hox) genes encode transcription factors that bind DNA and switch target genes on or off, setting up regional identity along the anterior–posterior axis. Because the same homeobox sequence is highly conserved, homologous Hox clusters are found across animals, and their order along the chromosome often mirrors the order of body regions they pattern. Mutations in these regulatory genes can transform one segment's identity into another's, showing that control is about regulating development rather than building structures directly. In plants, MADS-box genes help specify floral organ identity. Questions test whether you can link gene regulation to pattern formation across different organisms.

What earns the marks

• Homeobox (Hox) genes encode transcription factors that regulate the expression of other genes controlling body-plan development.
• Hox genes are highly conserved across different organisms, so homologous sequences are found in many animal groups.
• The order of Hox genes along the chromosome often corresponds to the order of the body regions they pattern along the axis.
• Mutations in regulatory/homeotic genes can alter segment identity, demonstrating genetic control of body-plan development.
• Gene regulation (switching target genes on or off) underlies pattern formation rather than genes directly building structures.
• MADS-box genes in plants specify floral organ identity, showing genetic control of body plans in different organisms.

Common mistakes and how to put them right

• Thinking Hox genes build body parts directly.

  Put it right by explaining they encode transcription factors that regulate other genes.

• Assuming body-plan genes are unique to each species.

  Put it right by noting the homeobox sequence is highly conserved across organisms.

• Confusing a homeotic mutation with a simple loss of a structure.

  Put it right by explaining it can change one segment's identity into another's.

Exam tips

• Read the stem carefully to identify whether it asks about gene identity, gene regulation or the consequence of mutation.
• Eliminate options that claim Hox genes directly construct structures; regulation of target genes is the key idea.
• Link any option about conservation or chromosomal order to the idea of controlling body-plan development across organisms.