Gene Control: Body Plans (OCR A Level Biology): Revision Note

Body Plans & Hox Genes

• The body plan of an organism is the basic pattern of the body, e.g.:

  • the polarity of an organism, i.e. where the head/tail and front/back of an organism develops

  • segmentation of organisms such as insects and mammals into distinct body parts, e.g. abdomen and thorax

  • the location at which body parts such as wings, limbs and internal organs develop

• Body plan is determined at the embryo stage of development, and is controlled by a family of genes known as homeobox genes

  • Most animals have a set of homeobox genes known as Hox genes

Homeobox genes

• Homeobox genes code for transcription factors

• Transcription factors ensure the correct development of body plan as follows:

  • they control which genes are being expressed at a particular time and in particular cells

  • gene expression can be switched on or off in specific locations in a tightly controlled sequence during early development

  • cells in the correct locations differentiate into the correct cell types, e.g. cells at the top of the spinal cord may differentiate into nerve cells for brain development

• Homeobox gene sequences in plants, animals and fungi are highly conserved

  • Mutations that cause changes in homeobox sequences can lead to organisms that are unable to survive, so their mutated alleles are not passed on

  • This strong negative selection pressure explains why homeobox sequences are conserved

• Homeobox genes can be seen as 'master genes' that control which genes function at different stages of development

Hox genes

• Hox genes are a subset of homeobox genes found in most animals

• They determine the identity of embryonic body regions along the head-tail axis

• These Hox genes are organised into groups known as Hox clusters

  • There is a linear order to the Hox genes in each Hox cluster and this order is directly related to the order of the regions of the body that they affect