Question

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An example of homeotic transformation in humans is the formation of cervical (neck) ribs. Normally, ribs are associated with thoracic vertebrae, but in some cases the last vertebra in the neck also develops ribs. Speculate on what might be causing this phenomenon (in general terms, specific genes are not important). Is an underlying mutation likely to be recessive or dominant? Briefly explain.

Answer 1

7Homeotic Transformation or Homeosis is the transformation of an organ or a part of the body into another organ or part, due to the mutation in the developmental genes or homeotic genes. It is studied under Evolutionary Developmental Biology. In animals, these homeotic genes usually control the development of organs on their anteroposterior axis. It was discovered that there is a DNA binding sequence present in proteins which are responsible for homeosis. This nucleotide sequence is called Homeobox and the domain with protein composed of 60 amino acids is called Homeodomain. Example- Antennapedia in Drosophila refers to the development of legs where antenna is usually present. Now in Humans, ribs are usually formed in the thoracic cavity(ribcage). The morphological development of the skeleton is under the control of HoxPG3, HoxPG4, HoxPG5. Mostly, we find that humans possess a rudimentary 7^th vertebra or last cervical vertebra (C₇ )as a result of an incomplete homeotic transformation. Loss of function in homeobox genes may result in mutation, leading to the development of ribs by the last cervical vertebra. This development of ribs is due to the mutation in the genes, resulting in the complete transformation of a rudimentary vertebra to a fully developed rib. Development of this rib pressurizes nerves and arteries, leading up to the arm and may cause Thoracic outlet syndrome. Since this rib doesn't serve any function and is not beneficial, it is usually eliminated by natural selection and mutation is recessive in nature.