Cervical rib
In the basic vertebrate form, ribs are not limited to the thorax, but are present throughout the vertebral column from the neck to the waist, as can be seen in the vertebral column (backbone) of the fish. Remnants of these cervical and lumbar ribs sometimes appear in humans. In Europe, the presence of an extra rib always causes a fuss. It seems to have its origin in the Old Testament in the second chapter of Genesis, verses 21-22: "(God) formed the woman from the ribs which he took out of the man". Although anatomy teaches that humans have 12 pairs of 24 ribs, about 1 in 10 adults has an extra rib. According to the Bornstein & Peterson (1966) Numerical variation in the pre-sacral vertebral column in three population groups. Am J. Phys Anthrop, 25: 139-46, 9% of the 1239 skeletons had 13 pairs of ribs; of the 9%, less than 1% had a seventh cervical vertebra and about 5% had a first lumbar vertebra that built the ribs. And 3% were due to an increased number of vertebrae. According to Galis (1999) Why do almost all mammals have seven cervical vertebrae? Developmental constraints, Hox genes, and Cancer. J Exp Zool, 285: 19-26, the probability of the seventh cervical vertebra building a rib is about 0.2%, a low number. As you know, this rib appeared in cervical vertebrae is called the cervical rib. The cervical ribs cause "thoracic outlet syndrome," which compresses the nerves (cervical plexus and brachial plexus) and blood vessels in the neck. In other words, stiffness and pain in the shoulders and arms, as well as numbness and coldness in those areas. In the old days of anatomy, this would be the end of the story, but not these days. Yes, genes.
Insects have a head, thorax, and abdominal segments. The genes that cause mutations in Drosophila that result in the growth of limbs on the head are called homeotic genes. It was discovered in 1984 that these genes (groups) share a common sequence region called the homeobox, which consists of 180 base pairs encoding 60 amino acids. Eventually, it was discovered that humans, not to mention fish and mice, also share this region! Now, since vertebrae and ribs are also derived from body segments, they are controlled by this homeotic gene. There are 8 homeotic genes in Drosophila, but 39 in mammals, and it is known that if HoxA4, HoxA5, HoxB5, HoxA6, and HOxB6 were all present and no other homeotic genes were present, the seventh cervical vertebra would not form a rib. Mutation or deletion of one of these five homeotic genes results in the formation of a rib, or cervical rib, on the seventh cervical vertebra.