3.7 Genes: Structural and Regulatory

Structural genes are responsible for body structures, such as hair, blood, and other tissues. Regulatory genes turn other genes on and off, an essential activity in growth and development. If the genes that determine bones, for example, did not turn off at a certain point, bones would continue to grow well beyond what would be acceptable for a normal life (Figure 3.20).

Two hands next to each other: one is average and the other is much longer and thinner.; A portrait of Abraham Lincoln dressed in formal clothing with his left hand on a stack of books.
FIGURE 3.20 Marfan Syndrome (a) The hand on the right shows normal finger growth. The hand on the left has much longer and thinner fingers due to Marfan syndrome, a hereditary disorder of the regulatory genes that control connective tissue. As a result of Marfan syndrome, uncontrolled bone growth leads to long and thin fingers and toes, long and thin arms and legs, and increased stature. Organs such as the lungs and heart can also be negatively affected. (b) In the 1960s, a scientific paper asserted that US president Abraham Lincoln (1809–1865) was afflicted with Marfan syndrome. This still-controversial assessment was based entirely on Lincoln’s unusual tallness and the length of his limbs.

Chickens have the genes for tooth development, but they do not develop teeth because those genes are permanently turned off. Humans have a gene for total body hair coverage, but that gene is not turned on completely. The human genes for sexual maturity turn on during puberty, somewhat earlier in girls than in boys. Regulatory genes can lead to lactose intolerance in humans (see “Anthropology Matters: Got Milk? The LCT Phenotype” in chapter 4). In this instance, the LCT gene is turned off for most human populations around the world after weaning, usually by about age 4. However, most humans of northern European and East African descent have inherited a different regulatory gene, which creates lactase persistence. A person who lacks the LCT gene and eats dairy products experiences great gastrointestinal discomfort. A person who retains the gene is able to digest lactose owing to the persistence of lactase, thus enjoying the nutritional benefits of milk.

An organism’s form and the arrangement of its tissues and organs are determined by regulatory genes called homeotic (Hox) genes. These master genes guide, for example, the embryological development of all the regions of an animal’s body, such as the head, trunk, and limbs (Figure 3.21). This means that in the process of development, particular sets of Hox genes are turned on in a particular sequence, causing the correct structure or part of a structure to develop in each region. Until recently, scientists thought that the genes that control the development of the key structures and functions of the body differed from organism to organism. We now know, however, that the development of various body parts in complex organisms—such as the limbs, eyes, and vital organs—is governed by the same genes. Hox genes were first found in fruit flies, but research has shown that a common ancestral lineage has given organisms—ranging from flies to mice to humans—the same basic DNA structure in the key areas that control the development of form. Flies look like flies, mice look like mice, and humans look like humans because the Hox genes are turned on and off at different places and different times during the development process.

An illustration of a chromosome with a hox gene and outlines of a fruit fly, mouse, goose, python, and human with their spines drawn in and the thorax highlighted. Hox genes have been identified in all animals, plants, and fungi. They are found as a unique cluster known as the Hox cluster or Hox complex. Unlike vertebrate animals, insects such as fruit flies have distinct body regions, including the head and the middle, or thorax. In fruit flies, the Hox gene that determines the thoracic region of the body during the larval stage of development is called Ant p. While the body regions of vertebrates, such as mice, are not as distinct as those of flies and other insects. Hox genes determine their body regions during embryological development. The Hox c 6 gene in mice delimits the thoracic region, which is indicated by the thoracic vertebrae. Other vertebrates, such as birds and reptiles, have a Hox c 6 gene, which determines the location of the thorax in the embryo. However, the location of the Hox c 6 gene varies with each animal, allowing for variation in the length of the cervical, or neck, region. If the Hox c 6 gene is lower in the body, as it is in the geese, the animal will have a much longer neck than if the gene is located close to the head, as it is in the pythons. Pythons, as result of this placement, have virtually no neck. Humans, being vertebrates, also have a Hox c 6 gene, which determines the location of the thoracic region. Humans have a neck of intermediate length when compared to geese and pythons; the Hox c 6 gene is closer to the head in humans than in geese, but is lower than pythons. The Hox c 6 gene is responsible not just for determining the development of the thorax; in humans, this gene determines the development of the entire thoracic region, including mammary glands.
FIGURE 3.21 Homeotic (Hox) Genes Discovered in 1983 by Swiss and American researchers, these regulatory genes are coded to produce proteins that turn on many other genes, in particular those that determine the regions of the body during prenatal development. Without these genes, or if there are mutations in these genes, body development may be altered. For example, a mutation in the Hox genes of a fruit fly can cause a leg instead of an antenna to grow from the head. In the figure, the gray shading refers to the thorax location for the four vertebrates, which is controlled by the Hoxc6 gene.

Glossary

  • structural genes
    Genes coded to produce particular products, such as an enzyme or hormone, rather than for regulatory proteins.
  • regulatory genes
    Those genes that determine when structural genes and other regulatory genes are turned on and off for protein synthesis.
  • homeotic (Hox) genes
    Also known as homeobox genes, they are responsible for differentiating the specific segments of the body, such as the head, tail, and limbs, during embryological development.