Chapter 3: Handy Genes

1. Many experiments were conducted during the 1950s and 1960s with chick embryos and they showed that two patches of tissue essentially controlled the development of the pattern of bones inside limbs.  Describe at least one of these experiments and explain the significance of the findings. Hint: see p. 58

During this time period, biologists Edgar Zwilling and John Saunders observed chicken embryos by cutting and moving various tissues to discover the effect upon the embryos’ development. With very precise microsurgery, Saunders and Zwilling discovered two patches of tissue that essentially control the manner in which the pattern of bones inside the chickens’ limbs develop. This patch of tissue controlled all limb development and when removed, halted development. When it was removed early, only the upper arm developed; when it was removed later, the arm almost completely developed with the exception of shorter, deformed digits in the chicken’s wing.

Mary Gasseling later performed an experiment in Saunders’s laboratory that built upon these findings. She removed a patch of tissue early in development from what would later become the pinky side of a limb bud and placed it on the opposite side of the limb but where the first finger would later form. The wing developed normally but now it also had a duplicate set of digits that were mirror images of the normal set. This effect was mimicked by dabbing Vitamin A upon the limb bud and even injecting it into the egg and letting the embryo develop. This patch of tissue that controlled development was named the zone of polarizing activity, or ZPA.  The proposed theory of the time was that a concentration of a certain corresponded to the formation of the “pinky” and “thumb “digits and was higher near the ZPA (where the pinky and thumb were formed) and

It was later discovered in the 1990s that what controlled this development was actually gene activity.  The same principles held true when tests were conducted with mouse genes and skate fin skeletal rods. They produced the same mirror-image duplication of appendages seen in the chickens.  These findings lead to the confirmation that all appendages are built by similar kinds of genes. They are significant because they suggest the “inner fish” concept that proposes the existence of a strong connection between various life forms. It also suggests that perhaps evolutionary transformation of fish fins into limbs did not involve the origin of new DNA and was instead a matter of genes working in new ways to form limbs with fingers and toes.

2. Describe the hedgehog gene using several animal examples.  Be sure to explain its function and its region of activity in the body.

The hedgehog gene was originally discovered in fruit flies as a gene that makes one region different from another, an effect very similar to the ZPA in chickens. Scientists then began searching for the hedgehog gene in other creatures including chickens, mice, and fish. Eventually, the chicken hedgehog gene was discovered and renamed the Sonic hedgehog gene. This gene was found in the ZPA of the chicken in the wing bud and when active, develops pinky and thumb digits. Every limbed animal has this gene and has been active in the ZPA tissue of every animal it has been studied in from grogs to humans. By the eighth week of human embryo development, the Sonic hedgehog gene will have become activated the ZPA (located in the developing arm) in a normal embryo. If not, it can lead to the development of extra fingers or the pinky and thumb being mirror duplicates of one another. This gene is crucial to building upper arms, forearms, wrists and digits. In the adult skate fin, experiments utilizing the mouse Sonic hedgehog gene have shown to glad to the development of skeletal rods with distinct, different shapes from the ones that were farther away. This experiment confirmed the function of the Sonic hedgehog gene in all limbed organisms to contribute to the development of distinct appendages.

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  1. Thank you Jill. This ZPA is important to understanding the sequence of gene activation in embryonic development. You need to edit the last sentence in para 1, however “The proposed theory of the time was that a concentration of a certain ??? corresponded to the formation of the “pinky” and “thumb “digits and was higher near the ZPA (where the pinky and thumb were formed) and . . . ”
    Also your response to number 2 needs some editing.
    Be careful with wording here, “This gene was found in the ZPA of the chicken”. Of course the gene is found in that tissue, the entire genome is found in every somatic cell. Perhaps you mean “the active” gene was found . . .? ML

  2. It is possible that each digit has its own combination of transcription factors that affects the gene expression for that particular finger? It’s fascinating that one type of gene can drastically affect the development of an embryo – from chicken to human.


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