In today's lesson students will explore how homeotic genes (hox genes) regulate embryonic development by looking at the results of a review of many hox gene studies of arthropods. Then, they will watch a short clip from Your Inner Fish that highlights studies of the regulation of the Sonic Hedgehog gene. Finally, students will revisit the arthropod models that they made yesterday and determine the hox genes that are responsible for the body design seen in their models. This lesson extends student knowledge beyond a basic understanding of Mendellian genetics so they can more deeply appreciate the complexity that controls the develop of the embryo. This is the second day of a two day lesson. Day one is at this link. Here is an overview of what students will learn today.
Show students the following images and ask them to describe what they see. First students should compare and contrast the heads of the two fruit flies. Next they should compare the body types of the two fruit flies. Ask students how the halteres on the two fruit flies differ.
Then have students speculate what has caused the differences between the two sets of flies. Students should write their hypotheses in their lab notebook.
Student should popcorn the first two portions of the student reading, summary and introduction, as a class. While the students are reading aloud, everyone should use their four colors of highlighters to indicate the four main ideas discussed in this reading. (Note: see student work example.)
Next, student pairs should analyze the images in the Hox Gene Summaries section and determine how mutations in hox genes affect the arthropod body plan by following the instructions included in the Reading Guide.
After twenty minutes, bring the entire class back together and review the importance of studying hox genes as a class.
(Note: The guided reading is based on the Review "Hox genes and the evolution of the arthropod body plan." It has been revised to an eleventh grade reading level.)
Students will watch video clips from Your Inner Fish about the derelict mammalian yolk genes to the Sonic Hedgehog gene and its effects on animal development.
First, play a portion of the second episode of Your Inner Fish (from 11:02 to 14:19). Have students complete the graphic organizer to compare the yolk genes of reptiles to the yolk genes of mammals.
Next, play a clip from the second episode of Your Inner Fish (from 34:31 to 40:58) to understand how skin organs are formed from a master gene, E.D.A
Finally, play a clip from the first episode of Your Inner Fish (from 35:34 to 45:49) have student complete the graphic organizer to compare the effects of manipulation of the Sonic Hedgehog gene.
Ask students to add the roles of these three genes to their reading guide.
Bring the class back together and show student the images of the hox mutants from the beginning of the period. Point out that the hox mutants have had a different hox gene expressed in cells in an area that normally does not have that gene expression. Ask students to explain how this can be.
Possible student answer: There was change in one of the hox genes that affected the expression of another gene. For example, there was a mutation in the Antennepedia gene which caused a gene that is responsible for the formation of legs to be expressed.
Have students to retrieve the arthropod models that they built yesterday. In their lab notebooks, students should draw the body plan of their model. Next they should list the genotype and phenotype of their arthropods. Using the graphs for the guided reading, they should determine which hox genes were responsible for the development of their model. They should list them in their lab notebook next to the gene to which they were regulating. Finally, they should write a short summary describing which hox genes were active during the development of the model.
Possible student answer: I made a fly. It has a body plan much like the fruit fly. t has a head, thorax, and abdomen. The homeobox gene responsible for the formation of the head is the lab and pb genes. The Ubx gene responsible for the formation of the thorax. The adb-A and adb-B genes were responsible for the formation of the abdomen. All of those genes were responsible in making sure the "styrofoam egg carton" gene was expressed which formed the head, thorax, and abdomen on my fly. My fly also has legs made of colored pipe cleaners. Three homeobox genes controls the expression of the colored pipe cleaner genes. They were ftx, antp, and imaginal discs. My fly also has twist tie antennae genes and waxed paper wing genes. The antennae genes are controlled by the Antp gene. The waxed paper wing gene is controlled by the imaginal discs gene. If something goes wrong with the expression of any of these hox genes while the fly embryo was developing, then my fly wouldn't look right.