{"id":1870,"date":"2017-01-30T23:37:06","date_gmt":"2017-01-30T23:37:06","guid":{"rendered":"https:\/\/courses.lumenlearning.com\/wm-biology2\/?post_type=chapter&#038;p=1870"},"modified":"2024-04-25T19:06:58","modified_gmt":"2024-04-25T19:06:58","slug":"hox-genes","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/wm-biology2\/chapter\/hox-genes\/","title":{"raw":"Hox Genes","rendered":"Hox Genes"},"content":{"raw":"<div class=\"textbox learning-objectives\">\r\n<h3>Learning Outcomes<\/h3>\r\n<ul>\r\n \t<li>Describe the roles that Hox genes play in development<\/li>\r\n<\/ul>\r\n<\/div>\r\n<p id=\"fs-idm4491456\">Since the early nineteenth century, scientists have observed that many animals, from the very simple to the complex, shared similar embryonic morphology and development. Surprisingly, a human embryo and a frog embryo, at a certain stage of embryonic development, look remarkably alike! For a long time, scientists did not understand why so many animal species looked similar during embryonic development but were very different as adults. They wondered what dictated the developmental direction that a fly, mouse, frog, or human embryo would take. Near the end of the twentieth century, a particular class of genes was discovered that had this very job. These genes that determine animal structure are called \u201chomeotic genes,\u201d and they contain DNA sequences called homeoboxes. Genes with homeoboxes encode protein transcription factors. One group of animal genes containing homeobox sequences is specifically referred to as\u00a0<strong><span id=\"term1028\" data-type=\"term\"><em data-effect=\"italics\">Hox<\/em>\u00a0genes<\/span><\/strong>. This cluster of genes is responsible for determining the general body plan, such as the number of body segments of an animal, the number and placement of appendages, and animal head-tail directionality. The first\u00a0<em data-effect=\"italics\">Hox<\/em>\u00a0genes to be sequenced were those from the fruit fly (<em data-effect=\"italics\">Drosophila melanogaster<\/em>). A single\u00a0<em data-effect=\"italics\">Hox<\/em>\u00a0mutation in the fruit fly can result in an extra pair of wings or even legs growing from the head in place of antennae (this is because antennae and legs are embryologic homologous structures and their appearance as antennae or legs is dictated by their origination within specific body segments of the head and thorax during development). Now,\u00a0<em data-effect=\"italics\">Hox<\/em>\u00a0genes are known from virtually all other animals as well.<\/p>\r\n\r\n\r\n[caption id=\"attachment_1677\" align=\"alignright\" width=\"449\"]<img class=\"wp-image-1677\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/01\/25210840\/Figure_27_01_04.png\" alt=\"This illustration shows the four clusters of Hox genes found in vertebrates: Hox-A, Hox-B, Hox-C, and Hox-D. There are 13 Hox genes, but not all of them are found in each cluster. In both mice and humans, genes 1\u20134 regulate the development of the head. Genes 5 and 6 regulate the development of the neck. Genes 7 and 8 regulate the development of the torso, and genes 9\u201313 regulate the development of the arms and legs.\" width=\"449\" height=\"346\" \/> Figure\u00a01. Shown here is the homology between\u00a0<em data-effect=\"italics\">Hox<\/em>\u00a0genes in mice and humans. Note how\u00a0<em data-effect=\"italics\">Hox<\/em>\u00a0gene expression, as indicated with orange, pink, blue, and green shading, occurs in the same body segments in both the mouse and the human. While at least one copy of each Hox gene is present in humans and other vertebrates, some\u00a0<em data-effect=\"italics\">Hox<\/em>\u00a0genes are missing in some chromosomal sets.[\/caption]\r\n\r\nWhile there are a great many genes that play roles in the morphological development of an animal, including other homeobox-containing genes, what makes\u00a0<em data-effect=\"italics\">Hox<\/em>\u00a0genes so powerful is that they serve as \u201cmaster control genes\u201d that can turn on or off large numbers of other genes.\u00a0<em data-effect=\"italics\">Hox<\/em>\u00a0genes do this by encoding transcription factors that control the expression of numerous other genes.\u00a0<em data-effect=\"italics\">Hox<\/em>\u00a0genes are homologous across the animal kingdom, that is, the genetic sequences of\u00a0<em data-effect=\"italics\">Hox<\/em>\u00a0genes and their positions on chromosomes are remarkably similar across most animals because of their presence in a common ancestor, from worms to flies, mice, and humans (Figure\u00a01).\r\n\r\n<em> Hox<\/em> genes are highly conserved genes encoding transcription factors that determine the course of embryonic development in animals. In vertebrates, the genes have been duplicated into four clusters: <em>Hox-A<\/em>, <em>Hox-B<\/em>, <em>Hox-C<\/em>, and <em>Hox-D<\/em>. Genes within these clusters are expressed in certain body segments at certain stages of development.\r\n\r\nIn addition, the order of the genes reflects the anterior-posterior axis of the animal's body. One of the contributions to increased animal body complexity is that\u00a0<em data-effect=\"italics\">Hox<\/em>\u00a0genes have undergone at least two and perhaps as many as four duplication events during animal evolution, with the additional genes allowing for more complex body types to evolve. All vertebrates have four (or more) sets of\u00a0<em data-effect=\"italics\">Hox<\/em>\u00a0genes, while invertebrates have only one set.\r\n<div class=\"textbox exercises\">\r\n<h3>Practice\u00a0Question<\/h3>\r\nIf a <em>Hox 13<\/em> gene in a mouse was replaced with a <em>Hox 1<\/em> gene, how might this alter animal development?\r\n\r\n[practice-area rows=\"2\"][\/practice-area]\r\n[reveal-answer q=\"319959\"]Show Answer[\/reveal-answer]\r\n[hidden-answer a=\"319959\"]The animal might develop two heads and no tail.[\/hidden-answer]\r\n\r\n<\/div>\r\nTwo of the five clades within the animal kingdom do\u00a0<em data-effect=\"italics\">not<\/em>\u00a0have\u00a0<em data-effect=\"italics\">Hox<\/em>\u00a0genes: the Ctenophora and the Porifera. In spite of the superficial similarities between the Cnidaria and the Ctenophora, the Cnidaria have a number of\u00a0<em data-effect=\"italics\">Hox<\/em>\u00a0genes, but the Ctenophora have none. The absence of\u00a0<em data-effect=\"italics\">Hox<\/em>\u00a0genes from the ctenophores has led to the suggestion that they might be \u201cbasal\u201d animals, in spite of their tissue differentiation. Ironically, the Placozoa, which have only a few cell types, do have at least one\u00a0<em data-effect=\"italics\">Hox<\/em>\u00a0gene. The presence of a\u00a0<em data-effect=\"italics\">Hox<\/em>\u00a0gene in the Placozoa, in addition to similarities in the genomic organization of the Placozoa, Cnidaria and Bilateria, has led to the inclusion of the three groups in a \u201cParahoxozoa\u201d clade. However, we should note that at this time the reclassification of the Animal Kingdom is still tentative and requires much more study.\r\n<div class=\"textbox tryit\">\r\n<h3>Try It<\/h3>\r\nhttps:\/\/assess.lumenlearning.com\/practice\/c88b930b-b836-4054-8786-49f09301d822\r\n<\/div>","rendered":"<div class=\"textbox learning-objectives\">\n<h3>Learning Outcomes<\/h3>\n<ul>\n<li>Describe the roles that Hox genes play in development<\/li>\n<\/ul>\n<\/div>\n<p id=\"fs-idm4491456\">Since the early nineteenth century, scientists have observed that many animals, from the very simple to the complex, shared similar embryonic morphology and development. Surprisingly, a human embryo and a frog embryo, at a certain stage of embryonic development, look remarkably alike! For a long time, scientists did not understand why so many animal species looked similar during embryonic development but were very different as adults. They wondered what dictated the developmental direction that a fly, mouse, frog, or human embryo would take. Near the end of the twentieth century, a particular class of genes was discovered that had this very job. These genes that determine animal structure are called \u201chomeotic genes,\u201d and they contain DNA sequences called homeoboxes. Genes with homeoboxes encode protein transcription factors. One group of animal genes containing homeobox sequences is specifically referred to as\u00a0<strong><span id=\"term1028\" data-type=\"term\"><em data-effect=\"italics\">Hox<\/em>\u00a0genes<\/span><\/strong>. This cluster of genes is responsible for determining the general body plan, such as the number of body segments of an animal, the number and placement of appendages, and animal head-tail directionality. The first\u00a0<em data-effect=\"italics\">Hox<\/em>\u00a0genes to be sequenced were those from the fruit fly (<em data-effect=\"italics\">Drosophila melanogaster<\/em>). A single\u00a0<em data-effect=\"italics\">Hox<\/em>\u00a0mutation in the fruit fly can result in an extra pair of wings or even legs growing from the head in place of antennae (this is because antennae and legs are embryologic homologous structures and their appearance as antennae or legs is dictated by their origination within specific body segments of the head and thorax during development). Now,\u00a0<em data-effect=\"italics\">Hox<\/em>\u00a0genes are known from virtually all other animals as well.<\/p>\n<div id=\"attachment_1677\" style=\"width: 459px\" class=\"wp-caption alignright\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-1677\" class=\"wp-image-1677\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/01\/25210840\/Figure_27_01_04.png\" alt=\"This illustration shows the four clusters of Hox genes found in vertebrates: Hox-A, Hox-B, Hox-C, and Hox-D. There are 13 Hox genes, but not all of them are found in each cluster. In both mice and humans, genes 1\u20134 regulate the development of the head. Genes 5 and 6 regulate the development of the neck. Genes 7 and 8 regulate the development of the torso, and genes 9\u201313 regulate the development of the arms and legs.\" width=\"449\" height=\"346\" \/><\/p>\n<p id=\"caption-attachment-1677\" class=\"wp-caption-text\">Figure\u00a01. Shown here is the homology between\u00a0<em data-effect=\"italics\">Hox<\/em>\u00a0genes in mice and humans. Note how\u00a0<em data-effect=\"italics\">Hox<\/em>\u00a0gene expression, as indicated with orange, pink, blue, and green shading, occurs in the same body segments in both the mouse and the human. While at least one copy of each Hox gene is present in humans and other vertebrates, some\u00a0<em data-effect=\"italics\">Hox<\/em>\u00a0genes are missing in some chromosomal sets.<\/p>\n<\/div>\n<p>While there are a great many genes that play roles in the morphological development of an animal, including other homeobox-containing genes, what makes\u00a0<em data-effect=\"italics\">Hox<\/em>\u00a0genes so powerful is that they serve as \u201cmaster control genes\u201d that can turn on or off large numbers of other genes.\u00a0<em data-effect=\"italics\">Hox<\/em>\u00a0genes do this by encoding transcription factors that control the expression of numerous other genes.\u00a0<em data-effect=\"italics\">Hox<\/em>\u00a0genes are homologous across the animal kingdom, that is, the genetic sequences of\u00a0<em data-effect=\"italics\">Hox<\/em>\u00a0genes and their positions on chromosomes are remarkably similar across most animals because of their presence in a common ancestor, from worms to flies, mice, and humans (Figure\u00a01).<\/p>\n<p><em> Hox<\/em> genes are highly conserved genes encoding transcription factors that determine the course of embryonic development in animals. In vertebrates, the genes have been duplicated into four clusters: <em>Hox-A<\/em>, <em>Hox-B<\/em>, <em>Hox-C<\/em>, and <em>Hox-D<\/em>. Genes within these clusters are expressed in certain body segments at certain stages of development.<\/p>\n<p>In addition, the order of the genes reflects the anterior-posterior axis of the animal&#8217;s body. One of the contributions to increased animal body complexity is that\u00a0<em data-effect=\"italics\">Hox<\/em>\u00a0genes have undergone at least two and perhaps as many as four duplication events during animal evolution, with the additional genes allowing for more complex body types to evolve. All vertebrates have four (or more) sets of\u00a0<em data-effect=\"italics\">Hox<\/em>\u00a0genes, while invertebrates have only one set.<\/p>\n<div class=\"textbox exercises\">\n<h3>Practice\u00a0Question<\/h3>\n<p>If a <em>Hox 13<\/em> gene in a mouse was replaced with a <em>Hox 1<\/em> gene, how might this alter animal development?<\/p>\n<p><textarea aria-label=\"Your Answer\" rows=\"2\"><\/textarea><\/p>\n<div class=\"qa-wrapper\" style=\"display: block\"><span class=\"show-answer collapsed\" style=\"cursor: pointer\" data-target=\"q319959\">Show Answer<\/span><\/p>\n<div id=\"q319959\" class=\"hidden-answer\" style=\"display: none\">The animal might develop two heads and no tail.<\/div>\n<\/div>\n<\/div>\n<p>Two of the five clades within the animal kingdom do\u00a0<em data-effect=\"italics\">not<\/em>\u00a0have\u00a0<em data-effect=\"italics\">Hox<\/em>\u00a0genes: the Ctenophora and the Porifera. In spite of the superficial similarities between the Cnidaria and the Ctenophora, the Cnidaria have a number of\u00a0<em data-effect=\"italics\">Hox<\/em>\u00a0genes, but the Ctenophora have none. The absence of\u00a0<em data-effect=\"italics\">Hox<\/em>\u00a0genes from the ctenophores has led to the suggestion that they might be \u201cbasal\u201d animals, in spite of their tissue differentiation. Ironically, the Placozoa, which have only a few cell types, do have at least one\u00a0<em data-effect=\"italics\">Hox<\/em>\u00a0gene. The presence of a\u00a0<em data-effect=\"italics\">Hox<\/em>\u00a0gene in the Placozoa, in addition to similarities in the genomic organization of the Placozoa, Cnidaria and Bilateria, has led to the inclusion of the three groups in a \u201cParahoxozoa\u201d clade. However, we should note that at this time the reclassification of the Animal Kingdom is still tentative and requires much more study.<\/p>\n<div class=\"textbox tryit\">\n<h3>Try It<\/h3>\n<p>\t<iframe id=\"assessment_practice_c88b930b-b836-4054-8786-49f09301d822\" class=\"resizable\" src=\"https:\/\/assess.lumenlearning.com\/practice\/c88b930b-b836-4054-8786-49f09301d822?iframe_resize_id=assessment_practice_id_c88b930b-b836-4054-8786-49f09301d822\" frameborder=\"0\" style=\"border:none;width:100%;height:100%;min-height:300px;\"><br \/>\n\t<\/iframe>\n<\/div>\n\n\t\t\t <section class=\"citations-section\" role=\"contentinfo\">\n\t\t\t <h3>Candela Citations<\/h3>\n\t\t\t\t\t <div>\n\t\t\t\t\t\t <div id=\"citation-list-1870\">\n\t\t\t\t\t\t\t <div class=\"licensing\"><div class=\"license-attribution-dropdown-subheading\">CC licensed content, Shared previously<\/div><ul class=\"citation-list\"><li>Biology 2e. <strong>Provided by<\/strong>: OpenStax. <strong>Located at<\/strong>: <a target=\"_blank\" href=\"http:\/\/cnx.org\/contents\/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8\">http:\/\/cnx.org\/contents\/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8<\/a>. <strong>License<\/strong>: <em><a target=\"_blank\" rel=\"license\" href=\"https:\/\/creativecommons.org\/licenses\/by\/4.0\/\">CC BY: Attribution<\/a><\/em>. <strong>License Terms<\/strong>: Access for free at https:\/\/openstax.org\/books\/biology-2e\/pages\/1-introduction<\/li><\/ul><\/div>\n\t\t\t\t\t\t <\/div>\n\t\t\t\t\t <\/div>\n\t\t\t <\/section>","protected":false},"author":17,"menu_order":22,"template":"","meta":{"_candela_citation":"[{\"type\":\"cc\",\"description\":\"Biology 2e\",\"author\":\"\",\"organization\":\"OpenStax\",\"url\":\"http:\/\/cnx.org\/contents\/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8\",\"project\":\"\",\"license\":\"cc-by\",\"license_terms\":\"Access for free at https:\/\/openstax.org\/books\/biology-2e\/pages\/1-introduction\"}]","CANDELA_OUTCOMES_GUID":"bb551b62-72e2-4a37-9868-a4b7e0a7a87a, 5c375795-aff9-4a6a-8ea6-76d7bd794dd0","pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":""},"chapter-type":[],"contributor":[],"license":[],"class_list":["post-1870","chapter","type-chapter","status-publish","hentry"],"part":21,"_links":{"self":[{"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/pressbooks\/v2\/chapters\/1870","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/wp\/v2\/users\/17"}],"version-history":[{"count":12,"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/pressbooks\/v2\/chapters\/1870\/revisions"}],"predecessor-version":[{"id":8428,"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/pressbooks\/v2\/chapters\/1870\/revisions\/8428"}],"part":[{"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/pressbooks\/v2\/parts\/21"}],"metadata":[{"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/pressbooks\/v2\/chapters\/1870\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/wp\/v2\/media?parent=1870"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/pressbooks\/v2\/chapter-type?post=1870"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/wp\/v2\/contributor?post=1870"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/wp\/v2\/license?post=1870"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}