{"id":4516,"date":"2019-06-24T12:56:43","date_gmt":"2019-06-24T12:56:43","guid":{"rendered":"https:\/\/pressbooks.ccconline.org\/acchumanbio\/chapter\/5-2-chromosomes-and-genes-3\/"},"modified":"2023-11-30T17:56:06","modified_gmt":"2023-11-30T17:56:06","slug":"5-2-chromosomes-and-genes-3","status":"publish","type":"chapter","link":"https:\/\/pressbooks.ccconline.org\/acchumanbio\/chapter\/5-2-chromosomes-and-genes-3\/","title":{"raw":"5.2\u00a0Chromosomes and Genes","rendered":"5.2\u00a0Chromosomes and Genes"},"content":{"raw":" \r\n

Identical Twins, Identical Genes<\/h1>\r\n[caption id=\"attachment_2358\" align=\"alignnone\" width=\"400\"]\"\" Figure 5.2.1 Identical twins share the same DNA since they came from a single zygote.<\/em>[\/caption]\r\n\r\nYou probably can tell by their close resemblance that these two young ladies are identical twins (Figure 5.2.1). Identical twins develop from the same fertilized egg, so they inherit copies of the same [pb_glossary id=\"5619\"]chromosomes[\/pb_glossary] and have all the same genes. Unless you have an identical twin, no one else in the world has exactly the same [pb_glossary id=\"5521\"]genes[\/pb_glossary] as you. What are genes? How are they related to chromosomes? And how do genes make you the person you are? Let's find out!\r\n
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Introducing\u00a0Chromosomes and Genes<\/h1>\r\n<\/div>\r\n[caption id=\"attachment_2110\" align=\"alignright\" width=\"424\"]\"\" Figure 5.2.2 Human male karyotype. There are 23 pairs of chromosomes per cell. The chromosomes in a pair are known as [pb_glossary id=\"5609\"]homologous chromosomes[\/pb_glossary].<\/em>[\/caption][pb_glossary id=\"5619\"]Chromosomes[\/pb_glossary]<\/strong>\u00a0are coiled structures made of\u00a0[pb_glossary id=\"277\"]DNA[\/pb_glossary]\u00a0and\u00a0[pb_glossary id=\"5813\"]proteins[\/pb_glossary]. They are encoded with genetic instructions for making\u00a0[pb_glossary id=\"519\"]RNA[\/pb_glossary]\u00a0and [pb_glossary id=\"5813\"]proteins[\/pb_glossary]. These instructions are organized into units called\u00a0[pb_glossary id=\"5521\"]genes[\/pb_glossary]<\/strong>. There may be hundreds (or even thousands!) of genes on a single chromosome.\u00a0Genes are segments of\u00a0DNA\u00a0that code for particular pieces of\u00a0RNA. Once formed, some RNA molecules go on to act as blueprints for building\u00a0proteins, while other RNA molecules help regulate various processes inside the cell.\u00a0Some regions of\u00a0DNA\u00a0do not code for\u00a0RNA\u00a0and serve a regulatory function, or have\u00a0no known function.\r\n
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Human Chromosomes<\/h1>\r\n<\/div>\r\nEach species is characterized by a set number of chromosomes. Humans cells normally have two sets of chromosomes in each of their cells, one set inherited from each parent. Because chromosomes occur in pairs, these cells are called [pb_glossary id=\"2103\"]diploid[\/pb_glossary] or 2N. There are 23 chromosomes in each set, for a total of 46 chromosomes per diploid cell. Each chromosome in one set is matched by a chromosome of the same type in the other set, so there are 23 pairs of chromosomes per cell. Each pair consists of chromosomes of the same size and shape, and they also contain the same genes. The chromosomes in a pair are known as [pb_glossary id=\"5609\"]homologous chromosomes[\/pb_glossary]<\/strong>.\r\n
\r\n\r\nAll human cells (except gametes, which are sperm and egg cells) have the 23 pairs of chromosomes as shown in Figure 5.2.2.\r\n\r\n \r\n\r\n<\/div>\r\nhttps:\/\/www.youtube.com\/watch?v=veB31XmUQm8&feature=youtu.be\r\n

Secrets of the X chromosome - Robin Ball, TED-Ed, 2019.<\/p>\r\n\r\n

Autosomes<\/h2>\r\nOf the 23 pairs of human chromosomes, 22 pairs are called autosomes (pairs 1-22 in the Figure 5.2.2), or autosomal chromosomes. [pb_glossary id=\"2112\"]Autosomes[\/pb_glossary]<\/strong> are chromosomes that contain genes for characteristics that are unrelated to biological sex. These chromosomes are the same in males and females. The great majority of human genes are located on autosomes.\r\n

Sex Chromosomes<\/h2>\r\n[caption id=\"attachment_2114\" align=\"alignleft\" width=\"336\"]\"Image Figure 5.2.3 The X and Y chromosomes, also known as the sex chromosomes, determine the biological sex of an individual.<\/em>[\/caption]\r\n\r\nThe remaining pair of human chromosomes consists of the\u00a0sex chromosomes<\/strong>, X and Y (Pair 23 in Figure 5.2.2 and in Figure 5.2.3). Females have two X chromosomes, and males have one X and one Y chromosome. In females, one of the X chromosomes in each cell is inactivated and known as a [pb_glossary id=\"5477\"]Barr body<\/strong>[\/pb_glossary]. This ensures that females, like males, have only one functioning copy of the X chromosome in each cell.\r\n\r\nAs you can see from Figure 5.2.3, the X chromosome is much larger than the Y chromosome. The X chromosome has about two thousand genes, whereas the Y chromosome has fewer than 100, none of which is essential to survival. Virtually all of the X chromosome genes are unrelated to sex. Only the Y chromosome contains genes that determine sex. A single Y chromosome gene, called SRY (which stands for sex-determining region Y gene), triggers an embryo to develop into a male. Without a Y chromosome, an individual develops into a female, so you can think of female as the default sex of the human species.\r\n
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Human Genes<\/h1>\r\n<\/div>\r\nHumans have an estimated 20 thousand to 22 thousand genes. This may sound like a lot, but it really isn\u2019t. Far simpler species have almost as many genes as humans. However, human cells use splicing and other processes to make multiple proteins from the instructions encoded in a single gene. Only about 25 per cent of the nitrogen base pairs of DNA in human chromosomes make up genes and their regulatory elements. The functions of many of the other base pairs are still unclear, but with more time and research their roles may become understood.\r\n\r\nThe majority of human genes have two or more possible versions, called\u00a0[pb_glossary id=\"5449\"]alleles[\/pb_glossary]<\/strong>. Differences in\u00a0alleles\u00a0account for the considerable\u00a0genetic variation\u00a0among people. In fact, most human genetic variation is the result of differences in individual DNA\u00a0base\u00a0pairs within alleles.\r\n
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Linkage<\/h1>\r\n<\/div>\r\nGenes that are located on the same chromosome are called\u00a0[pb_glossary id=\"2120\"]linked genes[\/pb_glossary]<\/strong>. Linkage explains why certain characteristics are frequently inherited together. For example, genes for hair colour and eye colour are linked, so certain hair and eye colours tend to be inherited together, such as dark hair with dark eyes and blonde hair with blue eyes. Can you think of other human traits that seem to occur together? Do you think they might be controlled by linked genes?\r\n\r\nGenes located on the sex chromosomes are called\u00a0[pb_glossary id=\"2121\"]sex-linked genes[\/pb_glossary]<\/strong>. Most sex-linked genes are on the X chromosome, because the Y chromosome has relatively few genes. Strictly speaking, genes on the X chromosome are\u00a0[pb_glossary id=\"5523\"]X-linked genes[\/pb_glossary]<\/strong>, but the term sex-linked is often used to refer to them. The diagram below is called a linkage map: a linkage map shows the locations of specific genes on a chromosome. The linkage map below (Figure 5.2.4) shows the locations of a few of the genes on the human X chromosome.\r\n\r\n[h5p id=\"490\"]\r\n\r\nFigure 5.2.4 Linkage Map for the Human X Chromosome. This linkage map shows the locations of several genes on the X chromosome. Some of the genes code for normal proteins. Others code for abnormal proteins that lead to genetic disorders.<\/em>\r\n
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5.2 Summary<\/span><\/h1>\r\n<\/header>\r\n
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