{"id":361,"date":"2019-06-24T13:07:17","date_gmt":"2019-06-24T17:07:17","guid":{"rendered":"https:\/\/pressbooks.bccampus.ca\/053humanbiology\/chapter\/5-13-non-mendelian-inheritance\/"},"modified":"2022-03-29T22:25:08","modified_gmt":"2022-03-30T02:25:08","slug":"5-13-non-mendelian-inheritance","status":"publish","type":"chapter","link":"https:\/\/pressbooks.bccampus.ca\/053humanbiology\/chapter\/5-13-non-mendelian-inheritance\/","title":{"raw":"5.14 Non-Mendelian Inheritance","rendered":"5.14 Non-Mendelian Inheritance"},"content":{"raw":"Created by: CK-12\/Adapted by Christine Miller\r\n\r\n \r\n\r\n[h5p id=\"71\"]\r\n\r\nFigure 5.14.1 Collage of Diverse Faces.<\/em>\r\n\r\nThis collage shows some of the variation in human skin colour, which can range from very light to very dark, with every possible gradation in between.\u00a0 As you might expect, the skin color trait has a more complex genetic basis than just one gene with two alleles, which is the type of simple trait that Mendel studied in pea plants. Like skin color, many other human traits have more complicated modes of inheritance than Mendelian traits. Such modes of inheritance are called [pb_glossary id=\"1558\"]non-Mendelian inheritance[\/pb_glossary],<\/strong>\u00a0and they include inheritance of\u00a0multiple allele traits, traits with codominance or incomplete dominance, and\u00a0polygenic traits, among others. All of\u00a0these modes\u00a0are described below.\r\n
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Codominance<\/h1>\r\n<\/div>\r\nYour ABO blood type is determined by certain cell surface markers called antigens which are found on your red blood cells. The alleles for blood type determine which antigens your body produces. You may have heard of A type, B type and AB type blood. Alleles A and B\u00a0 are neither [pb_glossary id=\"1452\"]dominant[\/pb_glossary] nor [pb_glossary id=\"1605\"]recessive[\/pb_glossary] to one another. Instead, they are codominant. [pb_glossary id=\"1863\"]Codominance[\/pb_glossary]<\/strong> occurs when two alleles for a gene are expressed equally in the phenotype of [pb_glossary id=\"1492\"]heterozygotes[\/pb_glossary]. In the case of blood type, AB heterozygotes have a unique phenotype, with both A and B antigens in their blood (type AB blood).\r\n

Multiple Allele Traits<\/h1>\r\n<\/div>\r\n\r\n[caption id=\"attachment_2681\" align=\"alignright\" width=\"382\"]\"ABO Figure 5.14.2 ABO blood types per genotype. <\/em>[\/caption]\r\n\r\nThe majority of human genes are thought to have more than two normal versions, or\u00a0alleles. Traits controlled by a single gene with more than two alleles are called\u00a0[pb_glossary id=\"1542\"]multiple allele traits[\/pb_glossary]<\/strong>. ABO blood type gives us an example of one of these multiple allele traits. There are three common alleles for this trait, A and B, which are codominant, as described above, and O type. The O type allele does not produce any antigens at all, so it is considered to be recessive to the other two alleles.\r\n\r\nWe indicate complete dominance with symbols that show that we are considering different version of the same gene by using upper and lower case of the same letter, such as D and d. We can similarly use our choice of allele symbols to indicate other types of dominance relationships between alleles. Since the three alleles for blood type are all version of the same gene, the common allele to show this is the letter I. Since the O type allele is recessive, it is usually indicated by the symbol i. The A and B type are both dominant, so they are I, and the versions of the codominant alleles are described by a superscript: IA<\/sup> or IB<\/sup>.\r\n\r\nAs shown in the table there are six possible ABO genotypes, because the three alleles, taken two at a time, result in six possible combinations. The A and B alleles are dominant to the O allele. As a result, both IA<\/sup>IA<\/sup> and IA<\/sup>i genotypes have the same phenotype, with the A antigen in their blood (type A blood). Similarly, both IB<\/sup>IB <\/sup>and IB<\/sup>i genotypes have the same phenotype, with the B antigen in their blood (type B blood). No antigen is associated with the O allele, so people with the ii genotype have no antigens for ABO blood type in their blood (type O blood).\r\n
<\/div>\r\nIncomplete Dominance<\/span>\r\n\r\nAnother relationship that may occur between alleles for the same gene is\u00a0[pb_glossary id=\"1510\"]incomplete dominance[\/pb_glossary].<\/strong>\u00a0This occurs when the dominant allele is not completely dominant. In this case, an intermediate phenotype results in heterozygotes who inherit both alleles. Generally, this happens when the two alleles for a given gene both produce\u00a0proteins, but one\u00a0protein\u00a0is not functional. As a result, the heterozygote individual produces only half the amount of normal protein as is produced by an individual who is homozygous for the normal allele.\r\n\r\nAn example of incomplete dominance in humans is Tay Sachs disease<\/a>. The normal allele for the gene in this case produces an\u00a0[pb_glossary id=\"1345\"]enzyme[\/pb_glossary]\u00a0that is responsible for breaking down\u00a0[pb_glossary id=\"1292\"]lipids[\/pb_glossary]. A defective allele for the gene results in the production of a nonfunctional enzyme. Heterozygotes who have one normal and one defective allele produce half as much functional enzyme as the normal homozygote, and this is enough for normal\u00a0development. Homozygotes who have only defective allele, however, produce only nonfunctional enzyme. This leads to the accumulation of lipids in the brain\u00a0starting\u00a0in utero<\/em>, which causes significant brain damage. Most individuals with Tay Sachs disease die at a young age, typically by the age of five years.\r\n\r\n \r\n\r\n[caption id=\"attachment_360\" align=\"aligncenter\" width=\"551\"]\"5.14.2 Figure 5.14.3 Three phenotypes of hair through the incomplete dominance model.<\/em>[\/caption]\r\n\r\nAnother good example of incomplete dominance in humans is hair type.\u00a0 There are genes for straight and curly hair, and if an individual is heterozygous, they will typically have the phenotype of wavy hair.\r\n
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Polygenic Traits<\/h1>\r\n<\/div>\r\n\r\n[caption id=\"attachment_360\" align=\"alignleft\" width=\"500\"]\"Like Figure 5.14.4 Human Adult Height. Like many other polygenic traits, adult height has a bell-shaped distribution.<\/em>[\/caption]\r\n\r\nMany human traits are controlled by more than one gene. These traits are called\u00a0[pb_glossary id=\"1590\"]polygenic traits[\/pb_glossary]<\/strong>. The alleles of each gene have a minor additive effect on the phenotype. There are many possible combinations of alleles, especially if each gene has multiple alleles. Therefore, a whole continuum of phenotypes is possible.\r\n\r\nAn example of a human polygenic trait is adult height. Several genes, each with more than one allele, contribute to this trait, so there are many possible adult heights. One adult\u2019s height might be 1.655 m (5.430 feet), and another adult\u2019s height might be 1.656 m (5.433 feet). Adult height ranges from less than 5 feet to more than 6 feet, with males, on average, being somewhat taller than females. The majority of people fall near the middle of the range of heights for their sex, as shown in Figure 5.14.4.<\/span>\r\n
\r\n\r\nEnvironmental Effects on Phenotype<\/span>\r\n\r\n<\/div>\r\n\r\n[caption id=\"attachment_360\" align=\"alignright\" width=\"236\"]\"Image Figure 5.14.5 Due to the effects of UV radiation, the skin on the upper part of the arm is much darker in color than the\u00a0 skin that was protected by a watch strap.<\/em>[\/caption]\r\n\r\nMany traits are affected by the environment, as well as by genes. This may be especially true for polygenic traits. Adult height, for example, might be negatively impacted by poor diet or childhood illness. Skin color is another polygenic trait. There is a wide range of skin colors in people worldwide. In addition to differences in genes, differences in exposure to ultraviolet (UV) light cause some variation. As shown in Figure 5.14.5, exposure to UV light darkens the skin.\r\n
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Pleiotropy<\/h1>\r\n<\/div>\r\nSome genes affect more than one phenotypic trait. This is called\u00a0[pb_glossary id=\"1586\"]pleiotropy[\/pb_glossary]<\/strong>. There are numerous examples of pleiotropy in humans. They generally involve important\u00a0proteins\u00a0that are needed for the normal\u00a0development\u00a0or functioning of more than one organ system. An example of\u00a0pleiotropy in humans occurs with the gene that codes for the main\u00a0protein\u00a0in collagen, a substance that helps form\u00a0bones. This protein is also important in the ears and\u00a0eyes.\u00a0Mutations\u00a0in the gene result in problems not only in bones, but also in these sensory organs, which is how the gene's pleiotropic effects were discovered.\r\n\r\nAnother example of pleiotropy occurs with sickle cell anemia<\/a>. This recessive genetic disorder occurs when there is a mutation in the gene that normally encodes the red blood cell [pb_glossary id=\"1373\"]protein[\/pb_glossary] called hemoglobin. People with the disorder have two alleles for sickle cell hemoglobin, so named for the sickle shape (pictured in Figure 5.14.6) that their red blood cells take on under certain conditions (like physical exertion). The sickle-shaped red blood cells clog small blood vessels, causing multiple phenotypic effects, including stunting of physical growth, certain bone deformities, kidney failure, and strokes.\r\n\r\n[caption id=\"attachment_360\" align=\"aligncenter\" width=\"385\"]\"Image Figure 5.14.6 For comparison, the sickle-shaped red blood cell on the left is shown next to several normal red blood cells.<\/em>[\/caption]\r\n\r\n
\r\n\r\nEpistasis<\/span>\r\n\r\n<\/div>\r\nSome genes affect the expression of other genes. This is called\u00a0[pb_glossary id=\"1462\"]epistasis[\/pb_glossary]<\/strong>. Epistasis is similar to dominance, except that it occurs between different genes, rather than between different alleles for the same gene.\r\n\r\nAlbinism is an example of epistasis. A person with albinism has virtually no pigment in the skin. The condition occurs due to an entirely different gene than the genes that encode skin\u00a0color. Albinism occurs because a protein called tyrosinase, which is needed for the\u00a0production of normal skin pigment, is not produced, due to a gene\u00a0[pb_glossary id=\"2105\"]mutation[\/pb_glossary].\u00a0If an individual has the albinism\u00a0mutation, he or she will not have any skin pigment, regardless of the skin color genes that were inherited.\r\n
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Feature: My\u00a0Human Body<\/h3>\r\n<\/div>\r\nDo you know your ABO blood type? In an emergency, knowing this valuable piece of information could possibly save your life. If you ever need a blood transfusion, it is vital that you receive blood that matches your own blood type. Why? If the blood transfused into your body contains an antigen that your own blood does not contain, antibodies in your blood\u00a0plasma\u00a0(the\u00a0liquid\u00a0part of your blood) will recognize the antigen as foreign to your body and cause a reaction called agglutination. In this reaction, the transfused red blood\u00a0cells will clump together. The agglutination reaction is serious and potentially fatal.\r\n\r\nKnowing the antigens and antibodies present in each of the ABO blood types will help you understand which type(s) of blood you can safely receive if you ever need a transfusion. This information is shown in Figure 5.14.7 for all of the ABO blood types. If you have blood type A, this means that your red blood cells have the A antigen and that your blood plasma contains anti-B antibodies. If you were to receive a transfusion of type B or type AB blood, both of which have the B antigen, your anti-B antibodies would attack the transfused red blood cells, causing agglutination.\r\n\r\n \r\n\r\n[caption id=\"attachment_360\" align=\"aligncenter\" width=\"553\"]\"Image Figure 5.14.7 Antigens and antibodies in ABO blood types.<\/em>[\/caption]\r\n\r\n
\r\n\r\n \r\n\r\n<\/div>\r\nYou may have heard that people with blood type O are called \"universal donors,\" and that people with blood type AB are called universal recipients. People with type O blood have neither A nor B antigens in their blood, so if their blood is transfused into someone with a different ABO blood type, it causes no immune reaction, meaning they can donate blood to anyone. On the other hand, people with type AB blood have no anti-A or anti-B antibodies in their blood, so they can receive a transfusion of blood from anyone. Which blood type(s) can safely receive a transfusion of type AB blood, and which blood type(s) can be safely received by those with type O blood?\r\n
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5.14 Summary<\/span><\/h1>\r\n<\/header>\r\n
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