{"id":76,"date":"2019-11-08T18:58:21","date_gmt":"2019-11-08T23:58:21","guid":{"rendered":"https:\/\/pressbooks.bccampus.ca\/sfubisc202\/?post_type=chapter&p=76"},"modified":"2022-06-15T13:58:50","modified_gmt":"2022-06-15T17:58:50","slug":"non-mendelian-genes-and-gene-interactions","status":"publish","type":"chapter","link":"https:\/\/pressbooks.bccampus.ca\/sfubisc202\/chapter\/non-mendelian-genes-and-gene-interactions\/","title":{"raw":"Non-Mendelian Inheritance","rendered":"Non-Mendelian Inheritance"},"content":{"raw":"

Non-Mendelian Inheritance<\/h1>\r\nSo far in this course, we have seen that we can predict how traits are passed from parents to offspring using Mendel's Laws of Inheritance (Self test: See if you can define all three, then refresh your memory with the glossary below) Unfortunately, in nature things are seldom so simple, and there are many cases where patterns of inheritance do not follow Mendel's laws.\u00a0In this chapter, we will take a look at several examples of non-Mendelian inheritance.\r\n
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Mendel's Laws of Inheritance<\/strong><\/p>\r\n\r\n<\/header>\r\n

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Non-Mendelian Dominance<\/h3>\r\nIn Mendel's peas, every trait Mendel described exhibited\u00a0complete dominance<\/strong>, meaning one allele would always mask the expression of the other.\u00a0Since Mendel's time, other types of Dominant\/Recessive relationships have been described, including\u00a0Co-Dominance <\/strong>and\u00a0Incomplete Dominance<\/strong>.\r\n

Incomplete Dominance<\/h4>\r\nTo fully understand non-Mendelian modes of dominance, we need to first understand why <\/em>alleles give rise to their associated traits. For example, we know that in Mendel's peas, flower colour is controlled by a single gene with two alleles. One allele (Dominant) produces purple flowers, while the other (Recessive) produces white flowers. Why is this?\r\n\r\nIt turns out that this gene codes for a protein that is needed to produce [pb_glossary id=\"117\"]anthocyanin pigment[\/pb_glossary]. The recessive allele codes for a non-functioning protein, and thus homozygous-recessive peas produce flowers with no anthocyanin pigment at all, giving them their white colour. If a plant has even one functioning copy of the allele, it can produce enough anthocyanin to completely dye its flowers purple, which is why heterozygous pea plants are still purple. This mechanism was described fairly recently, in 2010 (150 years after Mendel first described the phenotype!) If you want to learn more about the specifics of how this gene works, check out this article from New Zealand's Science Learning Hub<\/a>.\r\n\r\nIn this case, one functioning allele produces enough pigment to produce the purple phenotype. What if this wasn't the case? What if a homozygous plant with two copies of the purple allele produced twice as much pigment as a heterozygous plant with only one copy? We will consider this scenario in the following exercise:\r\n\r\n[h5p id=\"24\"]\r\n\r\nThis scenario, where the phenotype that arises from a given set of alleles is\u00a0dose-dependent<\/em>, is an example of i<\/strong>ncomplete dominance<\/strong>, where the recessive allele is not completely masked by the dominant allele. This typically leads to a third phenotype in heterozygotes, which looks like a muted version of the homozygous-dominant phenotype.\r\n

Co-dominance<\/span><\/h4>\r\nCo-dominance occurs when two alleles of the same gene are fully expressed at the same time in a heterozygous organism. This leads to a heterogeneous<\/em> phenotype, where both parental phenotypes are expressed simultaneously and equally. This is distinct from incomplete dominance, where the phenotype of one allele only partially masks the phenotype of the other.\r\n\r\nHypothetical Case Study: Thorny Roses<\/strong><\/em>\r\n\r\nLet's imagine you are a geneticist in charge of breeding roses. The variety of rose you work with has two types of thorns, long (TL<\/sup>) and short (TS<\/sup>):\r\n\r\n\"Two\r\n\r\nLuckily, you are able to acquire pure-breeding lines for each phenotype. You cross these lines to produce F1 offspring, all of which have the following phenotype:\r\n\r\n\"F1\r\n\r\nNote that these F1 offspring both have short and long thorns in equal proportion. You then cross two F1 offspring and note the following phenotype ratios in the F2 offspring:\r\n\r\n\"Monohybrid\r\n\r\nThese offspring ratios tell you a few things about the thorn-size phenotype. Test your understanding using the following exercise:\r\n\r\n[h5p id=\"27\"]\r\n\r\nContrast this scenario with the non-Mendelian pea flowers we discussed above. Heterozygote pea flowers express an intermediate phenotype that resembles a blend of the parental phenotypes. In this scenario, heterozygote roses express both parental phenotypes simultaneously and equally<\/em>. Thus, we can say that the alleles for thorn-size are\u00a0co-dominant\u00a0<\/em><\/strong>to each other.\r\n

Allelic Series<\/h4>\r\nIn all the scenarios we have considered so far, genes always have two alleles. In nature, it is common for genes to have more than two alleles, each with their own associated phenotype. In these cases, we say the gene exhibits multiple allelism<\/strong>.\r\n
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Hypothetical Case Study: Blue-spot Butterflies<\/strong><\/em><\/p>\r\n\r\n<\/header>\r\n

\r\n\r\nImagine you are a geneticist studying the rare blue-spot butterfly (sadly not a real insect). This butterfly has three distinct wing phenotypes: Small, medium, and large.\r\n\r\n<\/div>\r\n<\/div>\r\n<\/header>\r\n
\r\n\r\n\"The\r\n\r\n<\/div>\r\n

Through previous testing, you know that this phenotype is controlled at a single locus. At first glance, this looks like it could be a classic case of incomplete dominance, where heterozygous individuals produce a \"blended\" phenotype. However, while rearing the butterflies in the lab, you realize you can establish\u00a0pure-breeding\u00a0<\/em>lines\u00a0<\/em>of each phenotype! What does this tell you about the system?<\/p>\r\n\r\n<\/div>\r\n[h5p id=\"26\"]\r\n

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Now that you have this new-found knowledge about the wing-size phenotype, you run the following test-crosses on pure-breeding lines of each phenotype:<\/p>\r\n\r\n

\r\n\r\n\"Test\r\n\r\n \r\n\r\nNow this is an odd pattern! Let's try to analyse it one piece of information at a time:\r\n
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  1. The first test cross tells us that WM<\/sup> is dominant over WS<\/sup><\/li>\r\n \t
  2. The second test cross tells us that WL<\/sup> is dominant over WS<\/sup><\/li>\r\n \t
  3. The third test cross tells us that WL<\/sup> is also dominant over WM<\/sup><\/li>\r\n<\/ol>\r\nIt appears that there is a hierarchy of dominance<\/em> between the three alleles, structured as WL<\/sup> > WM<\/sup> > WS<\/sup>.\u00a0What you've described here is an a<\/strong>llelic series<\/strong>, a special case of multiple allelism where there is a set hierarchy for all the alleles involved.\r\n\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n<\/header><\/div>\r\n
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    Summary<\/strong>: Non-Mendelian Dominance<\/h4>\r\n<\/header>\r\n
    \r\n\r\nHere are brief definitions of the three modes of non-Mendelian dominance discussed above\r\n