Chapter 4 Extensions of Mendalism.docx

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Molecular Biology and Genetics
Course Code
MBG 2040
Dennis Baker

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MBG – Chapter 4 Incomplete Dominance and Co-Dominance: When the heterozygote has a different phenotype than the other homozygotes. Semidominant – when the heterozygote’s phenotype is midway between the phenotypes of the 2 homozygotes. As in, red, PINK and white. When a heterozygote shows characteristics found in each of the associated homozygotes. Such as in human blood types, people who are type AB produce antibodies for both the A and B antigens. Codominant because both alleles contribute independently to the phenotype, neither allele is dominant or partially dominant over the other. Multiple Alleles: Some gene has 3 or 4 or more different alleles. Ex: coat colour in rabbits. Also example of human blood types. Allelic Series: Wild-type is completely dominant over all the other alleles, the rest are partially dominant over another, etc. Null allele – non-functional alleles Hypomophic – partially functional alleles, they are recessive to alleles that are more functional usually including the wildtype allele. Why are some mutations dominant and others recessive? Recessive mutations often involve a loss of gene function, when the gene no longer specifies a polypeptide oe when it specifies a non-functional or underfunctional polypeptide. They are thus typically loss-of-function alleles. They usually have little or no effect on heteroxygotes with one wildtype allele that carries out the function. Some dominant mutations can also involve a loss of gene function. If the wildtype allele isn’t enough to keep the levels of polypeptide to have the wildtype phenotype because one allele is mutant. Other dominant mutations actually interfere with the function of the wildtype allele by specifying polypeptides that inhibit, antag
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