The terms "dominant" and "recessive" imply some competitive interaction between alleles over which one will control the phenotype. This is not the case, however. Alleles do not interact in the nucleus. Instead, both alleles are expressed (in most cases), and the phenotype reflects the result. How, then, can one allele determine the phenotype to the exclusion of another?
Genes code for proteins, and their effect on the phenotype is through the proteins they create. By analyzing the quantity and characteristics of protein produced from each allele, it has been shown that, in many cases, recessive alleles code for defective proteins, or for very low levels of protein. If the other allele codes for normal functional protein, and if the organism can make do with half the normal level of protein (or can increase production from the normal allele), the defective allele will have no effect on the phenotype, and will therefore act in a recessive manner. This type of allelic change is called a loss-of-function mutation. This is the case with the albino allele. This allele codes for a nonfunctional enzyme in the path-
way that produces the pigment melanin. Even with only one functioning allele, the organism can still make enough melanin to obtain normal skin pigmentation. Thus, the functional allele will appear to be dominant. dominant controlling the phenotype when one
Alleles coding for defective protein can act in a dominant manner in allele is present other situations, however. If the resulting protein is not properly regulated by other cell components, it may perform actions that harm the cell. This is called a toxic-gain-of-function mutation. Huntington's disease is thought to be due to a toxic-gain-of function mutation, although it is not yet clear what the exact toxic mechanism is.
A defective protein can also have a dominant effect if its absence cannot be compensated for by the other functioning allele (this is called a dominant negative effect). This may occur when half the normal protein level is insufficient for normal function (a condition called haploin-sufficiency), or when the protein forms part of a multiprotein complex, which is therefore defective in its entirety. Examples include a variety of human collagen-structure disorders. Collagen is the most abundant and important structural protein in the body, and is critical for bone formation and growth. Defects in one of the subunits cause a variety of dominant disorders termed "osteogenesis imperfecta."
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