Dominant gene interactions

It was shown81 that for a wide variety of gene interactions the mutation component is 1 for a detrimental trait determined solely by balance between mutation and selection. If the trait has a quantitative basis, the principle still holds true. For example, if the disease is expressed only when a particular threshold of the underlying variable is exceeded, the mutation component is still 1. The dominance of the gene also does not matter even a rare recessive disease has a mutation component of 1. 

The dominant-recessive interactions of allelic genes, gene complementation, epistasis, and the modification an interactions of nonallelic genes have been the object of investigations in genetics for a long time. 

As a result of complementary interactions between dominant genes, it is possible for a new character to develop. This new character differs from the traits determined by each gene separately. For instance, in the formation of cyanide in white clover a complementary interaction is necessary between two genes, which is realized on two levels the formation of the linamarin substrate and the synthesis of the linamarase enzyme which converts linamarin into cyanide. Each of the processes is controlled monogenically by the nonallelic dominant genes Li and Ac (Atwood and Sullivan, 1943). 

In some cases the gene modifiers could, apparently, change the dominant-recessive interaction between isozymes (which is regularly of a codominant character). For instance, a gene modifier was found in D. melanogaster which controlled the dominant expression of the slow isozyme of alkaline phosphatase in heterozygous flies. In the F2 a segregation was obtained wherein the ratio of individuals with slow and fast isozymes was equal to 3 1 (Schneiderman et al., 1966). 

We should note once again the general mechanism by which resistance appears in target species is natural selection. Many factors may accelerate or slow this process a growth in frequency or number of resistance alleles their dominance, ability to penetrate, and expressivity their interaction with various genes the speed with which generations are replaced the number of individuals in each generation the character of the reproductive system (sexual or asexual) and a host of other factors [113]. 

A major reason for using merodiploids is to study the interaction between different alleles of the same gene. This often tells us a great deal about how a gene or the gene product functions. The two simplest types of interactions are dominant and recessive. A cell that is z+/Fz behaves like a z+ cell as far as the metabolism of j8-galactosidase is concerned. Therefore, the z+ allele is dominant to the z allele, or conversely, the z allele is recessive to the z+ allele. 

Alleles are classified by how they interact with each other. The red-hair allele is recessive —i.e., one such gene on each member of the chromosome pair is required for the trait to be manifest. The nonred-hair allele, in contrast, is dominant —i.e., a person with either one or two of these genes has nonred hair. It is customary to designate dominant alleles with capital letters and recessive alleles with lower-case letters. If r stands for the red-hair gene and R for its alternative, rr persons will have red hair, and Rr and RR will not. 

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