Only children who inherit two copies of the __ allele (homozygotes) have the disease. Thus, at the __ level, this disease's allel qualifies as recessive. However, the activity level of the lipid- metabolizing enzyme in heterozygotes is intermediate between that in individuals homozygous for the normal allele and that in individuals with this disease. The intermediate phenotype observed at the __ level is characteristic of incomplete dominance of either allele. Fortunately, the heterozygote condition does not lead to disease symptoms, apparently because 1/2 the normal enzyme activity is sufficient to prevent lipid accumulation in the brain. Extending our analysis to yet another level, we find that heterozygous individuals produce equal numbers of normal and dysfunctional enzyme molecules. Thus at the __ level, the normal allele and the __ allele are __. As you can see, whether alleles appear to be completely __, __, or __ depends on the level at which the phenotype is analyzed.
- incompletely dominant