Genetics as it applies to evolution, molecular biology, and medical aspects.
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Essentially the dominant allele is the one that codes for a protein that will be seen or detected if only one copy of that allele is present. Recessive alleles may not make a product or the product they make is not seen or detected until both recessive alleles for that locus (spot) are present. In many cases both alleles are expressed and contribute to the trait, but only one is actually recognized as being the dominant one because the protein it codes for is most easily detected.
For example, the PTC locus- being able to taste bitter on the PTC strips can be said to have a dominant/recessive relationship because the allele that allows people to taste bitter shows up 75% of the time in the population. The recessive allele still works, it just does not allow a person to discern that specific taste. Heterozygous individuals still taste bitter, but not as strongly as those who are homozygous for the bitter taste allele. The difference between the two alleles is one nucleotide. While people like to put a huge emphasis on dominant/recessive, as more is understood about the actual genetic code, we will find there to be different terms to describe how often specific alleles are "seen".
The loci used to identify people do not code for any proteins that we can actually see, yet we use these regions to test for criminals, parents, or other human specific situations. None of the alleles at these sites are dominant or recessive- but with population studies, they are detected at specific frequencies which leads to a mathematical determination of how often a specific pattern of alleles shows up in a population. As we learn more about dominant/ recessive relationships, they may become described in terms of allele frequencies with the dominant one just occurring more often in the population.
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