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The three genotypes in the question AA, Aa, and aa are different combinations of alleles for a gene corresponding to a specific location or locus, on a specific chromosome. All diploid, sexually reproducing organisms will have two of each allele because two parents are needed to produce progeny by the process of fertilization. Each gamete (sex cell) contains one set of chromosomes to pass on to the offspring. Thus, the offspring has two sets of chromosomes and two copies of each allele. At the molecular level, a particular allele is a specific sequence of nitrogenous bases in the nucleotides of a person's DNA. Allele A and a differ in the sequence of nitrogenous bases slightly. A person who is AA has two identical copies at the molecular level of the gene A. The same idea applies to the individual who is aa -they will have two identical copies of the base sequence for allele a. The person who is Aa has one copy of the sequence of bases for allele A and another copy of the alternate sequence of bases for allele a. The homozygous AA and heterozygous Aa genotypes will both exhibit a phenotype coded for by allele A. In the heterozygote Aa, the allele a is recessive and remains hidden and is not expressed in the phenotype. The homozygote aa however, will express the a allele's associated phenotype because there is not a copy of the dominant gene to overshadow it.
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