A heterozygous organism has two different alleles for a given gene. The opposite of a heterozygous organism is a homozygous organism which has two alleles that are the same for that specific gene. Genes can exist in more than one form and the different forms are called alleles. Alleles code for different types of the same characteristic.
Genes are found in the DNA along the chromosomes. In the nucleus, most living things have two chromosomes. Pairs of complementary chromosomes with the gene at the same location are called homologous chromosomes. Genes can exist in many forms, but an organism can have no more than two different alleles of the gene.
The structure and function of cells, and the organism as a whole, are determined by the genes. The genetic combinations, or genotypes, are what establish the possible appearance or behavior of the cell, phenotype. How the different alleles of the gene interact is what determines the phenotype.
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Alleles can be dominant or recessive. If one allele is completely dominant over another, the characteristic for that allele is what will be expressed by the organism whenever a dominant allele is present. Only when you have two recessive alleles present, homozygous recessive, will the recessive characteristic be seen. It is impossible to tell if soemthing is a homozygous or heterozygous organism for a dominant trait without genetic testing.
With complete dominance, one single dominant allele is often as effective as two copies in determining the trait. An organism that is heterozygous will appear the same as if it were homozygous dominant. This is only the case when one allele is completely dominant over other forms. While many characteristics show this model of inheritance, there are also many that do not.
Other organisms show partial, or incomplete, dominance when inheriting characteristics. In this model, the heterozygous organism is different from both homozygous forms. An example of incomplete dominance is clearly shown in snapdragons, where the allele for red is dominant to the one for white. When you cross red snapdragons with white, not only do you get red and white offspring, you can also get pink. The pink offspring are the heterozygous organisms.
With partial dominance, heterozygosity is shown by a phenotype that is intermediate between the two homozygous states. This is clearly shown by the snapdragon example where pink is intermediate to red and white. This is just one example of incomplete dominance, not all show such a distinct intermediate heterozygous state.