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Genes exist in different forms called alleles. Homozygous organisms have two identical alleles, while heterozygous organisms have two different alleles. Genotypes determine the phenotype, which is influenced by how different alleles interact. Dominant alleles are expressed over recessive alleles, while incomplete dominance results in an intermediate phenotype. The snapdragon is an example of incomplete dominance.
A heterozygous organism has two different alleles for a given gene. The opposite of a heterozygous organism is a homozygous organism which has two identical alleles for that specific gene. Genes can exist in more than one form and the different forms are called alleles. Allele code for different types of the same trait.
Genes are found in DNA along chromosomes. In the nucleus, most living things have two chromosomes. The pairs of complementary chromosomes with the gene in the same location are called homologous chromosomes. Genes can exist in many forms, but an organism cannot have more than two different gene alleles.
The structure and function of cells and of the organism as a whole are determined by genes. The genetic combinations, or genotypes, are what establish the possible appearance or behavior of the cell, the phenotype. How different alleles of the gene interact is what determines the phenotype.
Alleles can be dominant or recessive. If one allele is completely dominant over another, the characteristic of that allele is what will be expressed by the organism whenever a dominant allele is present. Only when two recessive alleles are present, homozygous recessive, will the recessive characteristic be seen. It’s impossible to tell whether something is a homozygous or heterozygous organism for a dominant trait without genetic testing.
With complete dominance, a single dominant allele is often as effective as two copies in determining the trait. A heterozygous organism will appear as if it is homozygous dominant. This is only the case when one allele is completely dominant over other forms. While many characteristics exhibit this pattern 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 over that for white. When you cross red and white snapdragons, you not only get red and white offspring, but you can also get a rose. The pink offspring are the heterozygous organisms.
With partial dominance, heterozygosity is shown by a phenotype intermediate between the two homozygous states. This is clearly shown by the example of the snapdragon where pink is intermediate between red and white. This is just one example of incomplete dominance, not all show such a distinct intermediate heterozygous state.
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