Genotypes and phenotypes

Considering the alleles of a gene present in an organism and the physical results, brings us to the terms genotype, phenotype, and trait. An organism's genotype is its specific combination of alleles for a given gene. So, for example, in the pea plants above, the possible genotypes for the flower-color gene were red-red, red-white, and white-white. The phenotype is the physical manifestation of an organism's allellic combination (genotype). For the pea plants, if the red allele is dominant and the white allele is recessive, only two phenotypes are possible. Both the plants with red-red and red-white genotypes will have the red phenotype, while the plants with the white-white genotype will have the white phenotype. A trait is the general aspect of physiology being shown in the phenotype. So, for example, the trait being discussed in this paragraph is the flower-color of the pea plant. The phenotype can be either red or white flower color, depending on the genotype.

Genotype and phenotype can also be demonstrated using the human examples from above. For cystic fibrosis, people with the normal-normal or normal-mutant genotypes have the normal phenotype, while people with the mutant-mutant genotype have the disease phenotype. For Huntington's, people with the normal-normal genotype have the normal phenotype, while people with the normal-mutant or mutant-mutant phenotypes develop the diseased phenotype.

Often, organisms will carry a dominant and a recessive allele of a gene. These organisms can be referred to as carriers of the recessive allele. For example, if a pea plant has a red flower-color allele and a white flower-color allele, then it is a carrier of the recessive white flower-color allele. In humans, people who have both a normal and a mutant allele for the CF gene are carriers for the mutant recessive CF allele. While carriers have the dominant/recessive genotype for a given gene, they only show the phenotype caused by the dominant version of that gene.

If someone has the phenotype caused by the "disease" allele of a gene, we say they are affected. People who do not have that phenotype are unaffected. So, for example, in a family of four where the mutant CF gene is present, the terminology would go as follows:

    Individual      Genotype       Phenotype
    ----------      ---------      ---------
      mother       normal-mutant   Unaffected (carrier)
      father       normal-mutant   Unaffected (carrier)
      child 1      normal-normal   Unaffected
      child 2      mutant-mutant   Affected

Remember that the key is that affected people show the disease phenotype. The term affected does not dictate whether a person has one or two bad alleles. The number of bad alleles needed for the diseased phenotype to manefest, depends on the dominant-recessive relationship of the alleles for the gene in question.

Organisms can be homozygous or heterozygous for a gene. Homozygous means that the organism has two copies of the same allele for a gene. An organism can be homozygous dominant, if it carries two copies of the same dominant allele, or homozygous recessive, if it carries two copies of the same recessive allele. Heterozygous means that an organism has two different alleles of a gene. For example, pea plants can have red flowers and either be homozygous dominant (red-red), or heterozygous (red-white). If they have white flowers, then they are homozygous recessive (white-white). Carriers are always heterozygous. People with CF are homozygous recessive. Since Huntington's disease is autosomal dominant, people with the disease can be either homozygous dominant or heterozygous.