Genotype and Phenotype

Differences in phenotype can result from differences in genotype. Two individuals may have different bases at a particular location in the coding region of a gene. While this will result in different codons in that part of the sequence, the two different codons may code for the same amino acid, which will in turn result in the same protein. Thus it can be said that the two individuals have different genotypes in their DNA, but that their phenotypes are identical. This example illustrates the first factor one must look for in predicting phenotypes: how the genotype affects the translation of DNA.

The second factor is the inheritance pattern of the genotype. If each parent transmits an identical gene sequence, A, to a child, then the genotype of the offspring will be AA. This is called a homozygous genotype. However, if the child inherits a different allele, a, from one of the parents, then its resulting genotype, Aa, contains two different sequences. Such genotypes are called heterozygous.

An Aa genotype can result in the same phenotype as either an AA or aa genotype, if one of the alleles acts in a dominant fashion. If the A allele is dominant over the a allele, then the phenotype of a heterozygous (Aa) individual will be the same as the phenotype of a homozygous dominant (AA) individual.

Huntington's disease, a nervous system disorder, follows a dominant inheritance pattern. The presence of one mutated Huntington gene will result in the conditions of the disease. Even if a patient has one normal Huntington gene on another chromosome, one mutated copy is enough to produce Huntington's disease. Thus, the Huntington allele is said to be dominant over the normal allele.

Another type of inheritance pattern is called recessive. Here, two copies of the mutant allele, a, must be inherited (resulting in genotype aa) to cause a change in phenotype. This is exemplified by cystic fibrosis, a recessive disorder marked by digestive and respiratory problems. In this case a heterozygous individual (genotype Aa) who carries one copy of the faulty gene will not display clinical symptoms, because the normal gene is dominant over the CFTR gene. The normal gene is able to express the protein that epithelial cells need for transporting chloride. If two parents are heterozygous for the mutated cystic fibrosis gene, there is a 25 percent chance that their child will inherit a mutated copy from each parent and will have the disease. Heterozygous carriers, who live normal lives, pass on the mutant gene to half of their children, enabling it to stay within the population for generations and to persist at relatively high frequencies.

Although it is convenient to illustrate inheritance concepts by talking about diseases, if we consider the diversity of phenotypes expressed by all organisms in the living world it is obvious that not all variation is bad and most genetic changes do not lead to disease.