Genotype

The term genotype was introduced in the developing field of genetics following the rediscovery and further application of Gregor Mendel's work during the early 1900s. The genotype is the genetic constitution or makeup of an individual. It can be used to refer to the overall assemblage of genes that an individual possesses at all genetic loci, the positions occupied by genes on a chromosome, or more typically, in the specific sense to refer to the genetic constitution at a precise genetic locus in an individual. In the second usage, we refer to the alleles, the alternate forms of a gene, which are present at a specific locus on each member of the chromosome pair on which the gene is located. The introduction of new genetic variation in the base pair structure of the alleles is the result of the process of mutation. Mutations, or mutant alleles, are alterations in the genetic material of plants and animals that can have positive, deleterious, or neutral change effects. Epidemiological studies and medical genetics typically focus on mutations having deleterious or disease-causing effects.

The genetics of the ABO blood group is familiar to most people and provides a useful tool for explaining genotype in relation to phenotype. There are four phenotypic expressions in this trait. Some people have type O blood because their blood does not react with the antibodies for either A or B antigens, the proteins on the red blood cells that cause antibodies to be made. Other individuals have type A or B blood because their blood reacts with antibodies for the antigens in these respective blood types. Finally, some people react to both type A and B antibodies and are deemed type AB. At the genetic locus for ABO on the chromosome number 9 (Chr 9) pair, there are a number of possible pair combinations of alleles A, B, and O that yield any one of six genotypes that account for the four phenotypic expressions. Type AB individuals have the genotype AB, which means on one member of the Chr 9 pair, they have an A allele, and on the other, a B allele. Type O individuals are genotypically OO (an O allele on each Chr 9). People with type A blood may be genotypically AA or AO, as type A is a dominant trait with respect to O. Type B is also dominant, yielding two possible genotypes, BB or BO. A and B are said to be codominant, as both traits are expressed in individuals with this genotype.

Some genetic disorders occur when an individual inherits a single disease-causing allele. These disorders are said to exhibit dominance. For most of these disorders, the disease gene is located on one of the autosomal, or non-sex-determining, chromosomes, and the disorders are labeled autosomal dominant (AD). This means that the mutant allele needs to be located on only one member of the chromosome pair for the disorder to occur (assuming no other complications that sometimes occur in the expression of the disease). Achondroplasia, a genetic disorder causing short stature predominantly due to problems converting cartilage tissue to bone, is an example of a disorder that is inherited in an AD fashion. This disorder is caused by a mutation in the fibroblast growth factor receptor 3gene.

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