A genotype is an organism’s complete set of heritable genes, or genes that can be passed down from parents to offspring. These genes help encode the characteristics that are physically expressed in an organism, such as hair color, height, etc. The term was coined by the Danishbotanist, plant physiologist and geneticistWilhelm Johannsen in 1903. The genotype is one of three factors that determine phenotype, along with inherited epigenetic factors and non-inherited environmental factors. Not all organisms with the same genotype look or act the same way because appearance and behavior are modified by environmental and growing conditions. Likewise, not all organisms that look alike necessarily have the same genotype. One's genotype differs subtly from one's genomic flash card sequence, because it refers to how an individual differs or is specialized within a group of individuals or a species. So, typically, one refers to an individual's genotype with regard to a particular gene of interest and the combination of alleles the individual carries. Genotypes are often denoted with letters, for example Bb, where B stands for one allele and b for another. Somatic mutations that are acquired rather than inherited, such as those in cancers, are not part of the individual's genotype. Hence, scientists and physicians sometimes talk about the genotype of a particular cancer, that is, of the disease as distinct from the diseased. An example of a characteristic determined by a genotype is the petal color in a pea plant. The collection of all genetic possibilities for a single trait are called alleles; two alleles for petal color are purple and white.
Phenotype
Any given gene will usually cause an observable change in an organism, known as the phenotype. The terms genotype and phenotype are distinct for at least two reasons:
To distinguish the source of an observer's knowledge.
Genotype and phenotype are not always directly correlated. Some genes only express a given phenotype in certain environmental conditions. Conversely, some phenotypes could be the result of multiple genotypes. The genotype is commonly mixed up with the phenotype which describes the end result of both the genetic and the environmental factors giving the observed expression.
A simple example to illustrate genotype as distinct from phenotype is the flower colour in pea plants. There are three available genotypes, PP, Pp, and pp. All three have different genotypes but the first two have the same phenotype as distinct from the third. A more technical example to illustrate genotype is the single-nucleotide polymorphism or SNP. A SNP occurs when corresponding sequences of DNA from different individuals differ at one DNA base, for example where the sequence AAGCCTA changes to AAGCTTA. This contains two alleles : C and T. SNPs typically have three genotypes, denoted generically AA Aa and aa. In the example above, the three genotypes would be CC, CT and TT. Other types of genetic marker, such as microsatellites, can have more than two alleles, and thus many different genotypes. Penetrance is the proportion of individuals showing a specified genotype in their phenotype under a given set of environmental conditions.
Mendelian inheritance
The distinction between genotype and phenotype is commonly experienced when studying family patterns for certain hereditary diseases or conditions, for example, hemophilia. Humans and most animals are diploid; thus there are two alleles for any given gene. These alleles can be the same or different, depending on the individual. With a dominant allele, the offspring is guaranteed to inherit the trait in question irrespective of the second allele. In the case of an albino with a recessive allele, the phenotype depends upon the other allele. An affected person mating with a heterozygous individual there is a 50-50 chance the offspring will be albino's phenotype. If a heterozygote mates with another heterozygote, there is 75% chance passing the gene on and only a 25% chance that the gene will be displayed. A homozygous dominant individual has a normal phenotype and no risk of abnormal offspring. A homozygous recessive individual has an abnormal phenotype and is guaranteed to pass the abnormal gene onto offspring. In the case of hemophilia, it is sex-linked thus only carried on the X chromosome. Only females can be a carrier in which the abnormality is not displayed. This woman has a normal phenotype, but runs a 50-50 chance, with an unaffected partner, of passing her abnormal gene on to her offspring. If she mated with a man with haemophilia there would be a 75% chance of passing on the gene.
