In one sense, the term “genotype”—like the term “genome”—refers to the entire set of genes in the cells of an organism. In a narrower sense, however, it can refer to different alleles, or variant forms of a gene, for particular traits, or characteristics. An organism’s genotype is in contrast with its phenotype, which is the individual’s observable characteristics, resulting from interactions between the genotype and the environment.
There is a complex connection between the genotype and the phenotype. Since the phenotype is the result of an interaction between genes and the environment, different environments can lead to different traits in individuals with a particular genotype.
In addition, different genotypes can lead to the same phenotype. This happens because genes have different alleles. For some genes and traits, certain alleles are dominant while others are recessive. A dominant trait is one that shows up in an individual, even if the individual has only one allele">allele that produces the trait.
Some aspects of eye color work this way. Brown eyes, for instance, are dominant over blue eyes. This is because a pigment called melanin produces the brown color, while having no pigment leads to blue eyes. Having just one allele for the dark pigment is enough to make your eyes brown. There actually are several different pigments that affect eye color, each pigment resulting from a particular gene. This is the reason why people can have green eyes, hazel eyes, or any of a range of eye colors apart from blue or brown.
When discussing genotype, biologists use uppercase letters to stand for dominant alleles and lowercase letters to stand for recessive alleles. With eye color, for instance, “B” stands for a brown allele and “b” stands for a blue allele. An organism with two dominant alleles for a trait is said to have a homozygous dominant genotype. Using the eye color example, this genotype is written BB. An organism with one dominant allele and one recessive allele is said to have a heterozygous genotype. In our example, this genotype is written Bb. Finally, the genotype of an organism with two recessive alleles is called homozygous recessive. In the eye color example, this genotype is written bb.
Of these three genotypes, only bb, the homozygous recessive genotype, will produce a phenotype of blue eyes. The heterozygous genotype and the homozygous dominant genotype both will produce brown eyes, though only the heterozygous genotype can pass on the gene for blue eyes.
The homozygous dominant, homozygous recessive, and heterozygous genotypes only account for some genes and some traits. Most traits actually are more complex, because many genes have more than two alleles, and many alleles interact in complex ways.
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We are all unique. Even monozygotic twins, who are genetically identical, always have some variation in the way they look and act. This uniqueness is a result of the interaction between our genetic make-up, inherited from our parents, and environmental influences from the moment we are conceived.
Understanding genotype and phenotype
Wilhelm Johannsen was a scientist working in Denmark in the late 19th and early 20th centuries. During a series of experiments, he observed variations in genetically identical beans. He concluded the variation must be due to environmental factors and coined the terms ‘genotype’ and ‘phenotype’ in 1911.
Genotype
Genotype is the genetic make-up of an individual organism. Your genotype functions as a set of instructions for the growth and development of your body. The word ‘genotype’ is usually used when talking about the genetics of a particular trait (like eye colour).
Phenotype
Phenotypeis the observable physical or biochemical characteristics of an individual organism, determined by both genetic make-up and environmental influences, for example, height, weight and skin colour.
How genotype affects phenotype
The term ‘genotype’ is usually used to refer to specific alleles. Alleles are alternative forms of the same gene that occupy the same location on a chromosome. At any given locus, there are 2 alleles (1 on each chromosome in the pair) – you get 1 allele from your mother and 1 from your father. The 2 alleles might be the same or they might be different. Different alleles of a gene generally serve the same function (for example, they code for a protein that affects eye colour) but may produce different phenotypes (for example, blue eyes or brown eyes) depending on which set of 2 alleles you have.
For example, the ability to taste PTC (a bitter tasting compound) is controlled by a single gene. This gene has at least 7 alleles but only 2 of these are commonly found.
An upper case ‘T’ represents the dominant allele that confers the ability to taste – ‘dominant’ means that anyone with 1 or 2 copies of this allele will be able to taste PTC. The non-tasting allele is recessive and is represented by a lower case ‘t’ – ‘recessive’ means that an individual will need 2 copies of the allele to be a non-taster.
Each pair of alleles represents the genotype of a specific individual, and in this case, there are 3 possible genotypes: TT (taster), Tt (taster) and tt (non-taster).
If the alleles are the same (TT or tt), the genotype is homozygous. If the alleles are different (Tt) the genotype is heterozygous.
It is actually rare that 1 gene determines 1 characteristic as in the case of PTC tasting (a monogenic trait). Most traits are complex and have genes that affect them at more than 1 locus (polygenic).
How environmental factors affect phenotype
Your genes carry the instructions for the growth and development of your body. However, your phenotype is influenced during embryonic development and throughout your life by environmental factors. Environmental factors are many and varied and include diet, climate, illness and stress
With the PTC tasting example, scientists estimate that the gene controls about 85% of the ability to taste. Environmental factors that play a role include how dry your mouth is or how recently you have eaten.
The degree to which your phenotype is determined by your genotype is referred to as ‘phenotypic plasticity’. If environmental factors have a strong influence, the phenotypic plasticity is high. If genotype can be used to reliably predict phenotype, the phenotypic plasticity is low.
Overall, the amount of influence that environmental factors have on your ultimate phenotype is a hotly debated scientific issue. It is often referred to as the ‘nature (genes) versus nurture (environment)’ debate. Scientists commonly study monozygous (identical) twins to investigate the genotype/phenotype relationship.
In conclusion, your genotype or genetic make-up plays a critical role in your development. However, environmental factors influence our phenotypes throughout our lives, and it is this on-going interplay between genetics and environment that makes us all unique.
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