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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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Temperament includes behavioral traits such as sociability (outgoing or shy), emotionality (easy-going or quick to react), activity level (high or low energy), attention level (focused or easily distracted), and persistence (determined or easily discouraged). These examples represent a spectrum of common characteristics, each of which may be advantageous in certain circumstances. Temperament remains fairly consistent, particularly throughout adulthood.
Similar temperaments within a family may be attributable to shared genetics and to the environment in which an individual is raised. Studies of identical twins (who share 100 percent of their DNA) and their non-twin siblings (who share about 50 percent of their DNA) show that genetics play a large role. Identical twins typically have very similar temperaments when compared with their other siblings. Even identical twins who were raised apart from one another in separate households share such traits.
Scientists estimate that 20 to 60 percent of temperament is determined by genetics. Temperament, however, does not have a clear pattern of inheritance and there are not specific genes that confer specific temperamental traits. Instead, many (perhaps thousands) of common gene variations (polymorphisms) combine to influence individual characteristics of temperament. Other DNA modifications that do not alter DNA sequences (epigenetic changes) also likely contribute to temperament.
Large studies have identified several genes that play a role in temperament. Many of these genes are involved in communication between cells in the brain. Certain gene variations may contribute to particular traits related to temperament. For example, variants in the DRD2 and DRD4 genes have been linked to a desire to seek out new experiences, and KATNAL2 gene variants are associated with self-discipline and carefulness. Variants affecting the PCDH15 and WSCD2 genes are associated with sociability, while someMAOAgene variants may be linked to introversion, particularly in certain environments. Variants in several genes, such as SLC6A4, AGBL2, BAIAP2, CELF4, L3MBTL2, LINGO2, XKR6, ZC3H7B, OLFM4, MEF2C, and TMEM161B contribute to anxiousness or depression.
Environmental factors also play a role in temperament by influencing gene activity. In children raised in an adverse environment (such as one of child abuse and violence), genes that increase the risk of impulsive temperamental characteristics may be turned on (activated). However, a child who grows up in a positive environment (for example a safe and loving home) may have a calmer temperament, in part because a different set of genes is activated
Scientific journal articles for further reading
Bratko D, Butković A, Vukasović T. Heritability of personality. Psychological Topics, 26 (2017), 1, 1-24.
Manuck SB, McCaffery JM. Gene-environment interaction. Annu Rev Psychol. 2014;65:41-70. doi: 10.1146/annurev-psych-010213-115100. PubMed: 24405358
Power RA, Pluess M. Heritability estimates of the big five personality traits based on common genetic variants. Translational Psychiatry (2015) 5, e604; doi:10.1038/tp.2015.96; published online 14 July 2015. PubMed: 26171985 PubMed Central: PMC5068715