Genotype and phenotype
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.
Monozygotic twins
Identical or monozygotic twins occur when a single egg is fertilised to form 1 zygote that divides into 2 separate embryos. As a result, identical twins share identical DNA but may show differences in their phenotype due to environmental factors.
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
Phenotype is 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.
Researching the link between genotype and phenotype
Professor Peter Dearden, from the University of Otago, is interested in how genotype makes phenotype. In this video, he talks about his laboratory’s research where they manipulate individual genes and observe any resulting changes in phenotype.
Jargon alert
Genotype: The genetic information, or DNA sequence, of an individual organism
Phenotype: The observable physical or biochemical characteristics of an organism, determined by both genetic makeup and environmental influences.
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
Genetics or environment
Dr Christine Jasoni, from the University of Otago, is interested in brain development. In this video, she talks about her research into whether brain wiring during development is controlled by genes or environmental factors.
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.
Related content
Peter Dearden is the director of Genetics Otago. He is working to better understand how genes influence embryonic development. In his experiments, he turns genes on and off in flies – with some interesting results! Learn more about Peter's research in the article The genotype/phenotype connection.
Christine Jasoni hopes to improve people’s lives through understanding brain development. Learn more about her research into how environmental factors affect this complex organ.
The articles DNA, chromosomes and gene expression and Meiosis, inheritance and variation take a closer look inside our cells to explain some of the ideas fundamental to finding out what makes each of us unique.
The timeline – Fertilisation to adulthood – gives information on the key stages in the development of a human.
Activity ideas
Try these activities with your students, they have been designed to address common alternative conceptions about genetics. Genetics: true or false? can be combined with Genetics web quest and used at the start and/or end of a genetics unit.
Useful link
Visit the Learn Genetics website to learn more about the gene for tasting PTC.
