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. 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.
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. 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.
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. Armadillos are thus ideal animals to use in such research, because they are born as quadruplets derived from a single fertilized egg.
This means that all four armadillo pups share the same genetic sequence. During the s, researcher Roger Williams decided to study armadillos after completing extensive experimentation with rats. Williams's results were compelling, because after inbreeding animals for this many generations, scientists generally assume that the animals are nearly genetically identical. Moreover, Williams had housed the animals in controlled environments with the same diet.
Why, then, did the rats' phenotypes differ so greatly? Williams grew increasingly curious about this issue, and he surmised that the genetic contribution alone was not enough to generate the observed phenotypic differences.
Williams thus decided to take a closer took at the role of the environment. Before beginning his research, however, Williams knew he must do as much as possible to remove genetic variation from the equation. Thus, he and his student Eleanor Storrs turned to armadillos and looked at a variety of phenotypes in sets of quadruplets.
In contrast, other heritable traits had very little variation at all. In their research, Storrs and Williams focused on the " norm of reaction " for various phenotypes in armadillos. The norm of reaction is the theoretical concept that a specific phenotype may have a range of manifestations.
In some cases, like human blood type, the range of phenotypes is strictly related to genotype , and the environment has little effect. For other phenotypes, like height in humans, the norms of reaction are much wider. The norm of reaction also depends on the level of organization under study, and it can be used to describe the various ways in which related organisms respond to their environment.
Organisms of the same species with different genotypes can show differing norms of reaction when different phenotypes are measured or when environmental variables are altered.
Therefore, a different norm of reaction exists for every combination of genotype, phenotypic trait , and environmental variable studied. In Williams and Storrs's research, how might four embryos that developed in the same mother armadillo have been exposed to different environments? One possibility is that the intrauterine environment is slightly different for each developing embryo.
For example, the position of the armadillo fetus in the uterus may play a role Culliton, , causing one fetus to be exposed to a different amount of light or a slightly different temperature than its siblings.
The blood supply from the mother may also vary between armadillo siblings Culliton, Similarly, researchers Herbert Hauser and Ron Gandelman wondered if the in utero positioning of a fetus with respect to the sex of neighboring fetuses influenced phenotype. Uterine position could therefore partially explain the phenotypic differences in the sets of armadillo siblings.
Another possibility is that the four individual armadillo embryos shared identical genetic sequences, but not the same intracellular environment. After all, the initial fertilized egg is not a uniform cell with an equal distribution of cytoplasmic components. Rather, the cytoplasm and intracellular proteins, mitochondria , and ribosomes are unequally distributed, which may cause variation.
Differences in the number of mitochondria, for example, may produce variations in energy production in different animals during development Culliton, In armadillos, the fertilized egg develops to the blastocyst stage, and then four primordial buds form in two stages, which results in the development of quadruplets.
Thus, while the primordial buds formed four genetically identical embryos, the cytoplasmic contents were divided unequally. This too may account for the phenotypic differences between the armadillo siblings. Environmental factors are certainly critical in defining phenotypes during early development, as in the armadillos, and they continue to influence phenotypes throughout an organism's life cycle. Nearly every aspect of our development and behavior is affected by both the personal experiences we gain through our environment and our genetic makeup.
For example, we obtain necessary amino acids through our diets, and the incorporation of these nutrients into our bodies is determined by our genes. It is also important to remember that genes are not a steadfast blueprint for heredity.
For instance, animals being adapted to intensive production systems in Denmark will not always be as well adapted in tropical conditions. Therefore it is more appropriate to describe the phenotype as a function of the genotype and the environment.
Similarly, your genotype gives the instructions to produce your phenotype—your type of hair, your height, your body build and most of your physical characteristics, but these also depend in part on your environment. During development our environment can modify the instructions our genes are sending.
Perhaps not enough nutritious food was available when our body was growing, or we had a bout of infectious disease malaria or were exposed to toxic chemicals air pollution ; each of these can change how our body develops. We can develop a degree of mismatch between our genotype genetic code and our phenotype physical characteristics.
We would say that the person has not achieved their genetic potential. Now suppose that you got a few faulty appliances. Even though you are using your vacuum, your carpets remain dirty. Through time, the dirt accumulates. The amount of dirt that builds depends on how dirty the air is in your house.
If the air in your house is clean, the vacuum is still sufficient. In your body, the vacuum could be a gene that controls cleaning your blood. If your blood is not cleaned effectively, it can lead to disease, but the disease manifests because of the quality of your environment.
Research has been showing that there are gene variants that interact with environmental exposures differently. It takes the combination of having the faulty gene variant and the exposure s to result in some diseases and disabilities, such as Parkinson's disease and others discussed on this page. This analogy is not perfect. Sometimes there are many genes that act together to influence the development of disease or disability. However, this analogy may help you understand some of the complex gene-environment interactions that influence our health.
Below are some known gene-environment interactions that increase the risk for specific disease outcomes. Organophosphate pesticides , commonly used in agriculture, are neurotoxins that have been implicated in the development of PD. This genetic susceptibility makes these people more susceptible to the development of PD when exposed. The PON1 gene codes for an enzyme paraoxonase, an aryl esterase, that metabolizes organophosphates. There are different variants in the monoamine oxidase A gene MAOA , which produces a protein that metabolizes neurotransmitters in the brain such as dopamine, norepinephrine and serotonin.
Gene-environment research by Fergusson and others demonstrates that children with variants in the MAOA gene who are also exposed to child abuse have a higher risk of developing antisocial behavior than abused children who do not carry the risk variant. Children in that study who carried a low-activity MAOA variant and experience abuse were more likely later to be criminal offenders, demonstrate hostility and exhibit conduct problems.
Two genes are known to detoxify exposures that cause oxidative stress, such as air pollution. The glutathione GST gene and the epoxide hydrolase EPHX1 genes both have variants associated with an increased risk of developing asthma, especially if a person is exposed to air pollution.
Most striking, however, is the interaction between these variants and exposure to air pollution. Children who have both high-risk variants and live close to a major roadway have a 9-fold increased risk of developing asthma. The aldehyde dehydrogenase 2 ALDH2 gene produces an enzyme important to alcohol metabolism. There are two different variants in the gene, one that produces a functioning enzyme and one that produces a nonfunctioning enzyme. Individuals who carry the nonfunctioning variant of the ALDH2 gene have substantial problems metabolizing alcohol, as the broken enzyme causes aldehyde to build up in the body, known as flushing syndrome.
It is believed that around million people worldwide carry one or both non-functioning variants. When an individual carries one of the variants heterozygotes , one nonfunctioning and one normal, they have a fold reduction in alcohol metabolism. These heterozygotes have a higher risk of esophageal cancer when exposed to moderate or heavy ethyl alcohol use as compared to someone who has both functioning variants.
Esophageal squamous cell carcinoma ESCC is an aggressive cancer with a five-year survival rate around 15 percent. To get water out of the faucet, you have to physically turn it on and off. If you forget to turn the water off, it can flood the kitchen with too much water, doing considerable damage.
Just as you physically controll a faucet, the epigenome controls if a gene is on or off. As such, if a gene that leads to cell growth is turned on at the wrong time, cancer will be fostered. There are some parts of our genome we always want to keep dormant. To ensure this, you turn the security system on and lock your doors as protection against outside intrusion and activity. Likewise, there are genes in our genome, conserved through our evolutionary history, that must stay off to conserve good health.
Just as the security system locks down your dormant house, the epigenome silences these unneeded genes. Some environmental exposures, such as BPA, can break down this security system and turn on genes that disrupt healthy systems.
Whereas the genome is the full code for all of the proteins that make up a human being, the epigenome, in its simplest form, is a system of tags that surround the genome and controls what it does. It is these tags that can turn a gene on or off, controlling if a gene produces its product. Through epigenetic mechanisms, cells can become specialized. This is how cells are specialized. For example, genes that allow cells to detect light are turned on in the eye but not the liver. Humans need different genes to function at different times, depending on if cells need to repair themselves, to fight off intruders, to divide into two cells, or to function as part of an organ.
Epigenetic mechanisms allow this to happen. Environmental exposures can change gene expression through epigenetic mechanisms. For example, worker bees and a queen bee are genetically identical. When a developing bee is fed royal jelly, an epigenetic modification is made to the reproductive genes and they turn on.
The genes are not changed, but whether they are active or not has. When the hormone enters a cell, it binds a receptor that then binds with a specific stretch of DNA. By binding, the gene is turned on transcribed , turning into a product. It is through this process that endocrine disrupting chemicals can alter gene function in pathways associated with infertility , obesity , cancer and osteoporosis. The epigenome is heritable between generations.
This way, the daughter cell is as specialized as the cell it came from. It is especially important that the epigenome is copied correctly in cells of developing fetuses and children.
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