Figure 1: Inheritance chart detailing how an individual may inherit blue or brown eyes depending on the alleles carried by their parents, with the brown eye color allele being dominant and the blue eye color allele being recessive. Other examples of genotype include:. What is the definition of a phenotype?
Whilst a phenotype is influenced the genotype, genotype does not equal phenotype. The phenotype is influenced by the genotype and factors including:. Phenotype examples Environmental factors that may influence the phenotype include nutrition, temperature, humidity and stress. Flamingos are a classic example of how the environment influences the phenotype. Whilst renowned for being vibrantly pink, their natural color is white — the pink color is caused by pigments in the organisms in their diet.
A second example is an individual's skin color. Our genes control the amount and type of melanin that we produce, however, exposure to UV light in sunny climates causes the darkening of existing melanin and encourages increased melanogenesis and thus darker skin. Observing the genotype, however, is a little more complex. Genotyping is the process by which differences in the genotype of an individual are analyzed using biological assays. 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.
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. 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.
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. Genes are actually quite active throughout our lives, switching their expression on and off in response to the environment and experience.
Environmental factors can affect and alter gene expression , while our genes can define how we respond to different environments. In an age in which scientists and the public are excited about the sequencing of the entire human genome , we need to temper that excitement, at least a little, and be careful not to believe all the hype surrounding genes' involvement in determining development and behavior.
While an organism's genetic makeup plays a critical role in its development, there is also a rich and complex interplay between the genome and cues from the environment.
It is not a question of which one affects us more, nature heredity or nurture environment ; instead, it is a question of how signals that are not hard-coded interact with our genetics to make us complex individuals. Indeed, it is actually superficial to debate whether nature or nurture is more important. In truth, the relationship between genetic determinants and the environment is so completely entwined that you cannot look at an individual and judge which contribution is more valuable.
Together, the continual interplay of both genes and ever-changing environmental factors determines who we are. Culliton, B. Sixty-four armadillos threaten a theory. Expressivity can vary even among members of the same family. Penetrance refers to whether the gene is expressed or not. That is, it refers to how many people with the gene have the trait associated with the gene. Expressivity determines how much the trait affects or how many features of the trait appear in the person.
Expression, which can be stated as a percentage, ranges from complete to minimal, or it may not be present. Various factors, including genetic makeup, exposure to harmful substances, other environmental influences, and age, can affect expressivity. Both penetrance and expressivity can vary: People with the gene may or may not have the trait and, in people with the trait, how the trait is expressed can vary. A trait that appears in only one sex is called sex-limited.
Sex-limited inheritance is distinct from X-linked inheritance, which refers to traits carried on the X chromosome. Sex-limited inheritance, perhaps more correctly called sex-influenced inheritance, refers to special cases in which sex hormones and other physiologic differences between males and females alter the expressivity and penetrance of a gene.
For example, premature baldness known as male-pattern baldness is an autosomal dominant trait, but such baldness is rarely expressed in females and then usually only after menopause. Genomic imprinting is the differential expression of genetic material depending on whether it has been inherited from the father or mother. For most autosomes, both the parental and maternal alleles are expressed. For example, expression of the gene for insulin -like growth factor 2 is normally expressed only from the paternal allele.
Genomic imprinting is usually determined by effects that occur normally in the development of gametes. Changes such as methylation of DNA may cause certain maternal or paternal alleles to be expressed to different degrees. A disorder may appear to skip a generation if genomic imprinting prevents the causative allele from being expressed.
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