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There are around 6.8 billion people all over the world and every one of them is different from each other. We all have unique and distinctive phenotypes. Whether it's the height, colour of the skin or the shape of our anatomy, we are all, in some way, different from the rest of the people in the world. We might be similar in skin colour but our height or weight may vary. So what makes us different to other billions of people? What raises this variation in the group of species? There are basically two different explanations behind this question which are Genetic factor and Environmental factor.
Genetic factor is related to the genotype of an organism. Humans are diploid organism that means we have two sets of chromosomes in our body, one from parental and one from maternal. Hence, an offspring has a half set of chromosome that came from the mother and the other half that came from the father. And these chromosomes and the genes involved are the result for the phenotypic features that offspring exhibits. Environmental factor is related to the surroundings, habitat that offspring lives on or any other factor that causes the change in phenotypic feature which is not due to their genotype. Environmental factor could be anything from sun, water, wind, food etc. And if these factors somehow affect the phenotypic characteristics of that offspring, they are said to be environmental factor. But these two factors doesn't necessarily affect the individual distinctly. The phenotypic variation in an organism may as well be due the genes along with the influence of environmental factor. For example, suns radiation can cause the variation in a gene leading to mutation and different phenotype. So, these factors could have their affect solely or combined.
Phenotype is basically the appearance or a physical feature of an individual. Variation is many different forms of a single characteristic. There are two forms of variation, continuous variation and discontinuous variation. Continuous variation is a variation which is continuous and doesn't have two distinct types that can be differed. If these variations are data tabled then they can be put into a class interval, for example, height or weight of population. This could vary from one value to another value like 40kg to 80 kg, but there aren't any two or more distinct types. Whereas, discontinuous variation have two or more distinct types (phenotypes) and they don't vary like in continuous. A very good example is a blood group type of an individual which are A, B, O and AB. They are all different from each other and they are distinct. Continuous variations are very likely to be affected by environmental factors, for example, diet of an organism affects the weight of an individual. Discontinuous factor, in the other hand, doesn't get affected by the environmental factors and they are mostly caused due to a genetic factor.
Fig. 0 http://davidguilbault.typepad.com/.a/6a00e552f1c77b88340111685820c4970c-800wi
(Darwin's finches showing variation in the shape of their beaks)
Although both factors cause phenotypic variation, almost all of the major variations we see are due to the genotype of individuals. The fate of a child is determined by the parents. Phenotypes of progeny is the due the genes on chromosome that he/she receives from their two parents. If the parents have a brown coloured eye then the chances are that their child will also have brown coloured eye. Every single individual apart from the identical twins, have got different genotypes. Hence, they all have different physical characters. A white flowered plant will have white flowered progeny and a blue flowered plant will have a blue flowered progeny. They both have different genotypes hence, resulting in different phenotypes. But in some cases, a blue flowered plant might produce some progenies which are not blue but white or a different colour. In fact, it is common to see a progeny having a different variation in phenotype characteristics to their parents. These variations arise from the sexual reproduction, to be more precise, meiosis. Sexual reproduction is a process which has an element of randomness. During meiosis, homologous chromosome crosses over to exchange the genetic material (fig 1). This creates a variation in genotype. Another, key process that occurs during meiosis is the independent assortment of chromosomes which further increases the chances of variation. These two key processes in meiosis creates a variation that we see in the progeny. This can cause a progeny to have an unexpected new phenotype.
Let us take this case for an example. A blue flowered plant produces a white flowered progeny. There can be different reasons behind this unexpected result. It could have a simple explanation that a blue flowered plant somehow pollinated with a white flowered plant because of wind, insects or any other factors and it inherited genes from the parent which was white coloured. Or more complex explanation, where a blue flowered plant pollinated with a blue flowered plant and still produced a completely different phenotype which is white. Had blue flowered plant pollinated with white flowered plant then the result wouldn't have been surprising at all. We could guess that the gene for blue colour is dominant and the gene for white is recessive, hence, the progeny shows the blue colour. For e.g. let's say Blue flower parent had gene (B) which was dominant and coded for blue colour and another parent, white flower had gene (b) which was recessive and coded for white colour. Since both of these genes were passed on to the next generation, then their genotype would be Bb and since we know B is dominant over b, we can say the progeny would have phenotype coded by the dominant genotype which is B and hence blue phenotype. But, if two blue flowered parents produces a white flowered plant it has much more complex reasons behind it. The most obvious cause behind it can be the mutation. Mutation are changes in DNA sequences or fault in meiosis that causes a change in the genotype and hence the phenotype of an organism. In this case, a mutation in parent genotype can cause the progeny to have that mutated gene and show an altered phenotype, in this case it is white phenotype instead of blue.
A gene mutation is generally caused due to the point mutation. i.e. addition, substitution or deletion of bases in DNA chain. This causes a frame shift and leads to an altered amino acid sequence and hence codes for a different protein. This directly influences the expression of wrong phenotype and causes phenotypic variation. There are two classes of mutation, spontaneous mutation and induced mutation. Spontaneous mutation can be caused by tautomerism, depurination, deamination and mispairing and induced mutation are caused by the mutagens. Chemicals like hydroxylamine and N-methyl-N'-nitro-N-nitrosoguanidine (NG) causes the mutation in DNA of cell. Mutation can be a single gene mutation where mutation occurs in a single gene or mutation could occur in more than one gene. Mutations occurring in more than one gene can lead to very complex and surprising phenotypes in the future generations.
Let's look at an example where a single gene mutation creates a phenotypic variation. One of the common phenomenons that can create a phenotypic variation is if the genes are linked. For example, the different conidiospores of Aspergillus nidulans have different colour which are green, yellow and white. Green is a wild type and yellow and white are the mutants formed because of the mutation in a single gene. Here, a change in the DNA sequence which codes for the green colour has been altered and now it codes for new colours yellow and white. Two different mutations has led to new phenotypes. Now, lets take this example of mutation to carry out further experiments to see if there is more variation. Suppose we make a cross between green and each of the mutant spores then we would expect the phenotypes of the progeny to be green and the colour of whichever mutant it was crossed with. The ratio would be 1:1 as it is a single gene mutation. But if we cross, two mutants, i.e. yellow and white, we expect two phenotypes with 1:1 ration. But we get three different phenotypes of progeny which are yellow and white but green as well. This is a classic example of linked genes. Here, the allele which codes for the wild type green colour is linked with other mutant allele. Hence when crossing over takes place during meiosis, both of the alleles are inherited together and we get three different phenotype. The new phenotype observed is called recombinant. As genes are very close to each other in chromosome, it gets inherited together during meiosis which produces the recombinants. It means the closer the alleles are there is more chance of two genes getting inheriting together thus creating new phenotypes. Further the genes are there is more crossing over taking place between them and separating the genes. So if the genes are linked, then we get a new phenotype creating a variation. It creates a far more complex variation if two or more genes that codes for different features were linked.
Now, let's look at an example of a mutation in more than one gene that causes phenotypic variation in fruit fly (Drosophila melanogaster). This species has got two kind of mutants, one which has no wings(mutant1) and another which has got short wings(mutant2). Gene responsible for formation of wing is W and the gene responsible for the length of the wing is L. Wild type has got genotype W+L+. Mutant1 which has no wings with genotype W-L+ and Mutant2 which has short wings with genotype W+L-. In this case, wild type alleles are dominant over mutant alleles for both gene(wing formation and wing length). Now, if we cross the wild type with mutant1 then we get heterozygous chromosomes and wild type phenotype because of the dominant character of wild type gene from wild type. We get the same result with cross form wild type and mutant2. But, if we cross the two mutant types we must predict a progeny with a mutant phenotype or new kind of phenotype. Surprisingly, this is not the case. If we cross the two mutants we still get the wild type progeny. If we cross mutant1 (W-L+) with mutant2 (W+L-.), the dominant genotype of the progeny would be W+L+ and hence, it would have a phenotypic feature of a wild type. This is known as Complementation. This happens because the mutant gene along with its mutated gene also has got wild-type copy of a gene and when two mutant parents produce F1 progeny, it inherits the wild-type copies of both genes. This is due the mutation in different genes. So complementation causes a phenotypic variation within a mutant organism to produce the original wild type phenotype.
Another circumstance that causes a phenotypic variation is if the genes are sex linked. We know that female has XX chromosome and male has XY chromosome. X chromosome is like any other autosomal chromosome and it carries many genes. But Y chromosome is very different to rest of the chromosomes. It is almost half of the size of the X chromosome and it doesn't carry any other gene then the sex gene. This proves to be a very significant in determining the phenotype of the progeny. Taking the same example of fruit fly, let's see what phenotypes we get it the genes were sex linked. Let us take two parents where mother had short wings and the father had no wings at all. So their genotype would be W+L- and W-L+ respectively. And we would expect the progeny to have a wild type phenotypes because of heterozygous genotype with wild type dominant genes due to Complementation just like above experiment. But we get a different result. We get wild type female with short winged male. Here, male progeny have inherited the same phenotypes as their mother. The reason behind can be solved with punnett test. Since, the genes are sex linked, genotypes will be
Female = (W+L-)
Male = (W-L+)
So, we can see from the punnett square, as Y chromosome from father doesn't carry any genes, the male progeny is bound to have a genotype of mother which in this case is W+L-, and this leads to phenotype having short wings. We should be expecting wild type phenotypes because of the complementation process, but as the genes are sex linked we get yet another different phenotypes causing a phenotypic variation.
Co-dominance or partial dominance is another process that leads to a phenotypic variation. Co-dominance means where neither of the allele is dominant or recessive, both genes are equally strong and both genes are expressed. Co-dominance occurs in our blood groups. We have got four alleles to determine the blood group, i.e. A, B, O and AB. Both A and B are dominant whereas O is recessive allele. So if a child get either AO or BO genes from parents then the child will have either A or B blood group. But if child gets AB genotype from two parents then child's blood group would be AB as both alleles are expressed because of the co-dominant factor. This way, co-dominance has created a new genotype, therefore a new phenotype character. Again, yet another phenotypic variation is introduced. This can be further illustrated with punnett square which makes it much more clearer.
Let's take an example where father has Blood group A and mother has B. Hence their alleles would be as following
Father(AO)= A, O
AB ( blood group AB)
BO (blood group B)
AO (blood group A)
OO(blood group O)
As we can see if a child gets allele A from father and allele B from mother its blood group will be AB. So co-dominance is another factor that introduces different phenotype.
So we can see how there are so many different causes that can cause a different phenotype. The main reason obviously being sexual reproduction but the mutation also proves to play a major role in creating different phenotypes. We regard mutation to be bad because it causes diseases but mutation can also be very helpful. One of the examples is sickle cell disease. We know sickle cell is caused due to mutation which leads to substitution of amino acids and change the shape of Red Blood Cell. But the heterozygous individual or the carrier of sickle cell gene were protected from malaria. This was exclusively true for people from Africa where malaria used to kill thousands of children. But those who were carriers for these genes were not affected by this disease. So not only we can say mutation is helpful, we can also argue that mutation is utterly important event caused by the natural process to have better physical attributes(phenotype) to cope with the new challenging circumstances. As strong as these genetic factors are the reason behind the variation we can't forget the role of environment affect. There are various other factor that genetic factor depends upon. For example, if the blue coloured flower were not pollinated at all by insects to white coloured flower then we wouldn't be getting a reproduction in the first place, let alone the variation. Hence, insect plays a very critical part in variation of some organism. Similarly there are other factors in other organism that plays important role in causing variation. These other environmental factors are pivotal in determining the variation in an organism.
Phenotypic variation is a positive factor in terms of evolutionary process. Without variation we wouldn't be getting new genotypes with new and better phenotypes. Nature has been always longing for the variation. Due to variation, organisms are able to adapt to changing environment for better chances of survival. So these natural phenomenons that take place on our chromosome are decisively essential. Meiosis along with mutation, linked genes, sex linked genes, complementation, co-dominance and various other factors induces the alteration in our genes causing phenotypic variation. Some changes may make an organism better equipped and stronger and some changes might cause them defect but nevertheless changes in genes are integral part in the life of an organism and phenotypic variation is genuinely significant.