There are many factors that could affect the phenotype of a flower. Two major factors that influence the change in flower colour are the environmental and genetic effects. Independent assortment in meiosis gives rise to genetic variation throughout a population, where each individual has different characteristic. Environmental factors such as light intensity, water, soil nutrients and weather temperature also affect growth. An experiment is carried out to examine what is the cause of colour change in bluebell. In order to plant both white and blue bluebells under constant environmental conditions, we uproot a white bluebell and plant it in an area with blue bluebells, so that they can absorb same amount of sunlight and water. If results show white flower turns blue, the colour change is due gene mutation, whereas if results show flower colours remain unchanged, then it is due to environmental effect.
White-flowered bluebells and blue-flowered bluebells are different in phenotype. However, this does not tell us whether they are different in genotype. This could be investigated by performing a cross between the white and blue plants. Let dominant 'B' gene as blue flower, and recessive 'b' gene as white flower. First, we have to use pure white flower and pure blue flower to perform the cross. To ensure that they are both homozygous, white bluebell should undergo self pollination, and the same for blue bluebell. The progenies obtained are used as true-breeding parents to perform testcross. White bluebell and blue bluebell are genetically identical if F1 progeny shows both white and blue plants. If F1 progeny shows only blue bluebells, they are genetically different.
We then perform crossing of the F1 generation with the homozygous recessive mutant. This is known as the Testcross which shows a ratio of 1:1 segregation of alleles at meiosis. Results show the wild-type and mutant phenotype in 1:1 ratio of white: blue, therefore proving the mutation is in a single gene.
In order to analyze whether the blue colour is the dominant or recessive, we can also perform self-cross of F1 progeny. There are three genotypes found in F2 progeny but only two phenotypes are expressed. The white mutant phenotype reappears in F2 progeny, thus proving the presence of b allele in F1 progeny. This concludes that B is dominant to b in the wild-type phenotype of F1.
Moreover, the phenotypic variation of blue flower to white can be caused by mutation in two different genes that are controlling the expression of petal colour. This could be examined by considering complementation test. This testcross is done by crossing two mutant genomes, the white bluebells, which are taken from different parts of the wood. Mutation is likely to occur in two genes when both the homozygous mutants give heterozygous progeny with normal phenotype. The recessive mutations in each parental genome is said to be complementing each other.
Results show that F1 progeny has blue wild type phenotype after crossing two white mutant parents, hence suggesting that the mutation is in two different genes. After determining the mutation occurs in different genes, we can now estimate the distance between the two mutations whether they are physically close together or far apart on the chromosomes. A recombination test can be done by crossing the F1 progeny with a recessive homozygote. The testcross produces two types of progeny: parental and recombinants. Parental type is a combination of genes that retain the alleles from parent strains, whereas recombinant type is a new genotype that is formed by crossing over of genes on homologous chromosomes.
By obtaining the 1: 1: 1: 1 ratio of genotypes and the formation of recombinant gametes, we can say that the two genes are unlinked, which means that they are located in different chromosomes. This genetic diversity is caused by either random assortment of chromosomes at metaphase plate or crossing over of genes on homologous chromosomes at metaphase 1. We can calculate the recombination frequency (RF) to have a stronger proof that the two genes are unlinked. RF value should be approximately 50%.
The testcross of white and blue bluebells possessed a recombination frequency of 50% which suggests that the normal gene and mutant gene are not linked, but located in different chromosomes. If RF value obtained is lower than 50%, the genes are said to be linked. This means that genes are located in the same chromosome, and are not segregated during random assortment, therefore recombination does not occur. We can clearly know that random assortment of genes occurs in bluebells by inbreeding the F1 generation. Phenotypic ratio obtained will be ratio of 9: 7 blue: white.
Several genes contribute to a phenotype as part of a biochemical pathway. Flower colouration is controlled by more than one gene that encodes specific proteins. These proteins act as enzymes which synthesize the flower pigments, giving a particular phenotype. The blue colour flower pigment of bluebell is produced in two steps. Along this metabolic pathway, colourless intermediate is synthesized from colourless precursor by an enzyme that is encoded by a certain gene. The final pigment will only be synthesized from the intermediate by another specific enzyme that is encoded by a certain gene.
According to this pathway, if mutation occurs in either the gene encoding enzyme W or the gene encoding enzyme B, functional enzyme will not be produced. Eventually there is no formation of final blue pigment, giving the bluebell white phenotype by default. Hence, if either of these genes is mutated, they will give the same phenotype that is the mutant white flowers. This is known as epistasis. When two or more genes are controlling a single phenotype, the expression of a gene at one locus can mask or suppress the expression of a second gene at another locus (Russell, 2006). A homozygous recessive gene 'ww' at locus W is epistatic to locus B, and a homozygous recessive gene 'bb' at locus B is epistatic to locus W. This homozygous recessive genotype at either locus stops the biochemical pathway from producing the final pigment that is responsible for the flower colouration.
This concludes that the blue pigment is only formed when both genes are wild-type and encode for functional proteins that function as enzymes. However mutation in genes does not explain the low frequency of the white bluebells in the population. In terms of genetic explanation, the white alleles are recessive thus the population is dominated by the blue flowers. Gene mutations in the white flowers may also affect the fitness of bluebells. The missing or defective genes do not produce proteins or enzymes that might be essential for life. In addition, selective pressures may affect the population by favouring the blue traits. Under natural selection, blue bluebells have adaptive traits which are more likely to reproduce and thrive successfully in the environment.
For instance, the productivity of blue flowers is higher because of their colour adaptation that attracts pollen vectors such as bees for insect pollination. Less insect pollination occurs in white bluebells due to their unattractive petal colour therefore contributing to the inefficient reproduction. Besides that, the blue colour of bluebells probably helps them to camouflage in the wood, preventing them from being eaten by herbivorous predators. White bluebells fail to hide themselves thus increases the risks of predation. Moreover, sun emits harmful UV radiation during the day. Blue flowers absorb few amounts of UV rays because they reflect blue-violet colour, whereas white flowers absorb higher amount of radiation which causes DNA damage, leading to their early deaths.
In summary, different aspects of environment and genetic have to be taken into account order to analyze the phenotypic variation. Over evolutionary period, genetic variation arises for better adaptation as the environment changes. Only individuals with advantages traits are able to survive successfully, producing offspring that inherits the allele. This gives rise to the phenotypic variation. Even two individuals of the same species have a contrast in phenotypes if they are living in different environmental conditions. Hence, phenotypic variation is significant in enabling evolution of species by natural selection.
