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An Analysis Of Phenotypic Variation Biology Essay

Phenotype is the visible physical or biochemical characteristics of an organism, as determined by both genetic makeup and environmental influences. In this analysis of phenotypic variation, i will be analysing white and blue flowered bluebells, where the blue bluebells are wild-type because Vast number of bluebells are blue but there is also some white- flowered bluebells, therefore white bluebells are a mutant. Bluebells are diploid plants that grow mitotically. They undergo meiosis to produce haploid gametes.

To test whether it was an environmental factor that determines the colour of the flower. Plant the white flowered plant in the same environment as the blue flowered plant and plant blue bluebells in the same environment as the white bluebells. Then observe any colour change over several generations. If there is no change then it means that the colour of the flower does not determined by the environmental factors.

To investigate whether white-flowered plants were genetically different to the blue flowered ones, we can self crossed white and blue –flowered plants and examine the F1 Progeny. Self crossing ensure that the parents are true breed (homozygote). If they are true breed then self crossed blue bluebells will only produce blue bluebells and this is also true if you do this to white- flowered plants. If the F1 progeny of the two crosses are different, then the two plants are genetically different. Genetic diagram below shows self crossed blue bluebells.

Self crossing blue bluebells

Parents: Blue ( B) X Blue (B)

F1 progeny: Blue (B)

B+

B+

B+

B+/ B+

B+/ B+

B+

B+/ B+

B+/ B+

To determine whether the colour change was due to a mutation in a single gene and which form of the gene was dominant. To do this we can cross white and blue flowered plants, then depending on the F1 progeny we can determine which alleles is dominant and which I recessive. If the F1 progeny is pale blue (between blue and white) then it means a trait is neither dominant nor recessive over another, but produces an intermediate expression.

Breeding white and blue bluebells:

B+

B+

b-

B+/b-

B+/b-

b-

B+/b-

B+/b-Gene: B

Alleles: B+ = Blue (wild-type) ; b- = White

Parents: B+/B+ X b-/b- (Homozygote)

Gametes: B+ and b-

F1 progeny: Genotype = B+/b- (Heterozygote)

Phenotype = Blue

All progeny from both crossed were phenotypically wild –type therefore produce blue- flowered plants. This means that the B+ allele is dominant.

When crossing F1 progeny with the homozygous recessive mutant the progeny obtained demonstrate the normal and mutant phenotypes ratio is 1:1. This proves that B+ allele is dominant and b- is recessive. Mutation in B gene cause white coloured flower.

Parents: B+/b- X b-/b-

Gametes: B+ and b- and b-

Progeny:

Phenotype

Blue

White

Ratio

1

1

Genotype

B+/b-

b-/b-

To test whether the mutation causing the white colour of different plant were in a same or different gene. We could carry out complementation test, to perform this test, two mutants are crossed, and the F1 is analyzed. If the F1 does not express the wild type phenotype, but rather a mutant phenotype, we can conclude that both mutations occur in the same gene.

b-

b-

b-

b-/ b-

b-/ b-

b-

b-/ b-

b-/b-

Parents: b-/ b- X b-/ b-

Phenotype: White (b-/b-)

In single gene pair, when inbreeding F1 progeny:

F1 progeny: = B+/b-

F1 Gametes: B+ and b-

B+

b-

B+

B+/ B+

B+/b-

b-

b-/B+

b-/b-F2 progeny:

Blue: White

3:1

The dominant gene (B) will hide the recessive gene (b). After self-fertilized the F1 generation and obtained the 3:1 ratio, the genes can be paired in three different ways for each trait; B+ B+, b-b-, and B+b-.

If the phenotype is caused by mutations on different genes, then the two mutations will complement each other and prevent the mutant alleles from being expressed. This is shown below.

One gene pair will be B the second gene pair C. B+ and C+ is an allele for blue colour and b- and c- is an allele for white colour.

All F1 progeny will be b- C+ .

B+ c-

The F1 progeny has a wild-type phenotype because each mutant parental genome is balancing for the recessive defect in the other. The process independent assortment gives rise to the different types of gametes. Test cross shows the genotypes of all the possible gametes.

Parents: b- C+ and b- c-

B+ c- b- c-

Phenotype

Blue

Blue

Blue

White

Ratio

1

1

1

1

Genotype

B+ C+

B+ C+

B+ c-

B+ c-

C+ b-

C+ b-

b- c-

b- c-

Gametes

B+C+

B+c-

C+b-

b-c-

The genotypes and phenotypic classes of all possible F2 progeny (F1 x F1) are shown in the Punnett Square below:

B+C+

B+c-

C+b-

b-c-

B+C+

B+ C+

B+ C+

(B+ C+)

B+ C+

B+ c-

(B+ C+)

B+ C+

b- C+

(B+ C+)

B+ C+

b- c-

(B+ C+)

B+c-

B+ c-

B+ C+

(B+ C+)

B+ c-

B+ c-

(B+c-)

B+ c-

b-C+

(B+ C+)

B+ c-

b- c-

(B+c-)

C+b-

C+ b-

C+ B+

(B+ C+)

C+ b-

c- B+

(B+ C+)

C+ b-

C+ b-

(C+b-)

C+ b-

c-b-

(C+b-)

b-c-

b- c-

B+ C+

(B+ C+)

b- c-

B+ c-

(B+c-)

b- c-

b-C+

(C+b-)

b- c-

b- c-

(b-c-)

B+ C+: B+c- : C+b- : b- c-

9 : 3 : 3 : 1

Recombination frequency is the frequency that a chromosomal crossover will take place between two genes (loci) during meiosis. Recombinant Frequency gives you an idea about whether two different mutations were physically close together or far apart on the chromosome.

RF = (Number of recombinants / Total progeny) x 100

For the test cross shown above the RF is 50% this means the genes are unlinked and there are far apart on the chromosome. If the RF is less than 50% then it means genes are linked and they are closer on the chromosome.

If a gene that codes for a protein that is responsible for the colouration of a flower is a mutant, then the enzymes which are produced by the protein will be affected, therefore the enzyme is denatured and the reaction is stopped. Assume if there is two enzymes that catalyse the reaction to produce blue colour flower, one gene is responsible for one enzyme then if there is a mutation in two different genes then that means that the both enzymes are denatured and no blue colour flower is produced therefore mutation in both genes causes white colour flower.

There is a low frequency of white bluebells, this is maybe because it is affected by other factors. These could be genetic or environmental. The genetic factor could be that there are other mutations on other genes, as the result of that white bluebells s less fit then the blue bluebells, hence white bluebell is less likely to survive. The environment factor could be pollination, due to the colour of white bluebells, fewer insects are attracted hence less pollination and the reproduction of white bluebells is lower than the pollination and reproduction of blue bluebells. this results in less white bluebell population..

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