Essay title: "Analysis of phenotypic
variation-Bluebells"
Student no: 086223452
Tutorial group: Group 3
Tutor: Dr. Chris Cane
Phenotypic variation is an important biological phenomenon, which is
related to the different distribution of chromosomes during meiosis. A variety of
crosses occur during time between organisms of the same type, such as animals
or plants in order to maintain the continuity of life.
Each chromosome in the nucleus holds huge numbers of genes, which are
translated to code amino acids, which will be part of a polypeptide chain, a
protein. That protein will express a character, a phenotypic characteristic.
Variation of these phenotypes through a population occurs because of the free
recombination of genes through crossing over, synapsis and exchange of genetic
material between non-sister chromatids in prophase -1, in meiosis- 1. As a
result, we have the formation of new combinations of alleles at different genes
along the chromosomes.
To study the phenotypic variation of a population is extremely necessary
to know the factors that cause this phenomenon in order to perform experiments
that indicate them. All diploid organisms follow the same principles, thus genes
are inherited in the exact same way.
For this analysis we need to have an example which will be taken under
experimental processes, such as flowers, called bluebells. Random distribution
of genetic variation through a population may be a result of genetic rather than
environmental effects because new characters that are revealed in an organism
are results of specific gene behaviour. On the other hand we should not exclude
the possibility of environmental factors that might lead an organism to a new
phenotype. To verify that, we could do simple experiments in these specific
organisms. First of all we should define the alleles which we refer to. The blue
coloured bluebells, which is the majority, is defined with B and the white
coloured bluebells (minority) with b. To examine if the presence of the new
colour among species is a result of environmental effects we could take a
number of white bluebells in the laboratory and change the environmental
conditions like changes in the soil where they grow, light or exposure to UV
radiation and temperature . We do the same with the blue bluebells and if the
white colour changes into blue or if the blue colour changes into white, then the
presence of different colour are due to environmental effects. If the colour of the
flowers remains the same in each case, then the colour is due to genetic factors.
We can also detect these effects by another way, which might be more effective
than the laboratory experiments. We will take a number of blue bluebells and
replant them in the area where white bluebells were planted. If the blue flowers
remain blue that means that the colour appearance is due to genetic effects. We
do the same with a number of white bluebells and if they remain white the
phenomenon is due to genetic effects. Furthermore to get more accurate results
we need to be sure that no other factors contribute in the colour appearance,
apart from bluebells themselves. In order to achieve that, we must prevent
pollination from insects or wind by building a fence, so that no other genes
involve in the experiment.
However a different phenotype is almost always related to a genetic
differentiation. This can be examined by the observation of the two phenotypic
types of a population. For example, white and blue coloured bluebells might
have differences in their karyotype. This could be only defined by observing
their offspring. A simple way is to self cross the blue flowers (homozygous)
until all the flowers they produce have blue colour. Again we do the same with
the white flowers. In our results we shouldn’t have different colours than the
parents’ colours. This is a simple representation of the self cross to show that
the offspring should have the same colour as the parents:
B=blue colour
Parents:
Gametes:
F1 progeny:
B
B
X
B
B
B
B
B
B
All flowers are blue
The same cross is done for the white flowers, thus these two types of
flowers are genetically different.
By doing these crosses there is a need to define if the mutation is in a
single gene. This is when "no wild type allele of that gene exists in the
individual and neither mutated copy of the gene will be able to perform the
normal function (Hartwell et al, 2008)". Therefore, we cross a number of
homozygous blue bluebells with a number of homozygous recessive white
flowers (and the opposite).
B=blue (wild type)
b=white (mutant)
Parents:
B
Gametes:
X
b
B
b
B
b
F1 progeny:
B
b
100% blue bluebells
The results in the F1 progeny show that the gene that controls the blue
colour in bluebells is dominant because is the one that appears in the phenotype,
while the white colour is recessive (White colour is the dominant in the opposite
cross).
Then we cross the offspring from the F1 progeny with the homozygous
recessive mutant. This is called Testcross:
Parents:
B
b
Gametes:
B, b
X
b
b
b
Genotype
B
b
b
b
Phenotype
Blue (normal)
White (mutant)
1
1
Progeny:
Ratio
The testcross results show a 1:1 ratio which means that the mutation is in
a single gene.
In addition by self crossing the F1 progeny we have a stronger prove that
the blue allele is the dominant:
F2:
Gametes
G
a
m
e
t
e
s
B
b
B
B
B
B
b
b
B
b
b
b
The results from the F2 progeny show that the phenotypic ratio is 3 blue:
1white. The mutant phenotype is present in F2 progeny, thus the b existed in F1
progeny and it is the recessive one.
We examined the probability of mutation in a single gene but we also
have to examine what happens when the mutation is in two different genes, in
order to analyse properly the causes of phenotypic variation. If the mutation is
in two genes that means that we have a combination of two haploid genomes,
which contain different recessive mutations which yield the wild type
phenotype. We need to cross a number of white flowers with other white
coloured flowers and see what happens in F1 progeny.
Genes: b, w
Alleles: -Blue=
Parents:
𝑏+ 𝑤 +
𝑏+ 𝑤 +
𝑏− 𝑤 +
𝑏− 𝑤 +
(white)
-White=
x
𝑏− 𝑤 −
𝑏− 𝑤 −
𝑏+ 𝑤 −
𝑏+ 𝑤 −
(white)
𝑏 −𝑤+
Gametes:
F1 progeny:
𝑏− 𝑤 +
𝑏+ 𝑤 −
𝑏+𝑤 −
(blue)
The results show that when we cross white flowers with white flowers
and get the wild type phenotype in the F1 progeny, which is the blue colour, this
means that the mutation is in different genes.
Furthermore, if we take the mutation in different genes as a fact it would
be important to know whether the mutations are close together or far apart on
the chromosomes. We need to cross the genotypes of the F1 progeny with the
recessive homozygous (white).
Parents:
𝑏− 𝑤 +
𝑏− 𝑤−
x
𝑏+ 𝑤 −
𝑏− 𝑤−
Gametes: 𝑏 − 𝑤 + ,𝑏 + 𝑤 − , 𝑏 − 𝑤 − , 𝑏 + 𝑤 +
Parental
𝑏−𝑤 −
Recombinants
Gametes
F1 progeny:
𝑏−𝑤 −
Gamete
Genotypic ratio: 1:1:1:1
𝑏−𝑤 +
𝑏+ 𝑤 −
𝑏− 𝑤 −
𝑏+𝑤 +
𝑏−𝑤 +
𝑏+ 𝑤 −
𝑏−𝑤 −
𝑏+𝑤 +
𝑏−𝑤 −
𝑏− 𝑤 −
𝑏−𝑤 −
𝑏−𝑤 −
We observe free recombination of the chromosomes and production of
recombinant gametes. This means that the genes are unlinked (located in
different chromosomes). In addition, the number of parental gametes is equal to
the number of recombinant gametes, thus the recombination frequency is 50%.
If we self cross the F1 progeny (blue) we will have a phenotypic ratio: 9:3:3:1,
which indicates the independent assortment of genes thus the genes, are
unlinked. The cross is shown below:
Gametes
F2:
𝑏−𝑤 +
𝑏−𝑤 +
𝑏+𝑤 −
𝑏−𝑤 −
𝑏+𝑤 +
𝑏−𝑤 +
𝑏+𝑤 −
𝑏−𝑤 −
𝑏+𝑤 +
𝑏−𝑤 +
𝑏−𝑤 +
𝑏−𝑤 +
𝑏−𝑤 +
𝑏−𝑤 +
𝑏+𝑤 −
𝑏−𝑤 −
𝑏+𝑤 +
𝑏+𝑤 −
𝑏+𝑤 −
𝑏+𝑤 −
𝑏+𝑤 −
𝑏−𝑤 +
𝑏+𝑤 −
𝑏−𝑤 −
𝑏+𝑤 +
𝑏−𝑤 −
𝑏−𝑤 −
𝑏−𝑤 −
𝑏−𝑤 −
𝑏−𝑤 +
𝑏+𝑤 −
𝑏−𝑤 −
𝑏+𝑤 +
𝑏+𝑤 +
𝑏+𝑤 +
𝑏+𝑤 +
𝑏+𝑤 +
G
a
m
e
t
𝑏+𝑤 −
𝑏−𝑤 −
e
s
𝑏+𝑤 +
If the parental gametes are more than the recombinant the genes are
linked and this means that they are on the same chromosome. However
independent assortment does not occur because in the self cross of F1 progeny
we do not have the phenotypic ratio: 9:3:3:1. For linked genes the
recombination frequency is less than 50%. When the genes are close together
the probability of chiasma is lower, thus the recombination frequency is lower.
When they are far apart there is a probability of chiasma close to 50%.
Generally, as we mentioned before, every single character of an
organism is controlled by one or more genes. If flower colouration is controlled
by one gene, that gene encodes a specific protein that functions as an enzyme in
the synthesis of a blue pigment. White pigment is mutant thus the protein which
is responsible for its synthesis is not effective. Furthermore we need to identify
the missing or defective protein in the white bluebells by a simple diagram:
Enzyme
Substrate
Enzyme
Intermediate complex
(White colour)
(White colour)
Product
(Blue colour)
The diagram shows that the gene which is responsible for the white
colour is mutated, therefore the encoded protein is defective and cannot work as
an enzyme, thus the white colour cannot become blue because the reaction is
impossible.
With the exact same way, if the flower colouration is controlled by two
genes, these genes will encode two proteins-enzymes responsible for the
synthesis of the blue pigments. The diagram is the same with the deference that
there are two enzymes for the flower colouration:
Enzyme A
Substrate
(White colour)
Enzyme B
Intermediate complex
(White colour)
Product
(Blue colour)
The diagram shows that the bluebells are white because there is a
mutation in the gene that encodes the protein which functions as enzyme A, thus
the enzyme is defective or might be missing and the reaction cannot proceed
further. If there is a mutation in the gene that encodes the protein- enzyme B,
the reaction stops and the bluebells remain white for the same reason.
The conclusion from the last diagram is that in order to have blue
pigments both genes are necessary (in their normal form) so that they encode
two proteins-enzymes which function effectively for the blue pigment synthesis.
However mutation in genes is not the only factor that causes the lower
number of the white bluebells in contrast to the high number of the blue
flowers. There are other effects probably related to environmental factors or
physiology adaptations such as birds’ preference in colour. Birds might be more
attracted to white flowers, thus they eat them as a result to have a low number
of white bluebells. In addition white flowers absorb higher amounts of U.V
radiation than the blue ones, thus their DNA is destroyed. Another factor is that
blue flowers remain in a high number because they might have antifreeze
proteins which give them the ability to survive in low temperatures or maybe
they have other specified proteins which protect them from high temperatures
and dry weather conditions.
In conclusion, it is obvious that genotypic variation is a complicated
phenomenon which requires many different aspects of gene behaviour and other
specific effects to be perfectly analysed.
REFERENCES
Books
1. Campbell and Reece, (2008). Biology: 14:262-275
2. Hartwell et al, (2008).From Genes to Genomes: 7: 207-227
Images
Image taken from: http://people.na.infn.it/~nicodem/research/cell_genes.jpg