Bluebells are diploid perennial plants that grow mitotically and produce flowers every year. The flowers produce haploid pollen and ovules, and these gametes can fuse after either self or cross pollination to give the progeny seed (zygote).
To test whether the abnormal colour is due to colour we must first consider environmental factors that affect colour of a bluebell. Examples may include the composition of the soil, its pH, light intensity and heat. To prevent genetic factors from interfering we must use flowers from the same parent (genetically identical) as that of the white flower and breed them in varying environments. We will also replicate the environment in which the initial white flower was found. If in any of the cases anything other than a white bluebell is produced then it is an environmental factor that is affecting the phenotype.
We start with pure breeding which are homozygous that will produce offspring which carry unambiguous parental traits which will stay invariable from generation to generation. Crossing of two different phenotypes, where one parent carries one form of trait and the other carries alternative traits which are pure bred, blue and White. The progeny of these reciprocal crosses is Blue due to a single gene which is called the F1 generation. Inbreeding of F1 generation results F2 generation which consist of two phenotypes in a ratio of 3:1. Self pollination of F1 gives rise to f2 generation with a 3:1 ratio of individuals resembling the two original parental types.
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b Bb Bb
b- Recessive mutant B dominant wild type
Figure 1: F1 with F1
As you can see from the table all the progeny's are blue. Using these F1 progeny we will cross with f1 progeny.
B BB Bb
b Bb bb
b- Recessive mutant B dominant wild type
Figure 2: Mendel's first ratio; illustrates the combination that can rise when f1 hybridundergoes formation and self fertilization. The f2 generation have 3:1 ratio of blue to white bells.
Blue bells are dominant that appeared in F1 hybrids whereas white bells 'antagonistic' appeared in F2 generation recessive as white bells trait was hidden in F1 Hybrid. Each plant carries two copies from of a unit of inheritance from its maternal and parental parent, which is known as genes. Bluebell plant contains two copies of a gene for bells colour. Allele is a series of different forms of genetic locus. The gene for bluebell has white and blue alleles. If a bluebell has two copies of each gene, how come it only passes one copy to its progeny and offspring has two copies?
There are two phenotype classes in the F2 generation but three genotypic Classes. Two identical alleles of pure breeding plants segregate during formation which results as a , each pollen carries only one of each pair of parental alleles.
Considering the behaviour of copies of two different genes. Each pair of allele assorts independently during the process of gamete formation determines the phenotype observed.
Two mutant genomes with recessive mutation require, if F1 progeny of cross show normal phenotype we infer complementation. Each mutant genome compensates for the functional defect of the other. Conclude, two genomes are defective in different genes.
Cross blue bells with white bells. The 9:7 ratio of blue to white F2 plants indicates that at least one dominant allele for each gene is necessary for the development of blue colour.
Testcross: cross F1 with white recessive homozygote. If two mutations on the same gene therefore two gene are responsible for the pigment.
BB CC bb cc
BC BC bc bc Gametes
bc BbCc BbCc
bc BbCc BbCc
All blue so two genes: b- Recessive mutant B dominant wild type C Dominant (pigment produced)
At meiosis, copies of different genes may assort independently and random assortment of chromosome generates 2x genetically different gametes for a heterozygous organism with x chromosome pairs.
Biochemical basis of flower colourisation can be blocked by mutations that show evidence that different genes controls enzymes each which catalyzes certain reaction. The order of the genes acting in the pathway may be discovered from the phenotypic classes. A cross between true breeding blue bells and white bells hybrids pale blue bells which could be result of blue and white pigments. If it is approved to self pollinate, the F1 pale blue would produce F2 progeny blue, white and pale blue in the ratio of 1:2:1. The biochemical explanation for this type is incomplete dominance; each allele of the gene specifies a form of protein molecule with an enzymatic role of a pigment production. In the homozygote for the 'white' allele, where there is no functional enzyme and thus no blue pigment, the bells appear white.
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A and B codes for the enzyme that makes a pigment. W+ B+ would make enzymes a blue which will make a blue pigment. W- B+ it would stay white because it needs a pale blue pigment to turn into blue. W- B- , it stays white since there is no pigment. W+ B- .
Mutations in two different can both cause the flowers to be white. A gene interaction in which the effects of an allele at one gene can hide the effects of another gene is known as epitasis. Crosses between two true breeding BB EE(blue) and one type of pure breeding bb ee (white) that create a F1 genration of dihybrid would be blue. Crosses between f1 dihybrids produce f2 generation creating three phenotypic classes as the two genotypic classes are without a dominant E allele. The ratio of recessive epitasis in the f2 generation is 9:3:4 as ee genotype mask the control of other gene for colour.
B E Be ?E be
BE BB EE BB Ee Bb EE Bb Ee
Be BB Ee BB ee Bb Ee Bb ee
?E Bb EE Bb Ee Bb EE bb Ee
be Bb Ee Bb ee bb Ee bb ee
Figure: recessive epitasis
To find out the colour of the flower, one gene locus may influence whether the pigment is blue ( BB or Ba) or white (bb), while another locus decide whether the pigment is produced (EE or ee) or not (bb). In a bb flowered plant, the flowers will be white, irrespective of the genotype of the other locus as BB, Bb, or bb. It is not the case that the b allele is dominant to the A allele: the B locus is said to show recessive epitasis to the A locus, because the b locus when homozygous for the recessive allele overrides the phenotypic expression of the B locus.
There are many factors that explain a low frequency of the white bluebells for example; if the flower isn't blue then pollinators will not be attracted so they may not get pollinated. Although they may be able to self-pollinate and then they can guarantee white offspring. Another factor could be if they have the blue pigment that could provide a protection against predators.