The phenotypes of the maize cobs fruit:
- Purple and wrinkled (Pw)
- Purple and smooth (PW)
- Yellow and wrinkled (pw)
- Yellow and smooth (pW)
PW Pw pW pw
PW
Pw
pW
pw
Phenotype of fruit from F1:
-
Purple and smooth 9 (The ratio being 9:3:3:1)
- Purple and wrinkled 3
- Yellow and smooth 3
- Yellow and wrinkled 1
The Purple and smooth kernels are the dominant alleles, which can be seen by looking at the cross above; these dominant alleles can also be seen as the majority of the phenotypes of the maize cob fruits are mainly purple and smooth.
Gregor Mendel crossed pure breeding plants with round seeds and yellow albumen to pure breeding plants with wrinkled seeds and green albumen, the result of this was the F1 generation plants all had round seeds and yellow albumen and Mendel predicted that they would be heterozygous for both traits (RrYy).
Meiosis (cell reduction) is the cellular process of gamete creation, this is where the sperm and eggs get the unique set of genetic information that will be used in the development and growth of the offspring of the mating; the rules of meiosis highly apply to the dihybrid which are codified in Mendel's first Law and Mendel's second law which is also known as the law of segregation and the law of independent assortment.
A dihybrid cross is a breeding experiment between P generation (parental generation) organisms that differ in two traits, for example in this dihybrid cross (shown below), a plant with the dominant traits of green pod colour and yellow seed colour is cross pollinated with a plant with the recessive traits of yellow pod colour and green seed colour.
If a true-breeding plant with green pod colour (GG) and yellow seed colour (YY) is cross-pollinated with a true-breeding plant with yellow pod colour (gg) and green seeds (yy), the resulting offspring will all be heterozygous for green pod colour and yellow seeds (GgYy).
A class practical was carried out on purple fruit maize cobs and yellow fruit maze cobs where the individual kernels (fruits) were able to be identified and counted and the results were then recorded according to the maize’s phenotype.
The table to the right shows the observed results gathered from the group’s observations of the selected fruit maize cobs...
These phenotypes were what the fruit maze cobs consisted of being Yellow Wrinkled, Yellow Smooth, Purple Wrinkled and Purple Smooth.
A phenotype is an organism's observable characteristics or traits, Phenotypes result from the organism's genes as well as the influence of environmental factors and the interactions between the two.
This 2nd table below shows the overall results gathered from the group and the calculated chi square test results carried out on the maize cobs...
The results reviewed and calculated from the table shown the null hypothesis was rejected which came to 237.16 which was above critical value on the table as the result was significant due to there being a big difference between the observed and expected individual results, the chi square table below shows this concluded decision...
There were 4 different phenotypes identified and shown in the table, in order to work out the degrees of freedom from the four phenotype rows, 1 row is subtracted which leaves 3.
Once this step was undertaken the null hypothesis value which was 237.16 was observed across row 3 on the degrees of freedom, however the null hypothesis was not able to be identified on the chi square table due which lead to the conclusion of the null hypothesis being rejected as it was above critical value on the table as the result was significant.
There are many reasons why the null hypothesis could have been rejected; a dihybrid cross involving these types of genes can produce a phenotypic ratio very different from 9:3:3:1. which was expected, however there could be more than two gene products which could be affecting the same phenotype, and these products may have complex hierarchical relationships.
When the alleles of two different genes separate during meiosis, they separate independently of one another unless the genes are located on the same chromosome (linked); this is the principle of independent assortment.
According to the principle of independent assortment, the color gene and the seed shape gene should not affect one another; they should behave independently which means that there are four possible outcomes of kernels.
Most phenotypic traits are influenced by the function of genes; for example the synthesis of a pigment might require an enzymatic pathway with multiple steps, due to each step being catalyzed by its own enzyme and each enzyme is encoded by a different gene, this means If an individual is lacking the functional enzyme for any step in the pathway (for example in a homozygous recessive mutant that lacks an allele to encode the functional enzyme) then the pathway is blocked and the pigment (color) will not be produced in the kernel regardless of the genotype.
Between the observed and expected results does not contain any correlation; the dihybrid ratio is meant to be 9:3:3:1 however there could be no correlation in these results because of random segregation for example.
Every individual possesses a pair of alleles for any particular trait and that each parent passes a randomly selected copy (allele) of only one of these to its offspring; the offspring then receives its own pair of alleles for that trait, whichever of the two alleles in the offspring is dominant determines how the offspring expresses that trait such as the colour of the corn kernel, the texture of it and even the shape.
Mutations can also cause the corn kernels to be different as the mutations can effect the genes; the genes could have been effected from disease for example as if a certain area where the maize’s are being produced has contaminated soil these contaminations of disease will spread to the cobs in that area which will cause mutations within the appearance and final out come of the cob.
Chromosomal crossover (or crossing over) is an exchange of genetic material between homologous chromosomes which can effect what genetic information is received by the offspring; in most eukaryotes a cell carries two copies of each gene know as an allele, each parent passes on one allele to each offspring.
An individual gamete inherits a complete haploid set of alleles on chromosomes that are independently selected from each pair of chromatids which are lined up on the metaphase plate; without recombination all the alleles for those genes linked together on the same chromosome would be inherited together however meiotic recombination allows a more independent selection between the two alleles that occupy the positions of single genes, as recombination shuffles the allele content between homologous chromosomes.
Recombination results in a new arrangement of maternal and paternal alleles on the same chromosome; although the same genes appear in the same order the alleles are different, this makes it possible to have any combination of parental alleles in an offspring which results in different genetic material being used which causes variations in appearances for example colour and texture.
References used: (All used on the 15.12.2011)
