ABSTRACT:
This
experiment demonstrates both cell division and genetics. Various stages
of mitosis, prophase, metaphase, anaphase and telophase are easily identified
from fish eggs and onion tip cells. Stages of meiosis including synapsis,
metaphase, and telophase, are located and identified in grasshopper testis.
Corn plants are used to demonstrate the genetics of corn breeding for a
monohybrid and dihybrid cross. The results of the monohybrid crosses for
color are significantly different than the expected ratios, suggesting
counting error, sampling error, or insertions of genetic information from
transposons. The kernels from the dihybrid cross also vary significantly
from the expected ratio. The observed ratio is 14.5:2:6.5:1 and the expected
ratio is 9:3:3:1. Some of the same sources of error discussed above may
also be operating in the dihybrid cross as well. Finally, wild-type and
dpy-20 nematodes demonstrate that a single mutation can affect the phenotype
of the organism. This experiment is relevant since it shows that observational
data may not correspond exactly with theoretical calculations in monohybrid
and dihybrid crosses. Limit words in the
abstract to one hundred if possible, but say what must be said about the
experiment including introduction, purpose, results, and implications.
This must be done in present tense.
INTRODUCTION:
The objectives of this particular laboratory experiment are to: 1) identify
the phases of mitosis from histological samples of onion tip and fish blastula
through the use of a microscope; 2) to distinguish histological features
of meiosis from grasshopper testis through the use of a microscope; and
3) to analyze the results of monohybrid and dihybrid crosses of corn through
counting kernels of corn. Please
note that the purpose and the method of examination are listed in this
section. This section can be very brief and to the point.
EXPERIMENTAL PROCEDURES:
Refer to the laboratory manual pages 25-37. Only
itemize the procedures if there is a change from the laboratory manual.
RESULTS:
In the section on cell genetics, mitotic stages from the fish blastula
are not easily observable under 10 and 40 power; however, at 100 power,
they are identified as prophase, metaphase, and telophase. The onion tip
was easily identifiable under all powers (except oil immersion) and all
stages of mitosis are viewed. The grasshopper testis is most readily seen
under 40 and 100 power. Cells in synapsis, metaphase, and telophase are
noted.
Fish Blastula- drawings, with label, scale
and preparation technique
The drawings represent each of the stages of
mitosis with the exception of anaphase, since it cannot be located.
Onion Tip Drawings - drawings, with label,
scale and preparation technique
The drawings represent each of the stages of
mitosis.
Grasshopper Testis - drawings, with label,
scale and preparation technique
The drawings represent synapsis, metaphase and
telophase of meiosis from grasshopper testis.
Corn Genetics
In the section on corn genetics, the experiment begins with two homozygous parents, one parent was purple and homozygous dominant for the alleles (PP). The other parent is yellow and homozygous recessive (pp) for the alleles. These parents are crossed and a heterozygous offspring (Pp), purple in color, is produced. A test cross is then conducted to determine the genetic constitution or genotype of the F1 generation as determined from the phenotype by mating back to the parent with the recessive alleles. The heterozygote F1 (Pp), purple in color is crossed with the homozygous recessive (pp) parent, yellow in color.
Punnett's Square Here
This monohybrid cross produces two colors of offspring, purple and yellow in an expected ratio of 1:1. Yet, while counting kernels from the monohybrid cross, the ratios are not the same.
Yellow 50 155 pp 50/50 = 1
The observed ratio is 2.1:1. There
are several reasons why these ratios are not the same. For instance, some
kernels appear very small and shrunken indicating early selection against
certain phenotypes, miscounting of kernels may occur, and sample sizes
are relatively small, suggesting sampling error.
Next, two heterozygote
F1 offspring are crossed to each other. Each F1 heterozygote is genetically
(Pp)
and purple in color. A Punnet's square is developed to find expected
genotypic and phenotypic ratios for the offspring. The genotypic ratio
is 1:2:1, while the phenotypic ratio is 3:1. This suggests that
75% of the F2 kernels will be purple in color, while 25% will be
yellow in color.
Punnett's Square Here
Observations do not reveal a 3:1 phenotypic ration, but rather a 4.5:1 phenotypic ratio.
Purple 198 242 PP, Pp 198/44 = 4.5
Yellow 44 242 pp 44/44 = 1
The dihybrid cross is carried out for two traits, plump (S-) vs. shriveled (ss), and purple (P-) vs. yellow (pp). The expected ratio is developed from the following Punnet's square.
Punnett's Square Here
The expected phenotypic ratio is 9:3:3:1. This reflect 9 plump and purple, 3 plump and yellow, 3 shriveled and purple, and 1 shriveled and yellow.
Kernel Shape and Color Total Calculation Proportion
Plump and Purple 406 406/28 14.5
Shrivelled and Purple 56 56/28 2
Plump and Yellow 182 182/28 6.5
Shrivelled and Yellow
28 28/28
1
The progeny is then counted to reveal that the expected ratio of 9:3:3:1 does not match the observed ratio of 14.5:2:6.5:1. These counts do not support the expected ratio, indicating observer error in counting, misclassification of shape and color due to transposon interference, zygotic and developmental selection against certain phenotypes, and statistical error due to a small sample size. Moreover, incidence of transposable element expression is approximately 55% in the ear of corn for the monohybrid cross. Expression is either large spots of opposite color, splotches of color interspersed in a second color, or stripes through the kernel.
Finally, two nematodes are studied microscopically for variation in phenotype due to a mutation called dpy-20. The first nematode in normal, wild type. This organism is long, thin and moves in a gliding fashion. Dpy-20 is fat, short and moves in a more linear motion. This demonstration reflects the fact that a single mutation in a gene can result in alterations of the phenotype of the organism. This section should include all of your results. All data must be reported. Inferences can be made later. Notice that I highlighted information called for in the laboratory manual, so that this information is distinct from background information.
DISCUSSION:
The cell division
section demonstrates the differences between mitosis, used for the replacement
of soma, or general body cells, as distinct and separate from the process
of mitosis for the production of gametes. The stages are essentially the
same, while the processes and ends are different. While mitosis results
in the production of two identical daughter cells, meiosis results in the
production of four non-identical daughter cells that have quite a bit of
variation. The slides demonstrate key stages for each of these processes.
The monohybrid cross illustrates the inheritance of alleles culminating in the genotype, and expressing itself in the phenotype for a single variable, color. Next, a dihybrid cross is performed for two traits, physical characteristics and color. While expected and observed ratios are not the same, several sources of discrepancy are possible. Observer error is noted in at least one instance. Sample sizes are very small. With an increase in sample size, the phenotypic proportions will be more closely matched. Phenotypic selection may occur, as some kernels are very small and shrivelled.
Next, transposons may insert genetic information into kernels of opposite color, thereby influencing protein synthesis and affecting classification of phenotype. All of these factors may be responsible for the variation between expected and observed ratios.
Finally, a wild-type and mutated dpy-20 nematode,
are studied in a nematode to demonstrate that a single mutation can lead
to alterations in the phenotype of an organism. In
the discussion section, you want to place this laboratory into a larger
theoretical framework. What is this lab intended to do? Did this lab accomplish
those ends? Did your experiment come out as planned? Or, did something
go wrong? How do you explain discrepancies?