Genes are molecular units of heredity which encode for different types of traits. Each organism has traits that are defined phenotypically and can be studied though the field of genetics. Genetics is the study of genes, heredity, and how they cause variation in different living organisms. Scientists study genetic pattern in different organisms to determine the different trends in a certain population. In every organism, we obtain one allele from each parent. Alleles are types of genes that can be identified on the chromosomes, which are in the nucleus of the cell.
Alleles are either dominant or recessive. Dominant alleles are the ones that are usually phenotypically expressed, while the recessive alleles are usually silenced by the dominant gene. For a dominant allele, only one allele is needed for that trait to be expressed. However if the organism has two recessive alleles, the recessive trait is phenotypically expressed. Dominant phenotypes can have two different types of genotypes. For example, if the trait A is dominant and the trait a is recessive, an organism that has genotype AA or Aa will express that dominant trait.
However, if the alleles are aa, then the recessive trait is expressed. When studying variation of genes in a species, researchers perform crosses. This requires mating between individuals and analyzing the resulting offspring to see which traits are phenotypically expressed. Geneticists’ uses Punnet squares to predict which phenotypes the future generations may have. By crossing species that contain different types of phenotypic variations, scientists are able to use this information to trace traits and alleles from the past generations.
Gregor Mendel, the father of genetics, came up with two laws that he discovered in his pea plant experiments. These two laws contributed greatly to what we know about genes today. The first law is the Law of Segregation. This law states that every organism has a pair of alleles. Within this pair of allele, one of it is randomly selected to pass on to the offspring. This supports the fact that the mother and the father has to give an allele to the offspring to form a pair of alleles. Depending on which allele is selected from each parent, the offspring will express different phenotypes.
The second law is the Law of Independent Assortment. This law states that separate genes for different traits is separated independently from one another and one of them is passed onto the offspring. This means that a particular gene in the pair that is passed on has nothing to do with the selection of gene for any other trait. During gamete formation, the alleles separate from one another creating variation. In a monohybrid cross, there is one specific gene of interest that is different between the parents. The parents have different alleles.
In a monohybrid cross, we expect to see a 3 dominant trait: 1 recessive trait ratio in the F2generation. In a dihybrid cross, uses one parent who is homozygous dominant in two different traits and another parent who is homozygous dominant to create an F1 generation that is heterozygous in these two different traits. Afterwards, the F1 generation is crossed with one another to create an F2 generation. This turns out to have a 9:3:3:1 ratio of the two traits that were tested. Every time a cross is performed in a lab, whether it is monohybrid or dihybrid.
These phenotypic ratios are predicted as crosses are being performed in the laboratory. Furthermore, traits can either be autosomal recessive traits or sex linked traits. Autosomal recessive traits are not affected by other genes. However, sex linked genes are carried on the sex chromosomes. For example, X linked genes are located on the X chromosome. Since women have two X chromosomes, the recessive trait would have to be present in order for the recessive phenotype to be expressed. On the other hand, men have only X chromosome.
This make it easier for men to have a recessive trait expressed phenotypically. Usually, women are frequent carriers of a certain X linked trait. It is more difficult for women to have a recessive trait than men. When testing whether a certain trait is sex linked, geneticists generally use a reciprocal cross. A reciprocal cross is used to test the role of parental sex and how it may or may not affect the resulting phenotype of their offspring. This can change the whole inheritance pattern of all the future generations.
For the reciprocal cross to happen, the parent organisms have to be true breeding. In a reciprocal cross, we first cross a male expressing a specific trait with a female that does not express that same trait. With the reciprocal cross, we would cross a female that expresses that trait with a male that does not express that same trait. For these two experiments to work, the trait for both of these crosses have to be the same. After performing these crosses, we analyze the offspring to observe whether the reciprocal cross changes the phenotypic expression of that specific trait.
The Chi Squared Analysis uses the expected value and the observed value to determine the P value. The P. value shows the probability of something happening due to chance alone. This allows us to determine if the values we have observed is accepted or reject the null hypothesis. If the P value obtained during this experiment is below 0. 05, this shows that the value obtained is due to something other than chance which causes the value to be rejected. On the other hand, if the P value obtained is higher than 0. 5, it indicates that the difference between the expected and observed value is largely due to chance.
This allows the null hypothesis to be accepted. In this experiment, students learn how each law influences variation in the offspring of different species. Since the start of the 20th century, the common fruit fly, Drosophila melanogaster, has been a useful organism for the study of genetics due to its simplicity. The common fruit fly is easily cultured and has a relatively short generation time (about 10 days at 250C). This allows scientists to observe the different phenotypes present in each generation.
Fruit flies yields a large amount of offspring that can be collected as data in a short period of time. Because of drosophila melanogaster’s simple food requirements and easy handling in the laboratory, Drosophila can be maintained with minimal cost and effort. Drosophilas have four pairs of chromosomes. The X/Y sex chromosomes and the 2,3,4, autosomes. It is important to know the differences between the two adult sexes in order and collect the data accurately. The major sexual differences in Drosophila are apparent in the abdominal segment of the flies.
In males, the last abdominal segment of the male is much larger and rounded than that in the female. Another indicator is the presence of sex combs present in males. Male flies has a small, densely packed bristles call sex comb on the outer joints of both forelegs. Females lack sex combs. Therefore, if one sees sex combs on a fly, it is certain that the fly is a male. Female fruit flies remain virgins for approximately six hours after hatching but will mate after the six hour window. It is important for the female flies to be virgin, so one knows which fly genotypes are crossed.