Gregor Mendel created two main laws prior to his experiments with his growing pea plants. The first law he created is the law of segregation. It states that the two members of a gene pair (alleles) segregate (separate) from each other in the formation of gametes. Half the gametes carry one allele, and the other half carry the other allele. And the second, which is the law of independent assortment. It states that genes for different traits assort independently of one another in the formation of gametes.
These laws were discovered and formed using only pea plants. He used peas because peas are normally self-pollinating, but in this case, Mendel could control whether they self-pollinated or not. In his initial experiments, Mendel removed the stamens from some of the young flowers so that self-fertilization could not occur, then tied a bag around each flower so that cross-fertilization could not occur.
After the pistils became mature, he artificially cross-pollinated them by dusting the stigma of pea plants with pollen of other pea plants that had factors for some contrasting trait.
In this experiment, Drosophila melanogaster, fruit flies, will be used instead of pea plants. D. melanogaster continues to be widely used for biological research in studies of genetics, physiology, microbial pathogenesis and life history evolution. It is typically used because it is an animal species that is easy to care for, has four pairs of chromosomes, breed quickly, and lays many eggs. D. melanogaster is a common pest in homes, restaurants, and other occupied places where food is served.
If we believe that the law of dominance and segregation is accurate, then if we mate a red eyed female with a white eyed male, the characteristics would properly cross over to the offspring.
- Two transparent plastic tubes
- Food substance that can be put in tubes
- Magnifying lens
- Leave a fruit out in the open for 5 days. Two types of fruit flies should start to appear day by day: flies with white and red eyes. These are the only traits tested.
- Isolate one red eyed female virgin fly and one white eyed male fly in a suitable mating capacity which can be a tube with appropriate conditions.
- When the larvae have been created, take out the parents to prevent mating with the first generation. This would convolute the experiment if the parents were unnoticeably allowed to mate with the F2 generation.
- When the flies have grown up freeze them to immobilize them for a little while. This doesn’t harm them, it just freezes their wings for a little while.
- Then take them out onto a clean surface by gently shaking the tube downward. Make sure they are all out because each and every one of the flies can alter the outcome of the results.
- Separate the F2 generation by eye color and gender. Make a chart or a list that shows the exact number of flies in each gender group and separate them again with the color of their eyes.
- Analyze the results. (Why did this come out the way it did? What were the key factors that influenced the outcomes?)
- Then mate a male with a female from the F2 generation in a closed tube with food substances inside. Make sure that they are only from the F2 generation.
- After the female lays her eggs remove the parents from the tube to thwart future mating with the F3 generation.
- Wait about 14 to 21 days for the flies to grow into full adults.
- Once the larvae have grown into adults, freeze the tube again to immobilize them for a while.
- Then separate them again by gender and eye color as before.
- Male flies have a small black comb like fragment on their legs unlike the female fly.
- Female flies a visibly bigger than male flies.
- The flies in the F2 generation were all red eyed but were not all female
- After the dihybrid cross, there started to be more white flies.
It was observed a small but discrete variation known as white-eye in a single male fly in one of the bottles. I bred the fly with normal (red-eyed) females. All of the offspring (F1) were red-eyed. Brother–sister matings among the F1 generation produced a second generation (F2) with some white-eyed flies, all of which were males.
To explain this curious phenomenon, I developed the hypothesis of sex-limited—today called sex-linked—characters, which I assumed were part of the now called X-chromosome of females. Other genetic variations arose in my stock, many of which were also found to be sex-linked. Because all the sex-linked characters were usually inherited together, I gradually became convinced that the X-chromosome carried a number of discrete hereditary units, or factors.
I adopted the term gene, which was introduced by the Danish botanist Wilhelm Johannsen in 1909, and concluded that genes were possibly arranged in a linear fashion on chromosomes. Much to my credit, I rejected my skepticism about both the Mendelian and chromosome theories when I saw from two independent lines of evidence—breeding experiments and cytology—that one could be treated in terms of the other.
White Gene, abbreviated w, was the first sex-linked mutation ever discovered in the fruit fly Drosophila melanogaster. I collected a single male white-eyed mutant from a population of Drosophila melanogaster fruit flies, which usually have dark brick red eyes.
Upon crossing this male with wild-type female flies, I found that the offspring did not conform to the expectations of Mendelian inheritance. The first generation (the F1) produced 1,237 red-eyed offspring and three white-eyed flies, all males. The second-generation (the F2) produced 2,459 red-eyed females, 1,011 red-eyed males, and 782 white-eyed males.
Further experimental crosses led me to the conclusion that this mutation was somehow physically connected to the “factor” that determined gender in Drosophila. I named this trait white gene, now abbreviated w.
This is where the now called Y chromosome was introduced. The Y chromosome is one of two sex chromosomes in mammals, including humans, and many other animals. The other is the X chromosome. Y is the sex-determining chromosome in many species, since it is the presence or absence of Y that determines male or female sex.
In mammals, the Y chromosome contains the gene SRY, which triggers testis development. The DNA in the human Y chromosome is composed of about 59 million base pairs. The Y chromosome is passed only from father to son, so analysis of Y chromosome DNA may thus be used in genealogical research.
First, I crossed the white-eyed male he had found to a normal female, and I looked to see which trait was dominant in the F1 generation: all the progeny had red eyes. Now, would the white-eye trait reappear, segregating in the F2 progeny as Mendel had predicted? In the F2, there were 3470 red-eyed flies and 782 white-eyed flies, roughly a 3:1 ratio. Allowing for some deficiency in recessives, this was not unlike what Mendel’s theory predicted.
But in this first experiment, there was a result that was not predicted by Mendel’s theory: all the white-eyed flies were male! At this point, I had never seen a white-eyed fly that was female. The simplest hypothesis was that such flies were inviable (this might also explain the deficiency of recessives in the 3:1 ratio above).
If any of the F2 females carried the white-eye trait but did not show it, then it should be revealed by a test cross to the recessive parent. It was. Crossing red-eyed F2 females back to the original white-eyed male, I obtained 129 red-eyed females, 132 red-eyed males, and 88 white-eyed females, 86 white-eyed males. Again, this was a rather poor fit to the expected 1:1:1:1 ratio due to a deficiency in recessives.
The important thing, however, was that there were fully 88 white-eyed female flies. Clearly, it was not impossible to be female and white-eyed. Why, then, were there no white-eyed females in the original cross?
Garland, E. A. (2014, March 18). Thomas Hunt Morgan, 2.0. Retrieved January 3, 2015, from Encyclopeadia Britannica: http://www.britannica.com/EBchecked/topic/392256/Thomas-Hunt-Morgan/5014/The-work-on-Drosophila
Pearson Education. (2008, June 7). Concept 1: Reviewing Mendel’s Laws. Retrieved from BioCaoch Activity: http://www.phschool.com/science/biology_place/biocoach/inheritance/laws.html
Wikepedia: The free Encyclopedia. (2014, December 31). Drosophila melanogaster. Retrieved January 2, 2015, from Wikepedia: http://en.wikipedia.org/wiki/Drosophila_melanogaster
 (Pearson Education, 2008)
 (Drosophila melanogaster, 2014)
 (Garland, 2014)
 (Peterson, 2006)
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