Who is he (Biography)
Thomas Hunt Morgan was born on September 25, 1866. He was the eldest son of Charlton Hunt Morgan. Morgan showed an interest in natural history when we a young boy. He received a bachelor's degree from the State College of Kentucky in zoology and then entered John Hopkins University for graduate work in biology. This happened in 1886. Morgan studied under the morphologist and embryologist William Keith Brooks when he was at John Hopkins. Brooks was a great man according to Morgan. Morgan used experimental techniques to problems that related to embryology. He worked on this subject from 1893 until 1910. He tried to help as much as he could. In order to identify related events during development of the embryo, he observed and analyzed problems like the formation of embryos from separated blastomeres. Blastomeres are early embryonic cells. Morgan, like many biologists at his time thought the Darwinian Theory of Evolution was lacking in plausibility. Some parts seemed like they could not happen or were less likely to happen. Morgan thought that no single chromosome could carry specific traits. In 1909 and 1910 he argued about that in papers he published. He also said that the Mendelian Theory was hypothetical but it could account for certain things, such as predicting breeding results The problem with the theory was that it could not describe the exact process of heredity. The pairs of chromosomes seperating and going into different sperm or egg cells did not seem to be enough proof for Morgan to believe that segregation and independent assortment related. After he finished He later went on to work with fruit flies, otherwise known as Drosophila.
The Work with Drosophila (Fruit Flies)
Thomas Morgan wanted to study an organism with a unique trait so he could observe how the traits would be passed down. Morgan discovered that a single white-eyed male fly was living among the red-eyed flies. He decided to cross the mutant male and a red-eyed female. The offspring were all red-eyed. White-eyed flies finally reappeared in the next generation. It was the pattern of how recessive traits got passed down. He discovered that the white-eyed trait was seen only in males of the second generation. At the end of the experiment, he concluded that white eyes is a sex-linked recessive trait. This concluded with the idea that the gene for eye color is on the X chromosome.
Hypothesis
T.H Morgan didn't have a hypothesis at first. He wanted to study a unique trait's capability of inheritance. A hypothesis for future results was made later after his first experiment. Morgan first bred the white-eyed male and the red-eyed female. This led to 1, 237 red-eyed flies. This was the F1 generation. There were three white-eyed males as well but Morgan ignored them and focused on why there was little white-eyed offspring.
Morgan then let the F1 generation breed among themselves, which led to some interesting results.
- 2459 red-eyed females
- 1011 red-eyed males
- 782 white-eyed males
There were no white-eyed females.
The trait seemed to be sex-limited because it only appeared in the grandsons. Later it gets showed that the trait was compatible with females as well as males. A white eyed male from the F1 generation mated with one of his daughters and produced offspring that involved white eyed females.
- 129 red-eyed females
- 132 red-eyed males
- 88 white-eyed females
- 86 white-eyed males
This showed that the trait can be passed to females if there is a suitable cross.
A Hypothesis for Future Results
White-eyed Males: Assume that the sperm of the white-eyed males carry the factor for the white eyes (W). That means that half of the sperm carries a sex factor which is X. The other half of the sperm lacks W because males are not homozygous for sex ( males are XY). This eventually shows that the symbol for the males with white eyes are WWX and if it was for his two kinds of sperm then his symbols would become WX and W.
Red-eyed Females: Assume that the eggs of the red-eyed females all carry a factor (R) for the red-eyed trait. All of the eggs carry one X each. This means that the symbol for red-eyed females is RRXX. If it was for each of her eggs then it would be RX and RX.
So Morgan did some probability and said this..
-If the white-eyed males breed with their red-eyed sisters then the following results will occur.Then Morgan used probability again for the F1 generation's offspring.The last formula showed that Mendel's ideas were implemented in Morgan's experiment. It showed the 3:1 ratio. It also was apparent that the whites were mainly in males.
One note should be that there are two types of red-eyed females. There is the pure red-eyed (RRXX) and the hybrid (RWXX) but there is only one class of red males (RWX).
In order for the results to make sense we have to assume that when the two classes of the sperm are formed in the F1 red-eyed male(RWX), R and X will go together. If they do not then there will be different results with different combinations. The hypothesis is more utilized than said so that it can be tested easily for the different combinations.
Results (The Four Verifications)
1. If a white-eyed male (WWX) and a white-eyed female (WWXX) mated then the offspring would be 50% white-eyed female and 50% white-eyed male. The germ cells of all of them would be
- Males: WX-W
- Females: WX-WX
All the offspring are white-eyed and there should be an equal number of males and females.
2. A said earlier, there are two classes of females, RRXX and RWXX. We can test to see what offspring would be produced with a white-eyed male. In one situation ( where the female is RRXX) all of the offspring would be red-eyed. Then in another situation (where the female is RWXX) where there are four different classes of offspring with different sexes.
3. The red-eyed F1 generation females should all be classified RWXX and should give with any white male the different four combinations in the previous verification.
4. The red F1 generation males (when they are RWX) should be heterozygous. When they are crossed with white females (WWXX) all the female offspring should be red-eyed, and all the male offspring should be white-eyed.
Importance
The importance of the Thomas's work with fruit flies helped further the human knowledge on genetic disorders, because fruit flies and humans shared about 70% of the same genetic disorders. These fruit flies were really helpful to Thomas's work and brought up a new idea on how to determine sex linked disorders. Fruit flies were surprisingly helpful toward his work because they had X linked characteristics. Today, his work helped avoid a lot of people with disorders and made the foundation on how to analyze and improve our human DNA. He also made the chromosomal theory of inheritance and for that Thomas was awarded the Noble Prize for Medicine. This theory supported the idea that all humans have their own individuality. Although he died on December 4th, 1945, his work helped and continues human genetics.
Source
- http://www.unc.edu/depts/our/hhmi/hhmi-ft_learning_modules/2011/fruitflymodule/
- http://www.dnaftb.org/10/bio.html
- http://www.britannica.com/biography/Thomas-Hunt-Morgan
- http://www.nature.com/scitable/topicpage/thomas-hunt-morgan-the-fruit-fly-scientist-6579789
- http://www.nobelprize.org/nobel_prizes/medicine/laureates/1933/morgan-bio.html
