in morgan's experiment what will be the percentage

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Thomas Hunt Morgan and Sex Linkage

One day in 1910, American geneticist Thomas Hunt Morgan peered through a hand lens at a male fruit fly, and he noticed it didn't look right. Instead of having the normally brilliant red eyes of wild-type Drosophila melanogaster , this fly had white eyes. Morgan was particularly interested in how traits were inherited and distributed in developing organisms, and he wondered what caused this fly's eyes to deviate from the norm. Morgan's fly lab (Figure 1) at Columbia University was already in the habit of breeding Drosophila so that the researchers there could observe the transmission of genetic traits through successive generations, so Morgan chose to do a simple breeding analysis to find out more about white eyes. Little did Morgan know that, with this white-eyed fly, he was about to confirm the chromosome theory. In doing so, Morgan would also be the first person to definitively link the inheritance of a specific trait with a particular chromosome.

Morgan Detects an Unusual Pattern of Inheritance

Morgan's early days of scientific training had taught him that, in order to find an answer, he must design an experiment that asked the right question. Thus, he first performed a test cross between the white-eyed male fly and several purebred, red-eyed females to see whether white eyes might also occur in the next generation. The members of the resulting F 1 generation had all red eyes, but Morgan suspected that the white-eye trait was still present yet unexpressed in this hybrid generation, like a recessive trait would be. To test this idea, Morgan then crossed males and females from the F 1 generation to probe for a pattern of white eye reoccurrence. Upon doing so, he observed a 3:1 ratio of red eyes to white eyes in the F 2 generation. This result is very similar to those reported for breeding experiments for recessive traits, as first shown by Mendel. Strangely, however, all of Morgan's white-eyed F 2 flies were male, just like their grandfather—there were no white-eyed females at all! Correlation of a nonsexual trait with male or female identity had never been observed before. Why, Morgan puzzled, would this particular trait be limited to only males?

Table 1 provides a brief summary of Morgan's observed results, as well as the expected outcomes for a recessive trait that shows a normal Mendelian pattern of inheritance. In the Mendelian example, the 3:1 ratio of red eyes to white eyes would be shared equally among males (♂) and females (♀). Morgan's data, however, looked very different.

Table 1: Expected Mendelian Ratios versus Morgan's Actual Results

*Morgan did observe 3 white-eyed males in the F 1 generation. His original paper suggested that these white-eyed males were evidence of "further sporting."

Morgan Explores Possible Explanations for This Pattern

Morgan was curious as to why female flies never had white eyes, and he considered several possible reasons for this phenomenon. One potential explanation was that white-eyed females never hatched, or that they died early in development . In other words, this hypothesis predicted that white eyes were lethal in female flies—therefore, among the progeny of a test cross of heterozygous (F 1 ) red-eyed females to white-eyed males, there should be no white-eyed females. Morgan conducted this very cross to see whether the results matched his predictions. Surprisingly, this cross yielded a 1:1:1:1 ratio of red-eyed females to white-eyed females to red-eyed males to white-eyed males. Based on these results, Morgan arrived at three important conclusions:

• The appearance of white eyes in females shows that this trait is not lethal in females.
• All possible combinations of white eyes and sex are possible.
• The white-eye trait can be carried over to females when F 1 females are crossed with white-eyed males.

So, why would white eyes show a bias toward males in the original F 1 x F 1 cross? Morgan knew of recent work by Nettie Stevens and E. B. Wilson that demonstrated that sex determination was related to the inheritance of an " accessory chromosome ," more recently known as the X chromosome . He further recognized that the inheritance of the sex determination chromosomes in Drosophila seemed to follow closely with the inheritance of the white-eye phenotype . But what was the exact relationship between eye color and sex?

Principles of Sex Determination

Morgan's test crosses, the context of morgan's discovery.

Morgan's conclusion—that the white-eye trait followed patterns of sex chromosome inheritance—was at once very specific and very grand. A few years prior to these test crosses, Mendelian ideas of inheritance had been enthusiastically discussed by many researchers in the context of new findings about chromosomes. Indeed, after observing meiotic reductive divisions and correlating them to chromosome counts in male and female offspring, cytologists Walter Sutton, Nettie Stevens, and E. B. Wilson had all promoted the idea that sex was determined via chromosome-based inheritance . Morgan, however, had long resisted the idea that genes resided on chromosomes, because he did not approve of scientific data acquired by passive observation. Furthermore, Morgan was not convinced that traits couldn't morph into new forms in an organism based on the blending of parental contributions, an idea leftover from pre-Mendelian scientists. Morgan was sure that Wilson and the other researchers who promoted the chromosome theory of inheritance were looking for an easy answer as to how independent assortment occurred in gamete formation, because he believed they ignored counterevidence in the face of excited conviction. In fact, he thought that the concept of genes was at best an invention intended to link the mysterious paths of chromosomes and discontinuous inheritance patterns. Morgan formalized his derision in a well-known publication (Morgan, 1909), wherein he called for a more experimental approach to the understanding of inherited factors and insisted that germ plasm should not be cast aside as a putative carrier of inherited traits.

Interestingly, within a year of this public criticism of chromosome theory, Morgan set out to test the idea of inherited chromosomal factors using Drosophila . Because Morgan was particularly interested in experiments designed to test hypotheses, he turned to the fly system to maximize data acquisition over short periods of time. Soon after launching these experiments, Morgan saw his white-eyed fly peering back at him through his hand lens. Then, many crosses later, Morgan became convinced by his own empirical evidence that traits could in fact be passed on in the same manner predicted by the inheritance of sex chromosomes . Morgan never looked back, and he developed a huge following of accomplished students over the next few decades. Indeed, for his work with Drosophila , Morgan was awarded the Nobel Prize in 1933.

References and Recommended Reading

Benson, K. R. T. H. Morgan 's resistance to the chromosome theory. Nature Reviews Genetics 2 , 469–474 (2001) doi:10.1038/35076532 ( link to article )

Morgan, T. H. What are "factors" in Mendelian explanations? American Breeders Association Reports 5 , 365–368 (1909) ( link to article )

———. Sex-limited inheritance in Drosophila . Science 32 , 120–122 (1910) ( link to article )

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Linkage And Recombination – Principles Of Inheritance And Variation Class 12 NCERT

CBSE Class 12- Principles Of Inheritance And Variation- Linkage And Recombination: Linkage and recombination are the phenomena that describe the inheritance of genes. Linkage and Recombination both are related to the genetic information inherited from parents to offspring. Linkage is the tendency of genes present close to each other on a chromosome to be inherited together more frequently than expected by chance. Recombination is the process by which genetic information from 2 parent chromosomes is mixed and exchanged during meiosis, resulting in new combinations of alleles in the offspring.

Morgan’s Experiment

Thomas Hunt Morgan studied the fruit fly Drosophila melanogaster to understand how sexual reproduction created differences. For the following reasons, he preferred to work with fruit flies or Drosophila melanogaster:

• It was simple to grow it in a lab using a synthetic medium.
• Demonstrate a brief lifespan (two weeks).
• Several offspring could result from a single mating.
• Distinct sexes for men and women.
• There are various kinds of hereditary differences.

Morgan created a dihybrid cross using males and females with red eyes and brown bodies and white eyes and yellow bodies. Surprisingly, self-crossing of the F1 generation led to an F2 generation that was not 9:3:3:1 in ratio. In terms of peas, the results showed a departure from Mendel’s dihybrid cross.

Also Read: Law of Inheritance

What is Linkage?

In his research with flies, Morgan discovered that two genes didn’t follow the law of division. The percentage of parental combination should be higher than the non-parental combination if the two genes are on the same chromosome. The term “linkage” refers to the actual physical joining of genes. The gene is said to be related if many features are found on the same chromosome.

However because several characters are found on the same chromosome, the Mendelian principle of independent assortment can be used to explain the linkage. In dihybrid crosses , genetic recombination can be seen (non-parental combination). Whether genes are closely or loosely coupled to chromosomes affects the likelihood of recombination.

A Sturtevant Morgan student identified the chromosome’s place of linkage. By the gene mapping procedure, he discovered the location based on the frequency of recombination. The link map is frequently employed in the Human Genome Projec t.

Types of Linkage

Linkage typically falls into one of two categories: complete or incomplete.

• Complete Linkage : A complete linkage occurs when a group of characters regularly appears together throughout the course of more than two generations. Only two types of gametes are produced as a result of this perfect connection. Example: Melanogaster Drosophila
• Incomplete Linkage : New gene combinations are created in the progeny or offspring when there is an incomplete linkage. This happens as a result of the connected genes crossing over or forming a chiasma.

Linkage and Crossing Over

• The linkage gene is passed down from one generation to the next and has a tendency to remain on the chromosome. Many linkages are made throughout the crossing-over.
• In the crossover, the linkage often declines while the parental type and age of the connection rise.
• A new variety is created through crossover, whereas a new variety is kept alive through connection.
• In a linked gene, cross-over occurs during prophase I of meiosis , when parts of sister and homologous chromosomes are exchanged.

Significance of Linkage

• Because of linkage, breeders are unable to combine advantageous traits in a single variety. Breeders of plants and animals find it difficult to combine various features as a result.
• By lowering the likelihood of gene recombination, linkage aids in the preservation of parental traits. It thus helps the organism maintain parental, racial, and other traits.

What is Recombination?

The interchange of genetic materials between different species, also known as genetic recombination or genetic reshuffling, results in the production of offspring with combinations of traits that differ from those that originate in either parent. In eukaryotes , meiosis-induced genetic recombination can produce a new genetic data sequence that can be approved from the parents to the offspring. Recombination typically happens in nature.

Genetic recombination in eukaryotes comprises the linking of homologous chromosomes during meiosis. The transfer of information across chromosomes may be used to monitor this.

Types of Recombination

Two different types of recombination exist:

• Homologous Recombination: During meiosis, this kind of recombination takes place between chromosomes with comparable sequences.
• Non-Homologous Recombination: This happens between unrelated chromosomes.
• Site-specific Recombination: This is seen in extremely brief episodes that typically have commonalities.
• Mitotic recombination : Interphase is when mitotic recombination takes place. However, this kind of recombination is typically bad and can cause cancers . Radiation exposure to the cells causes an increase in it.

One of the following three procedures results in recombination in the prokaryotic cells :

• Conjugation
• Transformation
• Transduction

Recombination of Linked genes

To further understand how related genes recombinant, let’s use the example of freckles and red hair. People have freckles and red hair because the genes for both are located on the same chromosome. It is uncommon for the DNA to be divided between the two genes during homologous recombination. Although homologous recombination happens frequently, the likelihood of the DNA coding for these two genes splitting up is relatively low, hence the two traits are typically inherited together. As a result, genes frequently pass down in pairs.

Difference Between Linkage and Recombination

We have read about linkage and recombination and how they differ, as well as about the various kinds of linkage and recombination, Morgan’s example, and other topics. Eye color and sex are both governed by the same chromosome. As a result, some genes are passed down through chromosomes from parents to children. In his research with fruit flies, Morgan found that while there were over a thousand fruit flies with red eyes, there was only one male fly with white eyes. Linkage describes the actual physical connection between genes. There are two categories of linkage: incomplete linkage and complete linkage. Based on the existence or absence of the non-parental combination, this division is made.

FAQs on Linkage and Recombination

Q1: who is the father of linkage.

T.H. Morgan is known as the father of linkage.

Q2: What is the history of linkage?

In 1905, the first experiment to show linking was conducted. At the time, it was unclear why particular qualities seemed to run in families. Genes are physical structures that are connected by physical distance, according to later research. A centimorgan is a common unit of genetic linkage (cM).

Q3: What is the importance of recombination?

Hereditary recombinations give a consistent DNA homogenization inside the species and, in this manner, the species’ respectability as a rudimentary design answerable for the protection and ascend in the degree of biological solidness of creatures in developing genealogies.

Q4: What are two causes of recombination?

There are two particular hereditary components that lead to recombination i.e. independent assortment and crossing over.

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“Sex Limited Inheritance in Drosophila” (1910), by Thomas Hunt Morgan

In 1910, Thomas Hunt Morgan performed an experiment at Columbia University, in New York City, New York, that helped identify the role chromosomes play in heredity. That year, Morgan was breeding Drosophila , or fruit flies. After observing thousands of fruit fly offspring with red eyes, he obtained one that had white eyes. Morgan began breeding the white-eyed mutant fly and found that in one generation of flies, the trait was only present in males. Through more breeding analysis, Morgan found that the genetic factor controlling eye color in the flies was on the same chromosome that determined sex. That result indicated that eye color and sex were both tied to chromosomes and helped Morgan and colleagues establish that chromosomes carry the genes that allow offspring to inherit traits from their parents.

Prior to Morgan’s fly experiments, other researchers were studying heredity. In 1865, scientist Gregor Mendel in eastern Europe published an article describing heredity experiments he had performed using pea plants. By mating pea plants, Mendel observed that the resulting offspring inherited characteristics, such as seed color and seed shape, in predictable patterns. Mendel hypothesized that there were heritable factors, later called genes, controlling the development of those characteristics.

By the early 1900s, other scientists aiming to explain heredity began to reapply Mendel’s theory. In the late nineteenth century, researchers discovered structures inside the nuclei of cells. Researchers called those structures chromosomes because of the way staining materials colored them. Staining chromosomes enabled researchers to observe chromosomes throughout development. In 1902, Walter Sutton, a researcher at Columbia University, and Theodor Boveri, a researcher at the University of Würzburg in Würzburg, Germany, each observed that chromosomes behaved in a manner that was consistent with Mendel’s theories. Boveri and Sutton hypothesized that chromosomes carried heritable factors, or genetic material. Researchers called Boveri and Suttons’ theory the Boveri-Sutton chromosome theory.

By 1904, Morgan had begun to study the processes that affect heredity and development at Columbia University. However, Morgan, like other scientists at that time, was reluctant to accept the Boveri-Sutton chromosome theory. Morgan argued that scientists had a bias towards associating phenomena, like the inheritance of traits, with known structures, like the chromosome. Similarly, he argued that if one gene didn’t explain a phenomenon, scientists could argue that any number of genes might. In 1910, Morgan published an article explaining why he was reluctant to accept the Bover-Sutton chromosome theory.

Later that year, Morgan made an observation that eventually provided evidence in support of the chromosome theory. In 1910, Morgan was studying Drosophila at Columbia University to find what he called mutants, or individual flies that had atypical, heritable characteristics, such as white eyes instead of the normal red eyes. In May of 1910, after breeding thousands of flies, he observed a single male fly with white eyes, which he called a white mutant. Typically, both male and female flies have red eyes. To explain the white eye mutation, Morgan bred the mutant fly and observed how the mutation was inherited throughout successive generations.

In 1910, Morgan published details of his research in an article titled “Sex Limited Inheritance in Drosophila." First, Morgan took the white mutant and bred it with pure red-eyed female flies. All of the females that resulted from that breeding had red eyes. Morgan then took those red-eyed females and mated them with the original white-eyed mutant male to determine whether or not the inheritance of eye color followed Mendel’s inheritance patterns. If Mendel’s patterns applied to Morgan’s flies, there would be one white-eyed fly to every three red-eyed flies in the resulting generation of flies, regardless of sex. Although Morgan did observe one white-eyed fly to every three red flies, that inheritance pattern was not shared equally across males and females. Most of the white-eyed flies were male. That result indicated that the flies did not follow Mendel’s ratio in a traditional sense.

After observing the white-eye inheritance pattern, Morgan hypothesized that a factor, or gene, controlling eye color was located on the X chromosome. Female flies have two X chromosomes, and males have one X chromosome and one Y chromosome. If a trait, like eye color, correlated with a specific factor on the X chromosome, then the trait was called X-linked. Because males only have one X chromosome, they display all X-linked traits. Females, on the other hand, often need an X-linked trait to exist on both X chromosomes to display that trait. Morgan hypothesized that, in his breeding experiment, the first generation of flies contained males only with white eyes because the gene controlling eye color was on the X chromosome. Males displayed the white eye trait because the trait was present on their only X chromosome. Females did not display the white eye trait because the trait was only present on one of their X chromosomes.

To test his hypothesis that the white-eyed trait was on the X chromosome, Morgan mated other specific groups of flies together and observed the offspring. Prior to doing so, Morgan predicted what the sex and eye color ratios of the offspring would be if his hypothesis were true. By comparing the observed results with the predicted results, Morgan determined that his hypothesis was supported. In one mating, Morgan took a red-eyed male and mated it with a white-eyed female. He predicted and observed that half of the flies would be red-eyed females and the other half would be white-eyed males. That mating showed that the occurrence of the white-eyed trait is limited to the X chromosome, as only male offspring were capable of displaying the white-eyed trait with a single copy of the trait. Morgan showed that inheritance of a trait could differ between sexes.

In the following years, Morgan and a group of scientists at Columbia University established the chromosome theory of inheritance, which described the role that chromosomes play in heredity. In 1911, Morgan published more details of his experiments with the white-eyed mutant, an account in which Morgan explicitly stated that chromosomes carry heritable factors, or genes. In 1915, Morgan, and his colleagues, Alfred Henry Sturtevant, Calvin Bridges, and Herman Joseph Muller published the book Mechanism of Mendelian Heredity . That book contained contemporary scientific information about heredity and included the results of Morgan’s white-eyed mutant experiments.

In 1933, Morgan won the Nobel Prize in Physiology or Medicine for his work establishing the chromosome’s involvement in heredity.

• Boveri, Theodor. “Über mehrpolige Mitosen als Mittel zur Analyse des Zellkerns (On multipolar mitosis as a means to analyze the cell nucleus).” Verhandlungen der physicalisch-medizinischen Gesselschaft zu Würzburg ( Proceedings of the physical-medical company at Wurzburg ) 35 (1902): 67–90. http://publikationen.ub.uni-frankfurt.de/frontdoor/index/index/docId/15991 (Accessed April 2, 2017).
• Kandel, Eric R. “Thomas Hunt Morgan at Columbia University.” Columbia University Living Legacies. http://www.columbia.edu/cu/alumni/Magazine/Legacies/Morgan/ (Accessed March 25, 2017).
• Mendel, Gregor Johann. “Versuche über Pflanzen-Hybriden (Experiments Concerning Plant Hybrids)” [1866]. In Verhandlungen des naturforschenden Vereines in Brünn ( Proceedings of the Natural History Society of Brünn ) IV (1865): 3–47. Reprinted in Fundamenta Genetica , ed. Jaroslav Krízenecký, 15–56. Prague: Czech Academy of Sciences, 1966. http://www.mendelweb.org/Mendel.html (Accessed March 25, 2017).
• Morgan, Thomas H. "Chromosomes and heredity." The American Naturalist 44 (1910): 449–96. http://www.jstor.org/stable/pdf/2455783.pdf (Accessed March 25, 2017).
• Morgan, Thomas H. "Sex Limited Inheritance in Drosophila." Science (1910): 120–2. http://www.jstor.org/stable/pdf/1635471.pdf (Accessed March 25, 2017).
• Morgan, Thomas H. “Random Segregation Versus Coupling in Mendelian Inheritance.” Science (1911): 384. http://science.sciencemag.org/content/34/873/384 (Accessed April 2, 2017).
• Morgan, Thomas H., Alfred H. Sturtevant, Hermann J. Muller, and Calvin B. Bridges. The Mechanism of Mendelian Heredity . New York: Henry Holt and Company, 1915. http://www.biodiversitylibrary.org/bibliography/22551#/summary (Accessed March 25, 2017).
• Nobel Prizes and Laureates. “The Nobel Prize in Physiology or Medicine 1933.” The Official Web Site of the Nobel Prize. https://www.nobelprize.org/nobel_prizes/medicine/laureates/1933/ (Accessed April 2, 2017).
• Sutton, Walter S. "The chromosomes in heredity." The Biological Bulletin 4 (1903): 231–50. http://www.biolbull.org/content/4/5/231.full.pdf (Accessed March 25, 2017).

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