Chromosomal Theory of Inheritance: Linkage and Genetic Recombination

Chromosomes are thread-like structures found within the nucleus of both animal and plant cells. Protein and a single molecule of deoxyribonucleic acid make up each chromosome (DNA). DNA is passed along from parents to children and carries the precise instructions that distinguish each living thing. The word "chromosome" is derived from the Greek terms "chroma" and "body" (soma). 

As chromosomes are cell structures, or bodies, that are heavily stained by several bright dyes employed in the study, scientists gave them this moniker. One copy of each chromosome is inherited from the female parent and the other from the male parent in humans and most other complex creatures. This helps to explain why some children get features from their mother and others from their father. Let’s know more about the chromosomal theory of inheritance and discuss some important questions.

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Keyterms: Chromosomes, Nucleus, Protein, Animal cells, Plant cells, Molecule, Deoxyribonucleic acid, DNA, RNA, Protein, Cell, Egg, Sperm

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What are Chromosomes?

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Chromosomes have a special structure that maintains DNA firmly wrapped around spool-like proteins called histones. DNA molecules would be too lengthy to fit inside cells if they were not packaged in this way. For example, all of the DNA molecules in a single human cell would extend 6 feet if untied from their histones and placed end-to-end. Cells must constantly divide to make new cells to replace old, worn-out cells in order for an organism to grow and operate effectively. It's critical for DNA to stay intact and equally distributed among cells during cell division. In the great majority of cell divisions, chromosomes are an important part of the process that guarantees DNA is appropriately copied and disseminated. Even yet, mistakes do happen from time to time.

Chromosome number or shape changes in nascent cells might cause major difficulties. Defective chromosomes are made up of linked parts of damaged chromosomes, for example, cause one type of leukaemia and some other cancers in humans. It's also critical that reproductive cells like eggs and sperm have the right number of chromosomes and that the structure of those chromosomes is accurate. If this is not done, the kids may not develop normally. People with Down syndrome, for example, have three copies of chromosome 21, rather than the two copies common in the general population. Living things have different numbers and shapes of chromosomes. One or two circular chromosomes are found in most bacteria. Linear chromosomes, like those seen in other animals and plants, are organized in pairs within the nucleus of the cell.

The only human cells without chromosome pairs are reproductive cells, or gametes, which have only one copy of each chromosome. When two reproductive cells fuse, a single cell with two copies of each chromosome is formed. This cell divides repeatedly, and its descendants divide as well, eventually forming a mature human with a complete set of paired chromosomes in nearly all of its cells. In addition to the linear chromosomes present in the nucleus, human and other sophisticated species' cells include a much smaller form of chromosome comparable to those found in bacteria. This circular chromosome can be found in mitochondria, which are powerhouse structures located outside of the nucleus.

Scientists believe mitochondria were formerly free-living microorganisms capable of converting oxygen into energy. When these bacteria-infected cells lacked the ability to tap into the power of oxygen, the cells kept them, and the bacteria transformed into modern-day mitochondria throughout time.

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Chromosomal Theory of Inheritance Detailed Video Explanation:

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Discovery of Chromosomes

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The tiny circular chromosome found in mitochondria has a unique inheritance pattern. During fertilization, only egg cells, not sperm cells, preserve their mitochondria. As a result, mitochondrial DNA is always inherited from the mother. A number of human diseases, including various types of hearing loss and diabetes, have been linked to DNA present in mitochondria.

In the late 1800s, scientists studying cells under a microscope discovered chromosomes for the first time. However, the nature and function of these cell structures were unknown at the time. Thomas Hunt Morgan's pioneering studies in the early 1900s helped researchers get a far better grasp of chromosomes. Morgan demonstrated that the X chromosome is related to gender and eye color in fruit flies, establishing a relationship between chromosomes and inherited features.

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Chromosomal Theory of Inheritance

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Boveri and Sutton proposed the chromosomal hypothesis of inheritance in the early 1900s. It is the most basic concept in genetics. Genes are the units of inheritance, according to this hypothesis, and they are located in chromosomes.

Long after Mendelian genetics, the Chromosomal Theory of Inheritance was developed. Society was not accepting of such radical changes in scientific concepts during Mendel's experimentation. They couldn't understand the existence of distinct variables like genes that would segregate without mixing because it contradicted their theory of evolution as a result of continual changes. Furthermore, the means of communication at the time were inadequate, preventing information from reaching the public.

Mendel's quantitative approach to proving biological laws was also unacceptably flawed. Scientists Vries, Correns, and Tschermak identified chromosomes inside the nucleus as time went on. When the cells were divided, Sutton and Boveri watched the behaviour of the chromosomes. This process grew easier with the developments in microscopy technology. As a result, they used chromosomal movement to show Mendel's laws. They demonstrated chromosomal segregation during cell division's Anaphase phase. The chromosomal theory of inheritance was born from the idea of chromosomal segregation mixed with Mendelian principles. T.H. Morgan advanced and proved the work by using Drosophila melanogaster to demonstrate how sexual reproduction caused differences.

Also read: Molecular Basis of Inheritance

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Chromosomal Theory: Linkage and Genetic Recombination

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Morgan noticed that when a pair of features was crossed, two genes did not segregate according to Mendel's law. When two genes were found on the same chromosome, the likelihood of a parental combination in the next generation was substantially higher than that of a non-parental combination. Linkage refers to the physical relationship of genes. The non-parental gene combinations in a dihybrid cross were referred to as genetic recombination. Following the discovery of linked genes, the frequency of connected genes influenced the manifestation of phenotypes in subsequent generations.

Sturtevant, a student of Morgan's, discovered the location of connected genes on a chromosome by measuring their frequency of genetic recombination using the gene mapping procedure. During the Human Genome Project, this method of constructing a connection map was widely used.

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Observations of Chromosomal Theory of Inheritance

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Mendel's rules are supported by the Chromosomal Theory of Inheritance. The following are the findings of this theory:

• During cell division or meiosis, homologous chromosome pairs move as separate entities that are independent of other chromosomal pairs.
• The pre-gametes from each homologous pair have a random distribution of chromosomes.
• Each parent produces gametes, which make up half of their chromosome complement.
• Despite the fact that female (egg) and male (sperm) gametes have different sizes and morphologies, they both have the same amount of chromosomes, allowing for equal genetic contributions from both parents.
• During fertilization, the gametic chromosomes combine to produce kids with the same number of chromosomes as their parents.

Also read: Differences between Chromosome and Chromatid

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Things to Remember

• An aquatic environment is one in which water plays a critical role in the formation and growth of all species that live there. This is made up of plant and wildlife elements that come together to form life under a body of water. These bodies of water include the oceans, seas, rivers, lakes, bogs, streams, and lagoons.
• The bulk of marine aquatic environment animals have evolved to live in saltwater. As a result, the osmosis process causes their bodies to enlarge when they are in less salty water, such as freshwater in rivers. As a result, marine aquatic ecosystem organisms cannot flourish in freshwater.
• The most common aquatic animals are nekton, plankton, and benthos. Lakes, oceans, ponds, rivers, swamps, coral reefs, wetlands, and other popular aquatic habitats include, among others, lakes, oceans, ponds, rivers, swamps, coral reefs, wetlands, and other popular aquatic environments.
• The aquatic ecosystem includes freshwater ecosystems such as lakes, ponds, rivers, oceans, and streams, as well as marshes and swamps. Marine ecosystems include oceans, intertidal zones, reefs, and the seafloor. In addition, water-dependent animals, plants, and microbes call the aquatic ecosystem home.
• Two of the roles and types of aquatic ecosystems are i. It helps with nutrient recycling and water purification. ii. It contributes to groundwater replenishment. The characteristics of aquatic ecosystems are made up of abiotic and biotic components. Abiotic influences include depth, nutrition, temperature, salinity, flow, temperature, and other abiotic characteristics, while biotic factors include live animals.

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