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CONCEPT OF HEREDITY AND EVOLUTION
Law of Inheritance Heredity or hereditary is the process of passing the traits and characteristics from parents to offsprings through genes. The offspring get their features and characteristics that is genetic information from their mother and father. Genetics is a branch of science that studies the DNA, genes, genetic variation, and heredity in living organisms. Heredity is very evidently seen in sexual reproduction. This is because, in this process, the variation of inherited characteristics is high. Gregor Johann Mendel was a scientist who is recognized as the father and Founder of genetics. Mendel conducted many experiments on the pea plant (Pisum sativum) between 1856 and 1863. He studied the results of the experiments and deducted many observations. Thus, laws of inheritance or Mendel’s laws of inheritance came into existence. Before learning about Mendel’s laws of inheritance, it is important to understand what the experiments performed by Mendel were. Mendel’s Experiments on Pea Plant
- Selection of Traits: Mendel chose seven distinct traits in pea plants for his experiments, such as seed color (yellow or green), seed shape (round or wrinkled), flower color (purple or white), and plant height (tall or short).
- Pure Breeding: Mendel started with plants that were purebred for a particular trait, meaning they consistently exhibited one specific trait over several generations. This ensured that the traits were not a result of a mixture of genetic information.
- Cross-Pollination: Mendel then performed controlled crosses, transferring pollen from the male reproductive organ (anther) of one plant to the female reproductive organ (stigma) of another. This process, known as cross-pollination, allowed Mendel to control which plants mated and produced offspring.
- First Filial Generation (F1): The offspring resulting from the cross-pollination of purebred parents were called the F1 generation. Mendel observed that all the F1 plants displayed only one of the parental traits, indicating dominance of one trait over the other.
- Self-Fertilization of F1 Plants: Mendel allowed the F1 plants to self-fertilize. The resulting seeds gave rise to the Second Filial Generation (F2).
- Observations and Ratios: In analyzing the traits of the F2 generation, Mendel observed that the traits controlled by dominant and recessive alleles followed specific ratios. For example, in the case of seed color, the ratio of yellow to green seeds was approximately 3:1. Results of Mendel’s Experiments The results of Mendel’s experiments on crossing a pure tall pea plant with a pure short pea plant.
In the F1 generation, Mendel observed that all plants were tall. There were no dwarf plants. In the F2 generation, Mendel observed that 3 of the offsprings were tall whereas 1 was dwarf. Similar results were found when Mendel studied other characters. Mendel observed that in the F1 generation, the characters of only one parent appeared whereas, in the F2 generation, the characters of the other parent also appeared. The characters that appear in the F1 generation are called dominant traits and those that appear for the first time in the F2 generation are called recessive traits. He concluded that;
- The genes that are passed from the parents to the offsprings exist in pairs. These pairs are called alleles.
- When the two alleles are the same, they are called homozygous. When both the alleles are different, they are called heterozygous.
- Dominant characters are described using capital letters and recessive using small letters. For example, the dominant genes for tallness in a pea plant are written as TT and recessive genes as tt. The heterozygous genes are written as Tt where the plant appears tall has the recessive gene which might express itself in the future generations.
- The appearance of the plant is known as the phenotype whereas the genetic makeup of the plant is called the genotype. So, a plant with Tt genes appears tall phenotypically but has a recessive gene.
- During gametogenesis, when the chromosomes become half in the gametes, there is a 50% chance of either of the alleles to fuse with that of the other parent to form a zygote. Based on these observations, Mendel proposed three laws.
- The Law of Dominance : The offspring always exhibits a dominant trait. From the two alleles received from parents, the only dominant allele is expressed. This law states that in a heterozygous condition, the allele whose characters are expressed over the other allele is called the dominant allele and the characters of this dominant allele are called dominant characters. The characters that appear in the F1 generation are called as dominant characters. The recessive characters appear in the F2 generation.
on different chromosomes that are independently assorted into daughter cells during meiosis. This means that at the time of gamete formation, the two genes segregate independently of each other as well as of other traits. Law of independent assortment emphasizes that there are separate genes for separate traits and characters, and they influence and sort themselves independently of the other genes. This law also says that at the time of gamete and zygote formation, the genes are independently passed on from the parents to the offspring. How are information transferred from one generation to another There are two classes of genetic materials that are responsible for the transfer of information from one generation to another in animals: DNA or deoxyribonucleic acid RNA or ribonucleic acid It is in the DNA or RNA sequences that biological information is stored and passed on. DNA Most organisms contain DNA except some viruses which contain RNA as their genetic material. DNA was discovered by two scientists- Watson and Crick and their model of the structure of DNA are called the Watson and Crick model. DNA is in a double helix structure made up of nucleotides. The "backbone" of the double helix is composed of phosphates connected to a five-carbon sugar called deoxyribose. DNA molecule is a double helix consisting of two strands. Each strand of this helix is made up of nucleotides. Each nucleotide is made up of a phosphoric acid, a deoxyribose sugar and a nitrogenous base. Nitrogenous bases are of two types, viz. purines and pyrimidines. The purines are of two types, viz. adenine and guanine Pyrimidines are of two types viz. cytosine and thymine. The adenine always pairs with thymine with double hydrogen bonds Cytosine always pairs with guanine with triple hydrogen bond. The helices remain bound due to these hydrogen bonds.
RNA :
Unlike the DNA, RNA is a single-stranded genetic material. The nucleotide bases present in RNA are similar to those in DNA except that thymine is replaced by uracil and pairs with adenine. While DNA is the genetic material in most organisms, RNA is found in a few viruses. RNA is of three types depending on their function: tRNA or transfer RNA- helps transfer the amino acids from the mRNA to the ribosomes. mRNA or messenger RNA- helps to carry the codes for amino acids from the DNA to the ribosomes rRNA or ribosomal RNA- are found on the ribosomes and help in protein synthesis. rRNA : It is the component of the ribosome. It helps in protein synthesis.
- Transcription: Definition: Transcription is the process by which genetic information encoded in DNA is used to synthesize RNA molecules, specifically messenger RNA (mRNA). Process: It occurs in the cell nucleus, where the DNA serves as a template for the synthesis of complementary mRNA strands. The enzyme RNA polymerase catalyzes the formation of mRNA by matching complementary RNA nucleotides to the DNA template.
- Translation: Definition: Translation is the process in which the information carried by mRNA is used to build a corresponding protein. Process: It takes place in the ribosomes of the cell, where transfer RNA (tRNA) molecules bring amino acids to the ribosome based on the codons (three- nucleotide sequences) on the mRNA. The ribosome facilitates the linkage of amino acids to form a polypeptide chain, ultimately leading to the synthesis of a protein. A triple codon refers to a set of three consecutive nucleotides (base triplets) in messenger RNA (mRNA) that code for a specific amino acid during the process of translation. The genetic code is a set of rules that dictates how sequences of these triplets, known as codons, are translated into amino acids. Each codon corresponds to a specific amino acid or serves as a start or stop signal for protein synthesis. For example, the codon AUG serves as the start codon, initiating the process of translation. There are also triplets, such as UAA, UAG, and UGA, which are stop codons, indicating the end of the protein synthesis process.
- Translocation:
Definition: Translocation has different meanings depending on the biological context. In the context of genetics, translocation refers to the movement of a chromosomal segment from one location to another. Genetic Translocation: This can occur between non-homologous chromosomes or within the same chromosome. It can lead to genetic disorders or contribute to genetic diversity. Cellular Translocation: In cellular biology, translocation can also refer to the movement of molecules or cellular structures from one location to another within a cell. For example, proteins may be translocated from the cytoplasm to specific organelles or from the endoplasmic reticulum to the Golgi apparatus. In summary, transcription involves the synthesis of mRNA from DNA, translation involves the synthesis of proteins from mRNA, and translocation can refer to the movement of genetic material within chromosomes or the movement of molecules within a cell. Each process plays a crucial role in the central dogma of molecular biology, which describes the flow of genetic information from DNA to RNA to protein. Chromosome Chromosomes are thread-like structures made up of DNA and proteins, found in the nucleus of eukaryotic cells. They carry genetic information in the form of genes, which are segments of DNA that provide instructions for building and maintaining an organism. The number and structure of chromosomes vary among different species.
Klinefelter’s syndrome: This syndrome is characterized by an extra sex chromosome. These individuals are called ‘supermales’. The syndrome is represented as XXY where the individuals have an extra Y chromosome. The individuals with this syndrome have similar features like those with Turner’s syndrome but they are more aggressive and tend to have a criminal behaviour. Other syndromes are Edward’s syndrome (trisomy of chromosome 18) and Patau’s syndrome (trisomy of chromosome 13). Diseases Caused by Alteration in Structure of Chromosomes These diseases arise due to some changes like deletion or rearrangement of parts of the chromosome. They are of various types: Deletion: As the name suggests, this disorder arises due to loss of a certain portion of the chromosome during cell division. The amount of deletion decides the severity of consequences. If there is a loss of certain important genes, it can be lethal to the offspring. Example of such a disorder is Cry-du chat syndrome. Duplication: When there is a duplication of some part of a chromosome or a replicated part of a homologous chromosome attaches itself to an arm of the chromosome, there can be repeat gene sequences. This is known as duplication. Example: Fragile X syndrome. Translocation: When a portion of the chromosome is translocated or moved to another chromosome, this phenomenon is called as translocation. They can be of two types: Reciprocal translocation is when segments from two different chromosomes are exchanged and Robertsonian when one entire chromosome attaches to another chromosome. The effect translocations depend upon which segment has been translocated and which area it has been translocated to. They often result in children born with disabilities or worse, miscarriages. Inversions: In this type of chromosomal structure defect, a part of the chromosome get inverted such that the gene sequence appears inverted. Its effects are not as severe as seen in other forms of structural defects. Mutation : Sudden change occurring in the genetic material is known as mutation. Due to transmission of parental genes to offspring, there is remarkable similarity between parents and their offspring. But if there is mutation in any nucleotide then there are changes in the characters of the offspring. Mutations are of two types, viz. minor and major. Minor mutations can also bring about considerable changes. E.g. Genetic disorders like sickle cell anaemia is caused due to mutation. Mutation is an everlasting process which leads to the process of evolution. It also offers proof for Darwin's theory of natural selection.
The benefits of science of heredity: Diagnosis of hereditary disorders. Treatment of incurable hereditary disorders. Prevention of hereditary disorders. Production of hybrid varieties of animals and plants. Industrial processes in which microbes are used. EVOLUTION : Evolution is an important part of Earth’s history. There are various theories of evolution like Lamarck’s and Darwin’s theory of evolution. Evolution sheds considerable light on the explanation of the distribution and characteristics of the flora and fauna of the world today. The term evolution means to evolve from an initial simple state to a more organized and complex form through natural processes. Darwinism vs lamarckism is thus as follows. Therefore, evolution is the change of something in a natural way, such as the origin and evolution of the earth, the evolution of the earth’s surface, the evolution of the oceans, the description of the mountains, the evolution of animals, etc. In ancient periods, people believed that all modern animals and plants existed from the beginning of the creation of the earth. As long as the earth exists, their existence will remain unchanged. But in the fifth century, BC Xenophane discovered some fossils and showed that there is a difference between the past and present organisms. The idea of evolution begins with this theory. Since then, scientists have gradually developed various theories about evolution. The main proponents of the theory of evolution are Democritus, Aristotle, and other Greek philosophers. And later Linnaeus, Buffon, Lamarck, St. Hilarie, Charles Darwin, Weismann, Hugo de Vries, and many other scientists published the theory of biological evolution subject to
According to Lamarck’s theory, the inheritance of acquired traits and the acquisition of new traits in each generation gradually led to the creation of new species from one species to another. Examples in favor of Lamarck’s theory
- Birds that live on land go into the water in search of food. As they swim constantly in the water, thin skin is attached to the space between their toes.
- According to Lamarck’s, giraffes originated from deer. Giraffes with long necks and legs have emerged in recent generations to eat the leaves of tall trees. The deer’s neck and forelegs have grown a little longer in each generation to eat the leaves of tall trees. And now there are giraffes with long necks and legs.
- The ancestor of the ostrich had active wings and could fly in the sky. But as a result of not using the wings for generations, now it has become an endangered organ.
- The ancestor of the snake had four legs like a chameleon. But the snake’s legs are now completely extinct as a result of continued misuse for underground adaptation. Criticism of Lamarck’s theory Through experiments, various scientists have proved that Lamarck’s theory is scientifically baseless. Although any part of the organism is well-formed or extinct as a result of use and misuse, those altered traits are never inherited. Some information related to this is discussed below.
- Drosophila, a type of fly, has not been able to give birth to a blind fly after conducting breeding in a completely dark room for 60 generations.
- Scientist Weismann cut off the tail of a pair of rats. Weismann also cut off the tails of their children after they were born. Thus, despite cutting the tail of rats for 22 generations, no tailless rats have been born. Based on this experiment, Weismann said that Lamarck’s theory has no scientific basis. Darwin’s theory of evolution Charles Darwin was born in England in 1831-1835 he traveled to various islands in the Atlantic Ocean. He later developed a groundbreaking theory about the expression of living things by observing his travel experiences and samples. His theory is called Darwinism or natural selection theory. In 1859, Darwin published his theory in his book “On the Origin of species by Means of natural selection”. Darwin’s theory is explained below. 1. Prodigality of production According to Darwin, reproduction at an excessive rate is an innate feature of an organism. As a result, the number of organisms increases at a geometric and mathematical rate. For example, an oyster produces about 120 million eggs. According to Darwin, if all the elephants produced from a pair of elephants survived, the number of elephants in 750 years would be 19 million. 2. The constancy of food and shelter Habitat and food are also limited due to the limited surface area.
3. Struggle for existence As the organism multiplies at a geometric and mathematical rate and the food and habitat is limited, the organism has to face tough competition to survive. Darwin called it the ‘struggle for existence’. The struggle for existence is mainly in two ways, (i) intraspecific struggle (the struggle between similar species) and (ii) interspecific struggle (the struggle between different species). Living things also have to struggle with droughts, floods, etc. It is called a struggle with an adverse environment. 4. Variation According to Darwin, two creatures on Earth cannot be exactly the same. That is, there must be some difference between the two creatures. Even between two children of the same parents, there are also some differences. Darwin believed that continuous triple struggles resulted in variations in the organism, which are transmitted to the offspring during reproduction and are ultimately established as characteristics of the organism. According to Darwin’s theory, a small series of changes are responsible for the emergence of different species. 5. Survival of the fittest According to Darwin, of all the organisms involved in the struggle for life, those who have small adaptive traits in their bodies win the struggle for life. Others become extinct from the earth. 6. Natural selection This is the most important aspect of Darwin’s theory. Through various struggles on the surface of the earth, Darwin called the survival of the fittest is natural selection. Nature selects the most suitable organism. Those who are chosen by natural selection, survive in greater numbers. They breed and their offspring inherit favorable varieties. 7. Origin of new species As those species accumulate within a particular group of species, the differences between the generation and the offspring are much greater. And in time a new species emerges. As a result of evolution, new species of organisms are born. Criticism of Darwin’s theory Although natural selection is a recognized process for explaining the causes of evolution, Darwinism has the following weaknesses. Darwin discusses the survival of the fittest but does not discuss the most qualified organism. It is true that by natural selection an organism emerges and other organisms become extinct. Although the consequences of natural selection have been discussed in evolution, the theory of extinction has not been properly explained. Darwin’s evolution theory only discussed the mutual struggle between new offspring. However, there has been no discussion of the impact of this struggle on previous organisms.
Content Lamarckism Darwinism
1. Environmental impact The adaptation of the organism varies according to the variation of the environment. And the organism survives in harmony with nature. In order for an organism to survive, it has to struggle with the environment. And the creatures that win the struggle survive in harmony with nature. 2. Increase in the organism Lamarck gave the idea of increasing the size of an organism. Darwin explained the increase in the number of organisms in his theory. 3. Cause of evolution Organ transformation, growth, and extinction occur for use or abuse. Transmission of acquired changes occurs from generation to generation. As a result, biological evolution occurs. In order to survive in the struggle of life, different variations are seen in the organism. This is the main reason for evolution. 4. Struggle of life Lamarck did not mention the struggle of life. According to Lamarck, internal demand is the cause of an increase or decrease in any organ. Three types of life struggle explained by Darwin such as intraspecific struggle, interspecific struggle, and struggle with the environment. 5. Concept of inactive organs Inactive organs were used in the past, but are now extinct due to abuse. Darwin’s theory does not give any explanation about inactive organs. 6. Transmission of properties Traits acquired in life are transmitted to the next generation. The characteristics of the most naturally selected organisms are passed to the next generation. 7. Use and abuse of organ Organs are well-organized due to continuous use. As a result of abuse, the organs become weak and inactive. Darwin did not explain the use and abuse of organs. 8. Natural selection theory Lamarck does not support the theory of natural selection. The theory of natural selection is most important in Darwinism. 9. Origin of the giraffe’s neck The necks of giraffe ancestors were small. Gradually there was a shortage of grasses and short trees. Hence, have to stretch their necks to eat the As per Darwin, giraffe ancestors had different neck lengths. This is the result of mutation. These mutations flow down through
Microbial Heredity and Evolution Microbial heredity and evolution form the cornerstone of the extraordinary diversity and adaptability observed in microbial life. The intricate mechanisms governing microbial heredity, the driving forces behind microbial evolution, and the implications of these processes in shaping the microbial world through an exploration of key concepts such as horizontal gene transfer, mutation, and selective pressures will be the basis of our discussion. Microorganisms, encompassing bacteria, archaea, fungi, and viruses, exhibit an unparalleled ability to adapt to diverse environments. This adaptability is rooted in the dynamic processes of microbial heredity and evolution. Unlike higher organisms, microbes can exchange genetic material through various mechanisms, leading to rapid evolutionary changes. Microbial Heredity: Vertical Gene Transfer: The conventional mechanism of heredity in which genetic information is passed from parent to offspring. Horizontal Gene Transfer (HGT): A pivotal process allowing microbes to acquire genes laterally from other organisms. Mechanisms of Microbial Evolution: Mutation: Mutation as the Catalyst: At the core of microbial evolution, mutation serves as the catalyst for genetic variation. Defined as the spontaneous and heritable changes in the DNA sequence, mutations introduce novelties into the microbial genome. These alterations can occur through various processes, such as replication errors, exposure to mutagenic agents, or horizontal gene transfer. Adaptive Advantage: While some mutations may be neutral or deleterious, others confer adaptive advantages that enable microorganisms to thrive in diverse environments. Natural selection acts upon these advantageous mutations, leading to their increased prevalence in subsequent generations. This continual cycle of mutation and selection forms the basis of microbial adaptation. Rapid Evolutionary Responses: Microorganisms, particularly bacteria and viruses, exhibit remarkable evolutionary plasticity due to their short generation times and large population sizes. This rapid turnover allows for swift responses to environmental changes through the accumulation of beneficial mutations. Consequently, microbial populations can swiftly adapt to selective pressures, including antibiotic exposure and changing ecological niches. leaves of tall trees. generations (2) & (3).
Rapid Evolution of Resistance: Antibiotic resistance is an accelerated evolutionary response by microorganisms to the selective pressures exerted by antibiotics. In the presence of these drugs, microbial populations undergo rapid genetic changes, favoring the survival and proliferation of individuals with genetic variations that confer resistance. Genetic Mechanisms of Resistance: Microbes employ various genetic mechanisms to develop resistance to antibiotics. Horizontal gene transfer, mutations, and the acquisition of resistance genes through mobile genetic elements play pivotal roles. These mechanisms allow microorganisms to swiftly adapt and confer resistance traits to subsequent generations. Selective Advantage and Survival: The crux of antibiotic resistance lies in its selective advantage. Resistant microbes gain a survival edge in environments where antibiotics are present, as they can withstand the lethal effects of these drugs. Natural selection acts decisively, favoring the propagation of resistant traits and leading to the prevalence of resistant strains within microbial populations. Microbial heredity and evolution are complex processes that underlie the remarkable diversity and adaptability of microorganisms. A comprehensive understanding of these mechanisms is essential for addressing challenges such as antibiotic resistance and harnessing microbial capabilities for biotechnological advancements. Continued research in this field holds the key to unlocking the mysteries of microbial life and exploiting its potential for the benefit of society. Know Some Terms Gene – It is the basic unit of inheritance. It consists of a sequence of DNA, which is the genetic material. Genes can mutate and can take two or more alternative forms. Alleles – The alternative forms of genes. They affect the same characteristics or traits in alternate forms. They are located on the same place of the chromosome. Chromosomes – These are thread-like structures made up of nucleic acids (DNA) and proteins. They are mostly found in the nucleus of the cells. They carry the hereditary or genetic information in the form of genes. Genotype – It is the complete heritable genetic identity of an organism. It is the set of alleles that are carried by the organism. It also includes non-expressed alleles. Phenotype – It is the description of the actual physical characteristics of an organism or the expressed form of the genotype. Dominant alleles – When an allele affects the phenotype of an organism, then it is a dominant allele. Capital letters represent dominant alleles. For example, “T” to express tallness. Recessive alleles – An allele that affects the genotype in the absence of the dominant allele is called a recessive allele. Small letters represent recessive alleles. For example – “t” for tallness. Homozygous – Each organism has two alleles for every gene (Each chromosome has one each). In homozygous, both the alleles are same. For Example, “TT” is the homozygous expression for tallness trait. Heterozygous – If the two alleles are different from each other, then they are heterozygous in nature. For Example, “Tt” is the heterozygous expression for tallness trait.
