Choromosomal Theory of Inheritance in Homo Sapiens

It is propesed by Walter Sutton and Theodre Boveri about Chromosal theory and it's experimental proof was ellaborated and signified under Article 24/1945.

1. Student’s Profile Name: PREETI SINGH Class: XII Sec: D Subject: Biology Submitted to: Ma’am Priya

2. Investigatory Project Topic : Chromosomal Theory of Inheritance

3. Table of Contents S.no. Topic Name Page no. 1 About Inheritance 1-2 2 From Gene to Chromosome - Genes: The basic unit of Inheritance 3-8 - Gregor Mendel and his experiments 9-13 - DNA: The carrier of Genes 14-18 - Sutton and Boveri 19-20 - Chromosomes: The colored bodies 21-23 3 The Chromosomal Theory of Inheritance 24-26 4 Verification of Sutton – Boveri Theory 27-33

4. What i s Inheritance ?? When living things reproduce they  pass on characteristics  to their offspring. This is known as inheritance. It refers to the process of transmission of genes from parent to offspring. Inheritance is the passing on of genetic traits from parents to their offspring, and these offspring get all the genetic information from their parents. Members of the same biological family tend to have similar characteristics – including physical appearance and the likelihood of developing certain genetic conditions . Inheritance describes the way these traits are passed down between generations, with genetic information passed from a parent to a child. Page 1

5. How does Inheritance happens?? Most of our cells contain two sets of 23 chromosomes. This is known as being diploid. The exceptions are the egg and the sperm. They only have one set of chromosomes each, known as being haploid. During sexual reproduction, the sperm cell combines with the egg cell to form a fertilized egg – the first cell of the new organism.  In this way, an individual inherit one set from each of our biological parents.  The fertilized egg now has two sets of 23 chromosomes, with one set inherited from each biological parent. It has the complete set of instructions needed to make more cells and eventually develop into a person . 5 Page 2

6. From Gene to Chromosome Genes: The basic unit of Inheritance A gene is defined as the fundamental unit of heredity. It is a segment of DNA that has information coded in it in the form of a nucleotide sequence. It has the ability to undergo mutation and crossing over . In 1909, Johansen introduced the term gene. Before him, Mendel had given the term ‘factor’ for the unit of inheritance that is responsible for expressing a trait. Johansen defined the gene as ‘the elementary unit of inheritance which can be assigned to a particular trait.’ The work of Morgan proposes that a gene is the shortest segment of a chromosome that can be separated by crossing over. Page 3

7. Characteristics d etermined b y Genes The human cell contains 23 pairs of chromosomes. The trait is one of the characteristics determined by one or more genes. Abnormal genes and genes that are formed due to new mutations also result in certain traits. Genes vary in size depending on the code or the protein they produce. All cells in the human body contain the same DNA. The difference between the cells occurs due to the different type of genes that are turned on and therefore produce a variety of proteins. Page 4

8. Genes occur i n Pairs Allele is the word that we use to describe the alternative form or versions of a gene. People inherit one allele for each autosomal gene from each parent, and we tend to lump the alleles into categories. Typically, we call them either normal or wild-type alleles, or abnormal, or mutant alleles. An allele is one of two or more versions of DNA sequence (a single base or a segment of bases) at a given genomic location. An individual inherits two alleles, one from each parent, for any given genomic location where such variation exists. Page 5

9. Dominant and Recessive Alleles Dominant Allele : Dominant alleles can influence a specific trait if a person has one or both copies of the allele, which can come from just one or both parents . Example: Huntington’s disease  is a dominant condition caused by an insertion mutation in the  HD  (sometimes called  HTT ) gene. Individuals only need to inherit one mutated copy of the gene to experience symptoms. Recessive Allele : Recessive alleles only show their effect if the individual inherits two copies of the allele – one from each parent. If an individual has only one copy of the recessive allele, they are generally considered to be a ‘carrier’ of the recessive allele . Example: C ystic fibrosis is a recessive condition caused by a mutation in the  CFTR  gene. That means a person needs a mutation in both  CFTR  alleles to have cystic fibrosis. If an individual has a mutation in one  CFTR  allele but not the other, they are a carrier of cystic fibrosis, but won’t develop the condition themselves. Page 6

10. Homozygous and Heterozygous Genes Homozygous Gene : It has two same copies of the same allele coding for a particular trait . It contains only one type of allele, either dominant or recessive. Only one type of gamete is produced. It can be either homozygous dominant or homozygous recessive. Heterozygous Gene : It contains two different copies of alleles coding for a particular trait . It c ontains different alleles for a trait. Both dominant and recessive. Two types of gametes are produced. Heterozygous alleles can show complete dominance, co-dominance or incomplete dominance. Page 7

11. Functions of Genes Genes control the functions of DNA and RNA. Proteins are the most important materials in the human body which not only help by being the building blocks for muscles, connecting tissue and skin but also takes care of the production of the enzyme. These enzymes play an important role in conducting various chemical processes and reactions within the body. Therefore, protein synthesis is responsible for all activities carried on by the body and are mainly controlled by the genes. Genes consist of a particular set of instructions or specific functions. For example, the globin gene was instructed to produce hemoglobin. Hemoglobin is a protein that helps to carry oxygen in the blood. Page 8

12. Gregor Mendel - The Father of Genetics Gregor Mendel  (born July 20, 1822, Heinzendorf, Silesia, Austrian Empire and died on January 6, 1884   ) was a botanist, teacher, and Augustinian prelate, the first person to lay the mathematical foundation of the   science  of  genetics , in what came to be called  Mendelism . The profound significance of Mendel's work was not recognized until the turn of the 20th century (more than three decades later) with the rediscovery of his laws.  Erich von Tschermak ,  Hugo de Vries  and  Carl Correns  independently verified several of Mendel's experimental findings in 1900, ushering in the modern age of genetics Page 9

13. Mendel’s Experiment Mendel settled on studying seven traits that seemed to be inherited independently of other traits: seed shape, flower color, seed coat tint, pod shape, unripe pod color, flower location, and plant height. He first focused on seed shape, which was either angular or round. Between 1856 and 1863 Mendel cultivated and tested some 28,000 plants, the majority of which were  pea  plants ( Pisum sativum ).This study showed that, when true-breeding different varieties were crossed to each other (e.g., tall plants fertilized by short plants), in the second generation, one in four pea plants had  pure breed   recessive   traits , two out of four were  hybrids , and one out of four were purebred  dominant . His experiments led him to make two generalizations, the  Law of Segregation  and the  Law of Independent Assortment , which later came to be known as Mendel's Laws of Inheritance. Page 10

14. Mendel’s Monohybrid and Dihybrid Cross Monohybrid Cross: Mono refers to single and hybrid means mixed breed. Monohybrid cross is used to study the inheritance of a single pair of alleles. It is used to study the dominance of genes . Genotypic ratio: 1:2:1 Phenotypic ratio: 3:1 Dihybrid Cross: Di refers to two or double and hybrid means breed. Dihybrid cross is used to study the inheritance of 2 different alleles. It is used to study Offspring assortment. Genotypic ratio: 1:2:1:2:4:2:1:2:1 Phenotypic ratio: 9:3:3:1 Page 11

15. Mendel’s Laws of Inheritance Law of Dominance: The law of dominance states that when parents with pure, contrasting traits are crossed together, only one form of the trait appears in the next generation. The trait which appears in the next generation is known as a dominant trait. The trait that do not express is called a recessive trait .. Law of Segregation: This law explains that the pair of alleles segregate from each other during meiosis  cell division  (gamete formation) so that only one allele will be present in each gamete. Page 12

16. Law of independent assortment: According to the law of independent assortment, the alleles of two more genes get sorted into gametes independent of each other. The allele received for one gene does not influence the allele received for another gene. Page 13

17. DNA: The Carrier o f Genes Deoxyribonucleic acid (DNA) is the molecule that carries the genetic information for the development and functioning of all living organisms. DNA is made up of two long, intertwined strands that form a spiral structure known as a double helix. Each strand is composed of a sequence of chemical building blocks called nucleotides, which consist of a phosphate group, a sugar group, and one of four nitrogen bases: adenine (A), cytosine (C), guanine (G), or thymine (T). 5 The specific order of these bases along the DNA strand encodes the genetic instructions that guide the growth, development, and reproduction of organisms. I n humans and other complex organisms, DNA is primarily found within the nucleus of cells, where it is tightly packaged into structures called chromosomes. A small amount of DNA is also present in the mitochondria, the cellular structures that generate energy for the cell. Page 14

18. Structure of DNA A  DNA   molecule  consists of two long polynucleotide chains composed of four types of  nucleotide  subunits. Each of these chains is known as a  DNA chain , or a  DNA strand .  Hydrogen bonds  between the  base  portions of the nucleotides hold the two chains together. What is a Nucleotide?? N ucleotides are composed of a five-carbon  sugar  to which are attached one or more phosphate groups and a nitrogen-containing base. In the case of the nucleotides in DNA, the sugar is deoxyribose attached to a single phosphate group (hence the name   deoxyribonucleic acid ), and the base may be either  adenine (A), cytosine (C), guanine ( G ),  or  thymine (T) . The nucleotides are covalently linked together in a chain through the sugars and phosphates, which thus form a “backbone” of alternating sugar-phosphate-sugar-phosphate . What is a Polynucleotide C hain?? The way in which the  nucleotide  subunits are lined together gives a  DNA  strand a chemical polarity. If we think of each  sugar  as a block with a protruding knob (the 5′ phosphate) on one side and a hole (the 3′  hydroxyl ) on the other , each completed chain, formed by interlocking knobs with holes, will have all of its subunits lined up in the same orientation.   Page 15

19. Double Helical DNA DNA — the   double helix —arises from the chemical and structural features of its two polynucleotide chains. Because these two chains are held together by hydrogen bonding between the bases on the different strands, all the bases are on the inside of the double helix, and the  sugar -phosphate backbones are on the outside . In each case, a bulkier two-ring  base  (a  purine ; see  Panel 2-6 , pp. 120–121) is paired with a single-ring base (a  pyrimidine ); A always pairs with T, and  G  with C . This  complementary  base-pairing  enables the  base pairs  to be packed in the energetically most favorable arrangement in the interior of the double helix. In this arrangement, each  base pair  is of similar width, thus holding the sugar-phosphate backbones an equal distance apart along the DNA  molecule . To maximize the efficiency of base-pair packing, the two sugar-phosphate backbones wind around each other to form a double helix, with one complete turn every ten base pairs Page 16

20. DNA Replication DNA replication is an important process that occurs during cell division. It is also known as  semi-conservative replication , during which DNA makes a copy of itself . DNA replication takes place in three stages: Step 1: Initiation The replication of DNA begins at a point known as the origin of replication. The two DNA strands are separated by the DNA helicase. This forms the replication fork. Step 2: Elongation DNA polymerase III reads the nucleotides on the template strand and makes a new strand by adding complementary nucleotides one after the other. For eg ., if it reads an Adenine on the template strand, it will add a Thymine on the complementary strand. While adding nucleotides to the lagging strand, gaps are formed between the strands. These gaps are known as Okazaki fragments. These gaps or nicks are sealed by ligase. Step 3: Termination The termination sequence present opposite to the origin of replication terminates the replication process. The TUS protein (terminus utilization substance) binds to terminator sequence and halts DNA polymerase movement. It induces termination. Page 17

21. Functions of DNA A s carrier of genetic Material: DNA is the genetic material which car­ries all the hereditary information. Genes are the small segments of DNA, consisting mostly of 250 – 2 million base pairs. A gene code for a polypeptide molecule, where three nitrogenous bases sequence stands for one amino acid. Polypeptide chains are further folded in secondary, tertiary and quaternary structures to form different proteins. As every organism contains many genes in its DNA, different types of proteins can be formed.  Proteins  are the main functional and structural molecules in most organisms. Replication process: Transferring the genetic information from one cell to its daughters and from one generation to the next and equal distribution of DNA during the cell division Mutations: The changes which occur in the DNA sequences Transcription Cellular Metabolism DNA Fingerprinting Gene Therapy Page 18

22. Walter Sutton and Theodore Boveri Walter Sutton Walter Stanborough Sutton  (April 5, 1877 – November 10, 1916 ) was an American  geneticist  and  biologist  whose most significant contribution to present-day  biology  was his theory that the  Mendelian laws of inheritance  could be applied to  chromosomes  at the cellular level of living organisms. This is now known as the  Boveri–Sutton chromosome theory . The  German  biologist  Theodor Boveri  independently reached the same conclusions as Sutton, and their concepts are often referred to as the  Boveri–Sutton chromosome theory . Sutton's hypothesis was widely accepted by most scientists, particularly  cytologists , at the time. [5]  The continued work of  Thomas Hunt Morgan  at Columbia brought the theory to universal acceptance by 1915 through his studies of  Drosophila melanogaster , the fruit fly, even as  William Bateson  continued to question the theory until 1921. Page 19

23. Theodore Boveri Theodor Heinrich Boveri  (12 October 1862 – 15 October 1915) was a German  zoologist ,  comparative anatomist  and co-founder of modern   cytology .  He was notable for the first hypothesis regarding   cellular  processes that cause  cancer , and for describing  chromatin  diminution in  nematodes .   Using an  optical microscope , Boveri examined the processes involved in the fertilization of the animal egg cell; his favorite research objects were the  nematode   Parascaris  and  sea urchins . Boveri's work with  sea urchins  showed that it was necessary to have all  chromosomes  present in order for proper  embryonic development  to take place .  This discovery was an important part of the  Boveri–Sutton chromosome theory . He also discovered, in 1888, the importance of the  centrosome  for the formation of the  spindle  during  mitosis  in animal cells, which he described as the  especial organ of cell division .  Boveri also discovered the phenomenon of  chromatin  diminution during embryonic development of the nematode  Parascaris Page 20

24. Chromosomes: The Colored Bodies Chromosomes are threadlike structures made of protein and a single molecule of DNA that serve to carry the genomic information from cell to cell. In plants and animals (including humans), chromosomes reside in the nucleus of cells . Each pair contains two chromosomes, one coming from each parent, which means that children inherit half of their chromosomes from their mother and half from their father. Chromosomes can be seen through a microscope when the nucleus dissolves during cell division.  Chromosomes vary in number and shape among living organisms. Most bacteria have one or two circular chromosomes. Humans, along with other animals and plants, have linear chromosomes . In fact, each species of plants and animals has a set number of chromosomes. A fruit fly, for example, has four pairs of chromosomes, while a rice plant has 12 and a dog, 39. In humans, the twenty-third pair is the sex chromosomes, while the first 22 pairs are called autosomes. Page 21

25. Parts o f a Chromosome Chromatid:   Each chromosome has two symmetrical structures called chromatids or sister chromatids which is visible in mitotic metaphase. Each chromatid contains a single DNA molecule At the anaphase of mitotic cell division, sister chromatids separate and migrate to opposite poles Centromere and kinetochore :  Sister chromatids are joined by the centromere. Spindle fibres during cell division are attached at the centromere The number and position of the centromere differs in different chromosomes The centromere is called  primary constriction Centromere divides the chromosome into two parts, the shorter arm is known as  ‘p’ arm  and the longer arm is known as  ‘q’ arm. The centromere contains a disc-shaped  kinetochore , which has specific DNA sequence with special proteins bound to them The kinetochore provides the c entre for polymerization of tubulin proteins and assembly of microtubules Telomere :  Terminal part of a chromosome is known as a telomere. Telomeres are polar, which prevents the fusion of chromosomal segments Page 22

26. Chromatin :   Chromosome is made up of  chromatin.  Chromatin is made up of DNA, RNA and proteins. At interphase, chromosomes are visible as thin chromatin fibres present in the nucleoplasm. During cell division, the chromatin fibres condense and chromosomes are visible with distinct features. The darkly stained, condensed region of chromatin is known as  heterochromatin.  It contains tightly packed DNA, which is genetically inactive The light stained, diffused region of chromatin is known as  euchromatin.  It contains genetically active and loosely packed DNA At prophase, the chromosomal material is visible as thin filaments known as  chromonemata At interphase, bead-like structures are visible, which are an accumulation of chromatin material called  chromomere.  Chromatin with chromomere looks like a necklace with beads Secondary constriction and nucleolar organizers:   Other than centromere, chromosomes possess secondary constrictions. Secondary constrictions can be identified from centromere at anaphase because there is bending only at the centromere (primary constriction) Secondary constrictions, which contain genes to form nucleoli are known as the  nucleolar organizer. Satellite :   It is an elongated segment that is sometimes present on a chromosome at the secondary constriction. The chromosomes with satellite are known as  sat-chromosome. Page 23

27. The Chromosomal Theory of Inheritance The chromosomal theory of inheritance was given by Boveri and Sutton in the early 1900s. It is the fundamental theory of genetics. According to this theory,  genes  are the units of heredity and are found in the chromosomes. Chromosomal Theory of Inheritance came into existence long after Mendelian genetics. During Mendel’s experimentation, the society was not acceptable to such drastic changes in their scientific ideas. They could not believe the existence of such discrete factors such as genes which would segregate without mixing as this did not support their idea of the constant changes leading to evolution. Moreover, the means of communication was poor at that time as a result of which, the information could not be conveyed to the masses. Also, Mendel’s mathematical approach to prove biological laws was unacceptable. As time passed, scientists Vries, Correns and Tschermak discovered chromosomes which existed inside the nucleus. Sutton and Boveri discovered observed the behavior of the chromosomes when the cells were divided . With the advancements in the microscope, this task became easier. Hence, they proved Mendel’s laws with the help of chromosomal movement. They showed the segregation of the chromosomes during the Anaphase of cell division. The idea of chromosomal segregation combined with the Mendelian principles gave rise to the chromosomal theory of Inheritance. Page 24

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29. Observations of t he Theory During the process of cell division-meiosis, the pairs of homologous chromosome move as discrete structures, which are independent of other pairs of chromosomes. There is a random distribution of chromosomes into the pre-gametes from each homologous pair. Each parent synthesizes gametes, which constitute only half of their chromosomal complement. Even though female (egg) and male (sperm) gametes differ in their size and morphology, they have the same number of chromosomes, submitting equal genetic contributions from each parent. The gametic chromosomes fuse during fertilization to produce offspring with the same number of a chromosome as their parents. Page 26

30. Verification of t he Sutton- B overi Theory Sutton and Boveri’s work was further carried forward and proved by T.H. Morgan , who used Drosophila melanogaster to show how  sexual reproduction  gave rise to variations . Thomas Hunt Morgan Thomas Hunt Morgan was an American evolutionary biologist, geneticist , embryologist, and science author who won the Nobel Prize in Physiology or Medicine in 1933 for discoveries elucidating the role that the chromosome plays in heredity.  He formulated the chromosomal theory of linkage. He defined linkage as the co-existence of two or more genes in the same chromosome and performed dihybrid crosses in Drosophila to show that linked genes are inherited together and are located on X-chromosome. Page 27

31. Drosophila Melanogaster Drosophila melanogaster  is a species of  fly  (an  insect  of the  order   Diptera ) in the family  Drosophilidae . The species is often referred to as the  fruit fly  or  lesser fruit fly , or less commonly the " vinegar  fly", " pomace  fly ",  or " banana   fly“ Why Drosophila was used by TH Morgan? It produces a large number of offspring and allows sufficient data to be collected. Drosophila has only four pairs of chromosomes that are easy to study. The Drosophila genome is significantly homologous to that of the human genome which makes the study of certain genes easier. They shows XX-YY type of sex determination. Sexual dimorphism is seen in Drosophila With a low-power microscope, many hereditary variations can be easily observed. Page 28

32. Morgan’s Experiment and Observations Page 29

33. Linkage and Recombination Linkage: Genes are said to be linked when genes for different traits are located in similar chromosomes and hence are tied to each other. It is a deviation from the Mendelian principle of independent assortment that is appropriate to be applied to the genes that are situated on different chromosomes . Recombination: Recombination is a process of producing new combinations of alleles by the recombination of DNA molecules. It is also referred to as genetic recombination, as there is an exchange of genetic material (DNA) between two different chromosomes or between different regions of the same chromosome. This process is observed in both eukaryotes and prokaryotes. It increases the genetic diversity of sexually reproducing organisms. Page 30

34. Types o f Linkage Complete Linkage: A linkage is said to be complete when two or multiple characteristics are inherited and normally surface in two or further generations in their parental or original combinations, they are known as complete linkage. These particular genes do not generate combinations that are non-parental. The genes that exhibit these linkages are located nearby in the same chromosome. Examples – genes for long wings and grey body in male Drosophila Incomplete Linkage: It is displayed by genes that generate some portion of non-parental combinations. These genes are situated at a distance on the chromosomes which can be attributed to the occasional or accidental deconstruction of chromosomal segments while crossing over. Page 31

35. Types of Recombination Homologous Recombination: This type of recombination occurs between chromosomes of similar sequences and is carried out during meiosis. Non-homologous Recombination: This occurs between chromosomes that are not similar. Site-specific Recombination: This is observed between very short sequences that usually contain similarities. Mitotic Recombination: Mitotic recombination occurs during interphase. However, this type of recombination is usually harmful and can result in tumors. It increases when the cells are exposed to radiation. Page 32

36. Difference between Linkage and Recombination Linkage Recombination Linkage is referred to as the Here , the exchange of DNA. physical connection of genes . takes place between two different present on same chromosome. chromosomes. If the genes are tightly linked, the If genes is loosely linked, of chances of linkage is high . chances of Recombination is high Linkage is the tendency of genes Crossing over is the exchange present in the chromosome to chromosomal sections and disrupt stay intact and transfer links to form new linkages the to next generation  resulting in recombination. Page 33

37. Conclusion The Chromosomal theory of Inheritance was indeed a revolutionary theory which mended the path to modern genetics. Apart from establishing a strong connection with the Mendellian Inheritance, it also brings out a simplified yet an essential aspect of heredity. This theory can easily explain us the mode and the technique of transfer of characters from parents to offspring.

38. Bibliography The content of this project has been taken from the following resources: www.byjus.com www.ncbi.in www.yourgenome.com NCERT Biology (Class 12 )