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, the molecular basis for inheritance. Each strand of DNA is a chain of Nucleotides , matching each other in the center to form what look like rungs on a twisted ladder.]] Genetics is the Science of Heredity and Variation in living Organism s. 1 ISBN 0-7167-3520-2. 2 854 pages. ISBN 0-7637-1511-5. Knowledge of the inheritance of characteristics has been implicitly used since Prehistoric times for improving crop plants and animals through Selective Breeding . However, the modern science of genetics, which seeks to understand the mechanisms of inheritance, only began with the work of Gregor Mendel in the mid-1800s.3 Although he did not know the physical basis for heredity, Mendel observed that inheritance is fundamentally a Discrete process with specific traits that are inherited in an independent manner — these basic units of inheritance are now called Gene s. Following the rediscovery of Mendel's observations in the early 1900s , research in 1910s yielded the first physical understanding of inheritance — that genes are arranged linearly along large Cellular structures called Chromosome s. By the 1950s it was understood that the core of a chromosome was a long molecule called DNA and genes existed as linear sections within the molecule. A single strand of DNA is a chain of four types of Nucleotide s; hereditary information is contained within the sequence of these nucleotides. Solved by Watson and Crick in 1953 , DNA's three-dimensional structure is a Double-stranded Helix , with the nucleotides on each strand physically Matched to each other. Each strand acts as a template for synthesis of a new partner strand, providing the physical mechanism for the inheritance of information. The sequence of nucleotides in DNA is used to produce specific sequences of Amino Acid s, creating Protein s — a correspondence known as the " Genetic Code ". This sequence of amino acids in a protein determines how it folds into a three-dimensional structure, this structure is in turn responsible for the protein's function. Proteins are responsible for almost all functional roles in the cell. A change to DNA sequence can change a protein's structure and behavior, and this can have dramatic consequences in the cell and on the organism as a whole. Although genetics plays a large role in determining the appearance and behavior of organisms, it is the interaction of genetics with the environment an organism experiences that determines the ultimate outcome. For example, while genes play a role in determining a person's Height , the Nutrition and Health that person experiences in childhood also have a large effect. HISTORY OF GENETICS See Also: History of genetics led him to the hypothesis that genes are located upon chromosomes.]] Although the science of genetics has its origins in the work of , who used this pattern of inheritance to explain the evolution of various traits within species (these changes are now understood to be the product of Natural Selection rather than a product of Soft Inheritance ). Mendelian and classical genetics The modern science of genetics traces its roots to the observations made by , his work suggested the utility of the application of statistics to the study of inheritance. The significance of Mendel's observations was not understood until early in the twentieth century, after his death, when his research was re-discovered by other scientists working on similar problems. The word "genetics" itself was coined in by Charles Darwin in the The Origin Of Species .genetic, ''a.'' and ''n. pl.'', Oxford English Dictionary, 2nd ed. (1989) Bateson publicly promoted and popularized usage of word "genetics" to describe the study of inheritance in his inaugural address to the Third International Conference on Plant Hybridization in London, England, in 1906 .5 :Although the conference was titled "International Conference on Hybridisation and Plant Breeding", Wilks changed the title for publication as a result of Bateson's speech. In the decades following rediscovery and popularization of Mendel's work, numerous experiments sought to elucidate the molecular basis of DNA. In 1910 Thomas Hunt Morgan argued that genes reside on chromosomes, based observations of a sex-linked white eye mutation in fruit flies. In 1913 his student Alfred Sturtevant used the phenomenon of Genetic Linkage and the associated recombination rates to demonstrate and map the linear arrangement of genes upon the chromosome. Molecular genetics Although chromosomes were known to contain genes, chromosomes were composed of both protein and DNA — it was unknown which was critical for heredity or how the process occurred. In 1928, in 1952 identified DNA (rather than protein) as the genetic material of viruses, further evidence that DNA was the molecule responsible for inheritance. James D. Watson and Francis Crick resolved the structure of DNA in 1953, using the X-ray Crystallography work of Rosalind Franklin that indicated the molecule had a helical structure. Their double-helix model paired a sequence of nucleotides with a "complement" on the other strand. This structure not only provided a physical explanation for information contained within the order of the nucleotides, but also a physical mechanism for duplication through separation of strands and the reconstruction of a partner strand based on the nucleotide pairings. Although the structure explained the process of inheritance, it was still unknown how DNA influenced the behavior of cells. In the following years many scientists sought to understand how DNA controls the process of Protein production within Ribosome s, eventually discovering the translation of DNA into Messenger RNA and uncovering the Genetic Code which links the nucleotide sequence of Messenger RNA to the Amino Acid sequence of protein. With this molecular understanding of DNA, an explosion of research based on this understanding of the molecular nature of DNA became possible. The development of chain-termination DNA Sequencing in 1977 enabled the determination of nucleotide sequences on DNA, and the PCR method developed by Kary Banks Mullis in 1983 allowed the isolation and amplification of arbitrary segments of DNA. 8 These and other techniques, through the pooled efforts of the Human Genome Project and parallel private effort by Celera Genomics , culminated in the sequencing of the human Genome in 2001. FEATURES OF INHERITANCE Discrete inheritance and Mendel's laws See Also: Mendelian inheritance At its most fundamental level, inheritance in organisms occurs by means of discrete traits, called " Gene s". 9 Chapter 2 (Patterns of Inheritance): Introduction This property was first observed by Gregor Mendel , who studied the segregation of heritable traits in Pea Plant s. 10 Chapter 2 (Patterns of Inheritance): Mendel's experiments In his experiments studying the trait for flower color, Mendel observed that the flowers of each pea plant were either purple or white — and never an intermediate between the two colors. These different, discrete versions of the same gene are called " Allele s". In the case of pea plants, each organism has two alleles of each gene, and the plants inherit one allele from each parent. 11 Chapter 3 (Chromosomal Basis of Heredity): Mendelian genetics in eukaryotic life cycles Many organisms, including humans, have this pattern of inheritance. Organisms with two copies of the same allele are called " Homozygous ", while organisms with two different alleles are " Heterozygous ". The set of alleles for a given organism is called its by expressing an intermediate phenotype, or Codominance by expressing both alleles at once. 12 Chapter 4 (Gene Interaction): Interactions between the alleles of one gene When parents breed to produce children, their children randomly inherit one of the two alleles from each parent. The outcome of these crosses can be visualized by use of a Punnett Square . These observations of discrete inheritance and the segregation of alleles are collectively known as " Mendel's First Law " or the "Law of Segregation". Assortment and interactions of multiple genes 's data from 1889 shows the relationship between offsping height as a function of mean parent height. While correlated, the remaining variation in offspring heights indicates environment is also an important factor in this trait.]] Organisms have thousands of genes, and in sexually reproducing organisms assortment of these genes are generally independent of each other. This means that the inheritance of an allele for yellow or green pea color is unrelated to the inheritance of alleles for white or purple flowers. This phenomenon, known as " Mendel's Second Law " or the "Law of independent assortment", means that the alleles of different genes get shuffled between parents to form children with many different combinations. (Some genes do not assort independently, demonstrating Genetic Linkage , a topic discussed later in this article.) Often different genes can interact in a way that influences the same trait. In the ", with the second gene epistatic to the first. 13 Chapter 4 (Gene Interaction): Gene interaction and modified dihybrid ratios Many traits are not discrete features (eg. purple or white flowers) but are instead continuous features (eg. human height and skin color). These " Complex Trait s" are the product of interactions of many genes.14 The influence of these genes is mediated, to varying degrees, by the environment an organism has experienced. The degree to which an organism's genes contribute to a complex trait is called " Heritability ".15 Chapter 25 (Quantitative Genetics): Quantifying heritability Measurement of the heritability of a trait is relative, though — in a more variable environment, the environment has a bigger influence on the total variation of the trait. For example, human height is a complex trait with a heritability of 89% in the United States. In Nigeria, however, where people experience a more variable access to good nutrition and health care, height has a heritability of only 62%.16 Abstract from NCBI THE MOLECULAR BASIS FOR INHERITANCE DNA and chromosomes See Also: DNA Chromosome of DNA. Bases pair through the arrangement of Hydrogen Bonding between the strands.]] The (A), Cytosine (C), Guanine (G), and Thymine (T). Genetic information exists in the sequence of these nucleotides, and genes exist as stretches of sequence along the DNA chain.17 Virus es are the only exception to this rule — sometimes viruses use the very similar molecule RNA instead of DNA as their genetic material.18 The full set of all hereditary material in an organism is called its " Genome ". DNA normally exists as a double-stranded molecule, coiled into the shape of a duplicates the genetic information by splitting the strands and using each strand as a template for synthesis of a new partner strand.19 Chapter 8 (The Structure and Replication of DNA): Mechanism of DNA Replication Genes are arranged linearly along the long chains of DNA sequence, called ; in eukaryotes Chromatin is usually composed of Nucleosome s, repeating units of DNA wound around a core of Histone proteins. |
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