Genetics is the branch of biology that studies heredity. The passing of biological information from parents to offspring occurs through the combination of genes that are unique to each individual person, animal or plant that reproduces through living cells. As a field of study, it is concerned with the origin of individual characteristics and the way that these are transmitted to offspring. Genetics has developed over the centuries from the view of Aristotle and others that the species are fixed. One ancient idea being that inside of the seed of a man was another tiny little man. Inside of his seed was another tiny little man and so on through the generations.
Charles Darwin’s ideas on evolution sparked controversy in part over how it was possible for species to develop into different animals from “lesser” species. The prevailing view in his time was that of Jean-Baptiste Lamarck (1744–1829) who had viewed heredity as proceeding according to natural laws, in particular through “acquired characteristics.” His theory which was also accepted by Darwin and by others until the discovery of the ideas of Gregor Johann Mendel (1822–84) was recognized at the beginning of the 20th century.
A particularly notorious result of the Lamarckian understanding of Darwinism occurred from its adoption into social and political ideas. Herbert Spencer adopted the idea of evolution directed by the acquired characteristics of human beings to his socioeconomic theory. Followers like William Graham Sumner brought his ideas to America where they were used with a moral component added to justify the capitalism of the Robber Barons and others in the late 19th century. The Supreme Court also incorporated the idea into its rulings. The effect of Social Darwinism was to say that the successful (the rich) are the evolutionarily advanced and the poor are those that should be left to die because society will be better for it. Other political theories also adopted this view. The rediscovery in 1900 of a paper published by Gregor Mendel, an Augustinian monk from Moravia set modern genetics on the path of genes as the bearers of inheritance.
Mendel’s work as a gardener revealed to him that pea plants inherited traits. The traits came from the parents to the offspring. Having practiced gardening in his youth between 1850 and 1863 he grew and test over 29,000 pea plants (Pisum sativum). He observed that all traits were not inherited by the offspring, just some of the traits of each parent. He was able to show the genetic inheritance of the pea strains. One in four of the pea plants had pure bred recessive alleles. Two out of four were hybrids and one out of four was the dominant purebred type. He show that the inheritance of traits followed laws. These laws have since become known as Mendel’s Laws of Inheritance. He published his theory (“Experiments on Plant Hybridization”) in an obscure journal in 1866 (Proceedings of the Natural History Society of Brunn) in Brunn, Moravia (now Brno, Moravia). His theory was ignored until 1900 when the importance of his work was recognized. His paper was republished in a journal with an international readership.
The development of statistics in the 20th century along with the gene theory enable a modern evolutionary synthesis to occur that when combined with the revealing of the structure of DNA and the mapping of the human genome has made it possible to understand the specific genetics of an individual and to trace the genetic heritage of people and animals to distant ancestors.
Genetic information is located in the base sequences in the genes that are attached onto the DNA double helix. The constitute codes are read as gene expressions when the cells decode the information in order to manufacture proteins that cells need to function. In order to synthesize a protein a complementary RNA molecule is produced. Its sequence orders a specific amino acid sequence of the protein.
The codes of the amino acids are 64-base triplets called codons. The codons encode the 20 possible amino acids. Most of the amino acids have more than one codon. The codons are synonyms if they encode the same amino acid. The “wobble” position is on the third base which can vary so that there are initiation codons and three stop codons.
Humans who have identical copies of a particular gene are described as being homozygous. Those with two different copies of a gene are described as heterozygous. As genes are copied they undergo mutations. Variant genes are called alleles. In describing alleles life scientists use the convention that dominant variant genes are designated with a capital letter, while recessive variants are designated with a lower case letter.
In the Mendelian genetic theory each inherited trait is a phenotype. All of the phenotypes are the result of specific genes or combinations of genes. In other words genes define the unique characteristics of individuals. All of the phenotypes are the sum of the genotype which constitutes the individual human or creature.
If the phenotypes produce hybrids then one phenotype will probably dominate another. Purebred lines of genes are strains of species that have been bred as exactly the same for generations. For example, breeds of dogs or cattle or grape vines are selectively bred for characteristics that aid agriculture or other needs. The breeding seeks to maintain the same phenotype so that, for example, a Concord grape or a Scottish Terrier is the same from generation to generation.
Crossbreeding of pure-breed types (homozygous) line will have different inherited characteristics. They will have offspring that will all have the same phenotype. The technical term for this cross-breeding is the F1 generation. The second generation (F2) will show the original phenotypes in a ratio of 3:1. The dominant phenotype will be the majority of the prodigy.
Purebred lines are organisms that have been inbred for many generations. Stock breeders breed cats, dogs, cattle, sheep, or other kinds of animals for a variety of purposes. There are strains of laboratory animals such as mice, rats, flies, and other animals or plants are used for experimentation with virtually fixed genomes that permit replication of experiments in any laboratory in the world.
Dominance within species arises between differences in the phenotype for one inherited characteristic. Hybrids produced from two different phenotypes produce offspring with only one phenotype. For example, experiments with short-winged fruit flies and long winged fruit flies have shown that when bred they produce only long-winged offspring.
Mendel’s studies of pea plants showed that the offspring inherited several phenotypes. Focusing in on only the color of the petals, he crossed plants with white petals with plants with violet petals. The offspring of the first generation F1 (first filial) all inherited violet petal flowers. This demonstrated that the violet color was dominant to the white petal color. He then allowed the F1 generation of offspring to self fertilize. The resulting F2 (second filial) offspring has both violet and white petal flowers. However, the colors were in a ratio of 3:1 (monohybrid ration) in which the violet color was three and the white color was one. Other experiments with crossbreeding show the same off spring ratio which led Mendel to conclude that the inherited trait was biological which today are called genes.
The monohybrid ratio is the basis for all pattern of inheritance in organism higher than peas. The 3:1 phenotype ration will be a 1:1 ration if an F1 individual is crossed with a homozygous recessive parent. The crossing will produce gametes with either a dominant or a recessive gene. A testcross is produced from a recessive parent crossed with the phenotype to determine if it is heterozygous. This ratio is important because in families with genetic diseases such as Huntington’s disease the dominant allele is rare. As a result, the individual with the allele will not likely be homozygous.
Instances of partial or incomplete dominance also occur. For example snapdragon with white petals is crossed with a red petal parent the result will be a pink flower rather than the dominant ratio (F1). The next generation (F2) will produce red, white, and pink petal flowers.
Co-dominance occurs with crossbreeding as well. It is a departure from standard Mendelian order. When two alleles of a gene product are two distinct and detectable then they product is the results of co-dominance. Human MN blood typing is a commonly cited example of this phenomenon. This blood type is distinct from the common A, B, O, AB typing. The antigen formed by responses to antibodies that are found on the red blood cells identifies this type. The MN blood groups have two types of antigens. These two types are produced by the inherited alleles. People who are genetically heterozygotes inheritance produces both antigens and are classed as MN.
When co-dominance occurs, both alleles contribute equally to the phenotype. In contrast, in cases of incomplete dominance, the two alleles contribute unequally to the phenotype. When Mendel studies the genetic characteristics of the succeeding generations of peas he developed a concept of genetic inheritance based upon only two different alleles. When his work was rediscovered, life scientists soon found that nature was much richer than Mendel’s theory had proposed. Indeed, there are cases of genes with three, four, and occasionally more alleles.
To investigate two alleles is a challenge, but the process becomes very complicated when more than two alleles have to be considered. In the case of the color of rabbit fur, the gene for color has four alleles.
The ratio produced is similar to incomplete dominance. There is a difference in that both alleles are dominant. This characteristic is found in blood groups that are inherited. In humans, the MN blood group is controlled by a single gene.
Dominance, co-dominance, and incomplete dominance affect human health. Some of the alleles carried on the genes of some humans affect their viability. In many cases the homozygous recessive does not survive. However, the heterozygotes may have a normal lifespan. If there is a phenotype that is observable such as yellow coats in mice, then the consequences can be observed. Yellow coats are dominant to black coats in mice. However, when two yellow-coated mice breed their offspring die in utero. This particular allele is lethal. There is a nuance that is significant in this instance. In yellow coated mice the allele that produces a yellow coat is dominant. However, in terms of viability, it is recessive. This is an important principle that occurs in many other alleles.
In the developmental genes, many that are mutations produce changes if one is present, but if two copies of the same allele are present, the outcome is lethal. In humans, Tay-Sach’s disease, Huntington’s disease and sickle cell anemia are diseases that manifest this genetic characteristic.
In some cases, semi-lethal genes are present. If a homozygote is present in a crossbreeding but is present in only reduced numbers, it is an alternation of the 3:1 ratio established by Mendel. Because many genes have a large number of alleles, there are also possibilities of variants that are not fatal.
The ideas developed by Mendel were developed in the first part of the 20th century. While the procedures have not been altered much since then, understanding DNA and the mapping of the human genome has created a huge number of polymorphic markers that enable useful genetic analysis.
Polymorphism in DNA sequences when identified allows alterations in DNA sequences to be understood. DNA polymorphisms are grouped into different classes and can be used in forensic genetics. They can also be used of other purposes as well.
The genetic code is universal. It applies to all organisms which use the same codons for the same amino acids. However, as ever in nature exceptions can be found. Genes are measures of the response of each individual to the environment. Genes are inherited from parents usually without any genetic accidents. However, in rare cases mutations occur. These are variations in the copying the genetic sequence(s) which serves as a code. The “mistake” mars the expected genetic outcome. Some mutations are nonconsequential. Others are beneficial. Others are harmful or even fatal.
Genetic defects can occur spontaneously, be chemically induced, or caused by excessive exposure to radiation. The vast numbers of individuals that are born without any genetic defects is an improbably high number.
Medical genetics is field of study that deals with genetic defects which cause diseases through heredity. Diabetes and hemophilia are two such inherited diseases. Genetic diseases are a diverse group of genetic disorders. They are caused by one or more genes. Among the disorders are neurofibromatosis, hemoglobinopathies, thalassemias, Charcot-Marie Tooth Disease, multifactorial disease, and other disease. To identify genetic diseases, genetic screening is used. Its use however, is surrounded with ethical issues.
Recent research has indicated that cancer is genetically related. However, it also seems to be the case that there are DNA repair genes and tumor suppressor genes as well which use an in-activator gene. Because altered patterns of RNA are often found in tumors, it is currently thought that mutations in the introns or exon splice sites are responsible. Genes that have been identified as cancer causing are termed oncogenes.
Other areas of genetic research or applications of genetic knowledge include gene therapy with the introduction into cells copies of the necessary genetic code that will end or reawaken the appropriate genetic code.
The biotechnology industry is also the product of modern genetic research. Plants and animals have been the subject of extensive research and now development from genetic engineering. A major goal is the production of biologically useful proteins. Recombinant DNA is used to produce natural proteins. In addition, cloned genes are to produce proteins with modified amino acid sequences. Transgenic is the creation of useful plants and animals with altered genomes by the transfer of new genes. Cloning may be the ultimate act of genetic development. However, genetic work is filled with ethical issues arising from a respect for life.
Genetic Code; Genetic Disorders; Genetic Testing and Counseling.
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