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Chapter 2

Chapter 2

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Konstantine Zakzanis

Chapter 2 – Heredity and the Environment Transactions among a vast array of hereditary and environmental factors begin before birth. Such transactions between genes and the environment make each newborn unique, and they continue to shape the individual's characteristics throughout his or her lifespan. − Genotype: The particular set of genes that a person inherits from her parents. With the exception of identical twins, no two individuals have the same genotype. − Phenotype: Created by the interaction of a person's genotype, or genetic makeup, with the environment; the visible expression of the person's particular physical and behavioural characteristics. The Process of Genetic Transmission In a woman's oviduct, the sperm (the male germ cell) and ovum (the female germ cell, or egg, it is 90000 times as heavy as the sperm that penetrates it) unite to create a new living organism called a zygote, that has the potential to develop into a human being. This beginning is the start of a 9-month long period that normally ends with a full-term baby 7 or 8 pounds and roughly 50 cm long. − Chromosomes: Thread-like structures, located in the central portion, or nucleus, of a cell, that carry genetic information to help direct development. When the egg and sperm unite 23 chromosomes from each of these cells unite together to create 23 chromosome pairs or 46 chromosomes. Half of the chromosomes come from the father and the other half from the mother, this is possible because each chromosome is homologous (similar in shape and function) to one another. These 23 homologous pairs of chromosomes are passed on to every cell in the body except the reproductive cells, which contain only 23 single chromosomes, this possible due to meiosis. − Meiosis: The process by which a germ cell divides to produce new germ cells with only half the normal complement of chromosomes; thus, male and female germ cells( sperm and ovum) each contain only 23 chromosomes so that when they unite, the new organism they form will have 46 chromosomes, half from each parent. − Crossing over: The process by which equivalent sections of homologous switch places randomly, shuffling the genetic information each carries during meiosis. How does the union of the egg and sperm cell become a complex human being? This happens by a process called mitosis (the process in which a body cell divides in two, first duplicating its chromosomes so that the new, daughter cells contain the usual 46 chromosomes) which occurs in all autosomes (chromosomes that contain matching pairs) and sex chromosomes. Thus, the zygote divides and continues to divide, each time producing new cells that have the full complement of 46 chromosomes, and gradually becomes a multi-cellular organism. Genes, DNA, and Proteins − Deoxyribonucleic acid (DNA): A ladder-like molecule that stores genetic information in cells and transmits it during reproduction, made up of nucleotides (a compound containing a nitrogen base, a simple sugar, and a phosphate group) that are held together by two long twisted parallel strands. Only bases that are compatible with each other will form a bond (adenine and thymine, cytosine and guanine). − Gene: A portion of DNA that is located at a particular site on a chromosome and that codes for the production of certain kinds of proteins. Genes never work in isolation, but always in combination with environmental influences. − Proteins: Fundamental components of all living cells, they are any of a group of complex organic molecules containing carbon, hydrogen, oxygen, nitrogen, and usually sulphur, and that are composed of on or more chains of amino acids. The Transmission of Traits Gregor Mendel (through his work with pea plants) worked out the mechanisms or laws of inheritance of characteristics. These two principles are known as: − Principle of segregation: states that each inherited trait comes from one's parent as a separate unit. − Principles of independent assortment: states that the inheritance of various traits occurs independently of one another. Allele: An alternative form of a gene; typically, a gene has two alleles, one inherited from the individual's mother and one from the father. Homozygous: Describing the state of an individual whose alleles for a particular trait from each parent are the same. Heterozygous: Describing the state of an individual whose alleles for a particular trait from each parent are different. − Co-dominance: A genetic pattern in which heterozygous alleles express the variants of the trait for which they code simultaneously and with equal force. − Dominant: Describing the more powerful of two alleles in a heterozygous combination. − Recessive: Describing the weaker of two alleles in a heterozygous combination. Sex Chromosomes: The 23 rd pair of chromosomes, which determine the individual's gender and are responsible for sex related characteristics; in females, this pair normally compromises two X chromosomes, in males and X and Y chromosome. − X-linked genes: Genes that are carried on the X chromosome and that may have no analogous genes on the Y chromosome in males. In females X- linked recessive genes are expressed much less frequently compared to males, because they have two X chromosomes, and have a chance of inheriting a dominant and counteracting allele on the other X chromosome. − Hemophilia: A disorder caused by an X-linked recessive gene, in which the blood fails to clot; found more often in males than in females. Many other x- linked recessive disorder are more common in men than in women, including colour blindness, certain form of night blindness, atrophy of the optic nerve, and so on. Interactions among genes Some traits that are influenced by genes do not tend to run in the families, development of such traits usually depends on a certain configuration of many genes, and that particular configuration is not likely to be passed on from parent to child. Furthermore, a single pair of alleles may influence more than one trait. Moreover they may do this not directly, but indirectly through their effect on the expression of still other genes, these genes are termed modifier genes. Genetic Disorders Disorder and its Cause Method of Current method of Nature Diagnosis treatment and prevention. Hemophilia: Blood Heredity: X- Blood tests Treated by transfusion of disease linked clotting factors. Genetic characterized by recessive counselling can help poor clotting ability trait. determine whether a couple risk bearing a child with this disorder. Diabetes mellitus: Heredity: Blood and Can often control it by special Body's inability to multi-gene urine tests diet alone, in other cases, oral metabolize exaggerated medication and/or insulin carbohydrates and by injections are required to maintain proper environment maintain the body's glucose levels. al factors. equilibrium. Phenylketonuria Heredity: Blood tests Genetic counselling can (PKU): Inability to recessive prenatally or indicate the risk that a couple convert allele at birth will have a PKU child. A phenylalanine to special diet can be instituted tyrosine; untreated, that will prevent the disorder's leads to mental toxic affects. retardation Sickle cell anemia: Heredity: Two Blood tests Blood transfusions have until Blood disease recessive recently been the only characterized by alleles in treatment. However, the malformation of red combination recent in utero treatment of a blood cells that are fetus for an autoimmune low in oxygen. disorder had brought hope that sickle cell anemia and other similar diseases may be treated successfully before birth. Down syndrome Heredity: Amniocentesi Special physical training, (trisomy 21): extra full or s, special education, including Physically and partial alphafetoprot speech therapy, surgical mentally retarded chromosome ein assay, corrections of problems with development; 21 chorionic villithe heart and with hearing are sometimes sampling, sometimes necessary. cardiovascular and chromosome respiratory analysis. abnormalities Turner (XO) Chromosoma Blood tests Hormone therapy can Syndrome: l promote development of Underdeveloped abnormality: secondary sex characteristics. secondary sex only one X Counselling; special education characteristics; Chromosome to lessen deficits in spatial infertility; short instead of understanding. stature; social two immaturity; webbed neck; cardiovascular and renal abnormalities Triple X (XXX) Chromosome Blood tests Special education to improve Syndrome: Some abnormality: cognitive skills. physical extra X abnormalities, chromosome including menstrual irregularities and premature menopause; some limitations on cognitive abilities Klinefelter's (XXY) Chromosoma Bloods tests Testosterone treatments can Syndrome: Some l enhance development of male female physical abnormality: secondary sex characteristics characteristics; extra X as well as sexual interest and sterility; mild to chromosome assertiveness. Special severe cognitive education to improve difficulties cognitive skills. XYY Syndrome: Chromosoma Blood tests Special education. Unusual height; l some cognitive abnormality: impairment; extra Y attention deficit chromosome Fragile X Syndrome: Heredity: Blood tests No known treatment. Physical breaking of abnormalities; an X mental retardation chromosome that deepens with near its tip time; psychological and social problems. A major reason why potentially harmful diseases survive is that they are not harmful in the heterozygous state, these individuals that carry the recessive allele survive and reproduce. The defective allele also survives from generation to generation, even though it is activated only 25% of the time. Some potentially harmful alleles may survive because they are actually beneficial
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