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Chapter 2


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Elizabeth Johnson

Notes From Reading C HAPTER  2: HEREDITY  AND THE  ENVIRONMENT  (PGS. 42­79) Introduction - Genotype – The particular set of genes that a person inherits from her parents - 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 PART ONE: The Process of Genetic Transmission - In the moist environment of a woman’s oviduct, the sperm and egg unite to create a new living organism that has the potential to develop into a human being, called a zygote - Ovum – The female germ cell, or egg; is the largest human cell - Sperm – The male germ cell; the smallest of all human cells, is the shape of a head, where the hereditary information is, with a whip-like tail, which it uses to propel itself through the woman’s reproductive system in search of the ovum Chromosomes and Genes - Chromosomes – Thread-like structures, located in the central portion, or nucleus, of a cell, that carry genetic information to help direct development - At the moment of human conception, when the sperm and egg unite, 23 chromosomes from each of these cells join together to create 23 chromosome pairs (46 altogether) - Pairing is possible since each chromosome is homologous (similar in shape and function) - Meiosis – The process by which a germ cell divides to produce new germ cells with only half the normal complement of chromosomes o 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 chromosomes switch places randomly, shuffling the genetic information each carries - 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 o Occurs in all Autosomes – The 22 paired non-sex chromosomes Genes, DNA, and Proteins - Deoxyribonucleic Acid (DNA) – Aladder-like molecule that stores genetic information in cells and transmits it during reproduction - From each nucleotide, one of four different nitrogen-containing bases projects out toward the base on its opposite side to form one of the staircase’s “riser” o Nucleotide – Acompound containing a nitrogen base, a simple sugar, and a phosphate group - Genes – Aportion of DNAthat is located at a particular site on a chromosome and that codes for the production of certain kinds of proteins - Proteins –Are fundamental components of all living cells and are any of a group of complex organic molecules containing carbon, hydrogen, oxygen, nitrogen, and usually sulphur, and that are composed of one or more chains of amino acids o Some proteins give cells their characteristic physical properties (bone cells) o Other proteins do other jobs like triggering chemical reactions, carrying chemical messages, fighting foreign invaders, and regulating genes PART TWO: Genetic Influences on Development The Transmission of Traits:ABasic Model - Original work that is the basis of modern genetics was done by Gregor Mendel - From his observation of pea plants, he worked out the basic principles that govern inheritance of characteristics from one generation to the next - Principle of Segregation – each inherited traits comes from one’s parent as a separate unit o I.e., Flower color, stem height - Principle of Independent Assortment – inheritance of various traits occurs independently of one another o I.e., Inheritance of flower color has nothing to do with inheritance of height - 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 (aa orAA) - Heterozygous – Describing the state of an individual whose alleles for a particular trait from each parent are different (Aa) - Co-Dominance – Agenetic 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 Genes on the Sex Chromosomes: Exceptions to the Rule rd - Sex Chromosomes – In both males and females, the 23 pair of chromosomes, which determine the individual’s gender and are responsible for sex-related characteristics o Females: XX | Males: XY - X-Linked Genes – Genes that are carried on the X-chromosome and that may have no analogous genes on the Y chromosome in males (therefore, more frequent in males) - Haemophilia –Adisorder caused by an X-linked recessive gene, in which the blood fails to clot o Female is homozygous for the recessive allele, her blood clotting be impaired Interactions among Genes - Modifier Genes – Genes that exert their influence indirectly, by affecting the expression of still other genes - Example: modifier gene that affects the early development of cataract, a condition in which the lens of the eye becomes clouded, obscuring vision o It is these kind of genes that determine whether the cloudiness forms along the periphery of the lens or in its centre Genetic Disorders Table 2-2: Some Disorders that are caused by Genetic Factors Disorder and Its Nature Cause Method of Diagnosis Current Methods of Treatment and Prevention Hemophilia – Blood Heredity; X- Blood test Is treated at present diseases characterized linked transfusions of clotting by poor clotting ability recessive trait factors. New gene-splicing techniques may make it possible to provide these factors without running the risk of the transmission of blood-borne infections from donated blood products. Genetics counselling can help determine whether a couple risk bearing a child with this Notes From Reading C HAPTER  2: HEREDITY AND  THE ENVIRONMENT  PGS . 42­79) disorder. Phenylketonuria Heredity; Blood tests Genetic counselling can (PKU) – Inability to Recessive prenatally or at birth indicate the risk that a couple convert phenylalanine to Allele will have a PKU child. tyrosine; untreated, Modern genetic techniques leads to mental can detect recessive alleles retardation before such a child’s birth, and immediately after birth a special diet can be instituted that will prevent the disorder’s toxic effects. Down Syndrome Heredity: extra Amniocentesis, Special physical training; (Trisomy 21) – full or partial alphafetoprotein special education, including Physically and mentally chromosome assay, chorionic villi speech therapy. Surgical retarded development; 21 sampling, corrections of problems with sometimes chromosome analysis the heart and with hearing are cardiovascular and sometimes necessary. respiratory abnormalities Turner (XO) Chromosomal Blood tests Hormone therapy can promote Syndrome – abnormality; development of secondary sex Underdeveloped only one X characteristics. Counselling; secondary sex chromosome special education to lessen characteristics; instead of two deficits in spatial infertility; short stature; understanding. social immaturity; webbed neck; cardiovascular and renal abnormalities Klinefelter’s (XXY) Chromosomal Blood tests Testosterone treatments can Syndrome – Some abnormality: enhance development of male female physical extra X secondary sex characteristics characteristics; sterility;chromosome as well as sexual interest and mild to severe cognitive assertiveness. Special difficulties education to improve cognitive skills. WHY HARMFUL ALLELES SURVIVE - They are not harmful in the heterozygous state – having both normal and recessive allele o Example: Phenylketonuria (PKU); as long as a person also possesses a normal allele, the PKU allele has no ill effects o If infants aren’t treated, toxic substances accumulate in their bodies, damaging the nervous system and causing mental retardation - Some harmful alleles may survive because they are beneficial in combination with a normal allele o Example: sickle cell anemia allele actually helps some people survive malaria o Sickle Cell Anemia – Adisorder, caused by a recessive gene, in which the red blood cells become distorted when low in oxygen, causing fatigue, shortness of breath, and severe pain, and posing a threat to life from blockage of crucial blood vessels CHROMOSOMAL ABNORMALITIES - Generally arise during the process of meiosis, when the eggs or the sperms are formed - The aberration proves lethal and the zygote produced by the union of sperm and egg spontaneously aborts - Sometimes when certain chromosomes are involved, a zygote is able to survive the abnormal condition, and a baby with chromosomal defect is born DOWN SYNDROME - Down Syndrome – Aform of chromosome abnormality, in which the person suffers disabling physical and mental development and is highly susceptible to such illnesses as leukemia, heart disorders, and respiratory infections - Is caused by a deviation in set of chromosomes labelled number 21, trisomy 21 - Most likely comes from the mother’s egg when her homologous pair of 21 chromosome fail to separate during meiosis - This error occurs more often when older women become pregnant - Age also matters: Down syndrome births are higher for men over 40, and women over 35 - Children are generally slow to learn to speak and often have difficulty articulating words and even producing complex sentences in adolescence. They also have trouble attending to, discriminating, and interpreting complex or subtle information in their environments. SEX-CHROMOSOMAL ANOMALIES - Turner Syndrome (XO) – Aform of abnormality of the sex chromosomes found in females, in which secondary sex characteristics develop only if female hormones are administered and in which abnormal formation of internal reproductive organs leads t
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