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Chapter 3

CH3 Hereditary Influences on Development.doc

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University of Guelph
PSYC 2450

CH3 Hereditary Influences on Development -challenges in presenting information in a clear way evident when trying to explain it to 4-year-old. -chapter approaches human development from hereditary perspective, seeking to determine how one's genotype (genes one inherits) expressed as phenotype ( observable or measurable characteristics). -review evidence for hereditary contributions to psychological attributes ex intelligence, personality, mental health, and patterns of behaviour. -implies: many noteworthy phenotypic characteristics influenced by genes passed to us by our parents Principles of Hereditary Transmission -must start conception: moment when an ovum released by woman's ovary and on way to uterus via fallopian tube fertilized by a man's sperm. -establish what is inherited at conception, can examine mechanisms by which genes influence characteristics we display. The Genetic Material -sperm cell penetrates lining of ovum, biochemical reaction repels other sperm, preventing them from repeating fertilization process -sperm cell begins to disintegrate, releasing genetic material; ovum releases its genetic material, and new cell nucleus forms around hereditary information -zygote single cell formed at conception from union of a sperm and an ovum. -only 1 / 20th the size of a pin -What hereditary material is present in human zygote? - new cell nucleus contains46 elongated, threadlike bodies ; chromosomes, each consists of thousands of chemical segments, genes-the basic units of heredity -chromosomes come in matching pairs. Each member of pair corresponds to other in size, shape, and hereditary functions it serves -each parent contributes 23 chromosomes to each of their children. -Genes actually stretches of deoxyribonucleic acid, DNA, complex "double-helix" molecule resembles twisted ladder , provides chemical basis for development -unique feature of DNA : it can duplicate itself - makes it possible for a one-celled zygote to develop into human being. Growth of the Zygote and Production of Body Cells -zygote moves through fallopian tube toward prenatal home in uterus, begins to reproduce itself through mitosis -divide into two cells, two become four, four become eight, eight become sixteen etc . -before each division, cell duplicates 46 chromosomes, duplicate sets move in opposite directions. - division of cell proceeds, resulting in two new cells, each has identical 23 pairs of chromosomes ( 46 in all) thus same genetic material as original cell -Mitosis continues throughout life, generating new cells that enable growth and replacing old ones that are damaged. The Germ (or Sex) Cells -germ cells: serve one special hereditary function- to produce gametes (sperm in males ; ova in females). -different type of cell reproduction than process of mitosis. -shares some characteristics of mitosis, but differs in ways that make resulting cells able to join w/ gametes to create unique cell that will become unique individual Production of Gametes through Meiosis -germ cells produce sex cells through meiosis -first duplicates 46 chromosomes. -event called crossing-over often takes place: Adjacent duplicated chromosomes cross and break at one or more points along their length, exchanging segments of genetic material -transfer of genes during crossing-over creates new , unique hereditary combinations. - next , pairs of duplicated chromosomes (some altered by crossing-over) segregate into two new cells that each contains 46 chromosomes. -Finally, new cells divide so each of resulting gametes contains 23 single, or unpaired, chromosomes. -conception= sperm w/ 23 chromosomes unites w/ ovum w/ 23 chromosomes, producing a zygote w/ full complement of 46 chromosomes. -Why is it, offspring of same parents sometimes barely resemble each other? meiosis makes us genetically unique Hereditary Uniqueness -independent assortment principle stating : each pair of chromosomes segregates independently of all other chromosome pairs during meiosis. -When pair of chromosomes segregates during meiosis, matter of chance which of two chromosomes will end up in particular parent cell. - because each chromosome pair segregates independently of all other pairs according to principle of independent assortment, many different combinations of chromosomes could result from meiosis of single germ cell. -laws of probability : each parent can produce 223- more than 8 million- different genetic combinations in sperm or ova. -father produce 8 million combinations of 23 chromosomes ; mother produce 8 million, any couple could theoretically have 64 trillion babies w/o producing two children inheriting same set of genes -odds of exact genetic replication in two siblings smaller than 1 in 64 trillion. Why? -crossing-over process alters genetic composition of chromosomes increasing the number of possible variations in individual's gametes -Each brother or sister inherits half of each parent's genes, -although two siblings never inherit same half, owing to random process which parental chromosomes (and genes) segregate into sperm and ovum , combine to produce each child Multiple Births -monozygotic (or identical) twins twins who develop from single zygote that later divides form two genetically identical individuals -Occasionally, zygote split into separate but identical cells, which become two individuals. - monozygotic (or identical) twins ; developed from single zygote , have identical genes. -occur in about 1 of every 250 births around world -should show very similar developmental progress if genes have much effect on human development. -occurring approx 1 of 125 births, : dizygotic (or fraternal) twins- pairs result when mother releases two ova at same time , each fertilized by different sperm -albeit fraternal twin born together, have no more genes in common than any other pair of siblings -fraternal twins often differ considerably in appearance, need not be same sex. -Canada and other developed countries, incidence of multiple births involving three or more infants increased - births in Canada involving three or more infants nearly tripled b/w 1979 and 1999, while total number of births decreased 7.7 percent -increase attributed to use of assisted reproductive technologies; fertility drugs, artificial insemination, in vitro fertilization, -Most of these multiple births involve siblings fraternal, not identical Male or Female? -chromosomal portraits, / karyotypes, reveal 22 of 23 pairs of human chromosomes (autosomes) similar in males and females -Sex determined by 23rd pair (sex chromosomes). -In males, 23rd pair consists of X chromosome and Y chromosome. -females, both of sex chromosomes are Xs -Throughout history, mothers often been belittled, tortured, divorced, beheaded failing to bear husbands a male heir -both social and biological injustice that fathers determine sex of their children. -When sex chromosomes of genetic (XY) male segregate into gametes during meiosis, half of sperm produced contain X chromosome , half Y chromosome -ova produced y genetic (XX) female all carry X chromosome. -Thus child's sex determined by whether X-bearing or Y-bearing sperm fertilizes ovum. What Do Genes Do? -How do genes promote development? -they call for production of amino acids, which form enzymes + proteins necessary for formation & functioning of new cells -regulate production of pigment , melanin in the iris - brown eyes have genes , call for much of this pigment, whereas lighter (blue or green) eyes call for less pigmentation -Genes guide cell differentiation, specialize for certain parts - Genes influence and influenced by biochemical environment surrounding them during development. -particular cell might become part of eyeball or elbow depending on what cells surround it during early embryonic development -some responsible for regulating pace and timing of development -specific genes are "turned on" or "turned off" by other regulatory genes at different points in life span -Regulatory genes, might "turn on" genes responsible for growth spurt in adolescents, then shut growth genes down in adulthood. -Environmental factors clearly influence how genes function - ex child who inherits genes for tall stature may / may not be tall as adult -maybe experience very poor nutrition for prolonged period early in life, could end up being only average / below average in height, despite genetic potential -Environment affects actions of genes at several different levels -nucleus contains chromosomes and genes -environment w/in nucleus may affect expression of genetic material. -internal environment that surrounds cell may affect gene's expression. -external environment affects expression of genetic material -some effects of external environment experienced by all , some experienced by a some people -former : "experience-expectant interactions," ; latter : "experience-dependent interactions" -important : realization genes do not simply "code" for human characteristics, but interact w/ environment at many levels to produce proteins eventually influence human characteristics. *-approach how genes influence development : consider major patterns of genetic inheritance: ways in which parents' genes expressed in their children's phenotypes. How Are Genes Expressed? -four main patterns of genetic expression: simple dominant-recessive inheritance, co dominance, sex-linked inheritance, , polygenic (or multiple gene) inheritance. Single-Gene Inheritance Patterns -Genes influence human characteristics in different ways. -Sometimes characteristics determined by actions of a single gene , sometimes by many genes working together: polygenic inheritance. -Understanding single-gene inheritance patterns can help build understanding of actions of genes and interactions w/ environment. -turn to understanding mechanisms at work when many genes interact to influence characteristics. Simple Dominant-Recessive Inheritance. -Many human characteristics influenced by only one pair of genes (called alleles): -Gregor Mendel contributed to our knowledge of single gene-pair inheritance by cross-breeding different strains of peas and observing outcomes. -major discovery : predictable pattern to way in which two alternative characteristics (ex , smooth seeds vs. wrinkled seeds, green pods vs. yellow pods) appeared in offspring of cross- breeding. -called some characteristics (ex , smooth seeds) "dominant" : appeared more often in later generations than opposite traits, which called "recessive" traits. - offspring's phenotype often not 'blend" of characteristics of mother and father. -Instead, one of parental genes often dominates other, child resembles parent who contributed dominant gene. -consider fact : about three-quarters of us have ability to see distant objects clearly ( 20/20 vision) whereas remaining one-fourth cannot , myopic (nearsighted). - gene assoc w/ normal vision is dominant allele; weaker gene for nearsightedness is recessive allele. -person who inherits one allele for normal vision ,one for myopia would display phenotype of normal vision - normal-vision gene overpowers (dominates) nearsightedness gene. -normal-vision allele dominates nearsightedness allele, represent normal-vision gene w/ capital N and nearsightedness gene lowercase n. -three possible genotypes for this characteristic:(1) two normal-vision alleles (NN), (2) two nearsightedness alleles (nn), and (3) one of each (Nn). -genotype for attribute consists of two alleles of same kind= homozygous for that attribute -NN individual homozygous for normal vision, will pass genes for normal vision to children. -nn individual homozygous for nearsightedness (only way one can actually be nearsighted is inherit two of recessive alleles) and pass nearsightedness genes -Nn individual heterozygous for visual trait = inherited alternative forms of allele. -person will have normal vision, N allele is dominant. -Can two individuals w/ normal vision produce nearsighted child? yes-if each parent is heterozygous for normal vision and carrier of recessive allele for nearsightedness -Because each of four combinations equally likely odds are 1 in 4 child of two Nn parents will be nearsighted. carrier heterozygous md1v1dual who displays no sig n of recessive allele their own phenotype but can pass gene to offspnng. \ Codominance. Alternative forms of a gene do not always follow pattern described by Mendel. - some codominant: phenotype they produce is compromise b/w two genes. -alleles for human blood types A and B equally expressive, neither dominates other. -heterozygous person who inherits allele for blood type A and one for type B equal proportions of A- antigens and B-antigens in their blood. -occurs when one of two heterozygous alleles stronger than other but fails to mask all of its effects. -sickle-cell trait example of "incomplete dominance". - about 8 % of African Americans (relatively few whites or Asian Americans) heterozygous for attribute, carrying recessive "sicklecell" allele - causes some of person's red blood cells to assume an unusual crescent shape - problem : tend to cluster together, distributing less oxygen throughout circulatory system. -overt symptoms of circulatory distress: painful swelling of joints , fatigue, rarely experienced by sickle-cell "carriers," unless experience oxygen deprivation as they might after physical exertion at high altitudes or ,under anesthesia -consequences severe for those who inherit two recessive sickle-cell genes -develop sickle-cell anemia, causes massive sickling of red blood cells , inefficient distribution of oxygen at all times -Many die from heart or kidney failure or respiratory diseases during childhood Sex-Linked Inheritance. Some traits sex-linked characteristics , determined by genes located on sex chromosomes. fact, -majority produced by recessive genes found only on X chromosomes - males inherit most sex linked characteristics - ex red/ green colour blindness - Males have XY (one inherited from mother) ; Women have XX -no corresponding gene on Y chromosome that might counteract effect of "colour-blind" allele. -genetic female who inherits one gene will not be colour-blind, for colour-normal gene on her second X chromosome will dominate colour-blind gene, -female cannot be colour-blind unless both X chromosomes contain recessive gene for colour blindness -roughly 8 white males in 100 cannot distinguish red from green; only 1 in 144 white females red/ green colour-blind -more than 100 sex-linked characteristic -hemophilia ( disease which blood does not clot), two kinds of muscular dystrophy; degeneration of optic nerve, certain forms of deafness and night blindness Polygenic Inheritance -most important human characteristics influenced by many pairs of alleles: polygenic traits - examples height, weight, intelligence, skin colour, temperamental attributes, susceptibility to cancer, -As # of genes that contribute to particular characteristic increases, number of possible genotypes and phenotypes quickly increases. -result, observable traits for polygenic traits are not either I or possibilities -observable traits follow pattern of continuous variation w/ few people having traits at extremes, -most people having traits in middle of distribution ( distribution follows bell curve) -When considering characteristic that follows dominant-recessive pattern but influenced by two genes w/ two alleles, number of potential genotypes increases to nine ; number of phenotypes increases to five -When we consider another characteristic influenced by three genes w/ two alleles each, number of potential genotypes increases 27 ; number of phenotypes increases to seven --distribution of observable phenotypes in population begins to resemble a normal curve -imagine increased complexity when considering some of many genes would follow other patterns of inheritance eg co dominance, incomplete dominance, or sex-linked inheritance -unknown numbers of genes, interacting w/ environmental influences, create wide range of individual differences in most important human attributes. Hereditary Disorders -congenital defect problem present (though not necessa rily apparent) at birth; defects may stem from genetic and prenatal influences or from complications of birth process -NEL Huntington's disease genetic disease caused by dominant allele , typically appears later in life , causes nervous system to degenerate -approximately 5 of every 100 infants have congenital problem of some kind -Huntington's disease present from moment of conception. -gradual deterioration of nervous system assoc w/ condition not apparent at birth, will not ordinarily appear until much later- usually after age 40. - DNA testing for disorder 98.8 percent accurate; fetal DNA tests (from amniocentesis or chorionic villus) can be performed for early detection. Chromosomal Abnormalities -When germ cell divides during meiosis, distribution of 46 chromosomes into sperm or ova sometimes uneven. -one of resulting gametes may have too many chromosomes, while other has too few. -If abnormal germ cells conceived, vast majority abnormalities lethal, fail to develop, or spontaneously aborted. -some chromosomal are not lethal. - Approximately 1 child in 250 is born w/ either one chromosome too many or one too few Abnormalities of the Sex Chromosomes Many chromosomal abnormalities involve 23rd pair- sex chromosomes. -males born w/ extra X or Y chromosome, producing genotype XXY or XYY, -females may survive if inherit single X chromosome (XO) or three (XXX), four (XXXX), five (XXXXX) X Abnormalities of the Autosome -autosomes 22 pairs of human chromosomes identical in males and females. -most common type : abnormal sperm or ovum carrying extra autosome combines w/ normal gamete to form zygote that has 47 chromosomes (2 sex chromosomes and 45 autosomes) -extra chromosome appears along w/ one of 22 pairs of autosomes to yield three chromosomes of that type, trisomy. -most frequent of autosomal abnormalities (Canada, occurrence about 1 in 900 births;), Down syndrome, or trisomy 21, -condition which child inherits all or part of extra 21st chromosome -mentally retarded, IQs average 55( normal children is 100) -mildly or moderately mentally retarded. - may have congenital eye, ear, heart defects -usually characterized by distinctive physical features : sloping forehead, protruding tongue, short stubby limbs, slightly flattened nose almond-shaped eyes -youngsters reach many of same developmental milestones as normal children, but at slower pace -Development progress appears to be best when parents strive to include Down syndrome children in family activities, patient , work hard to properly stimulate them, Genetic Abnormalities -Parents who themselves healthy often amazed to learn their child could have hereditary defect. -genetic problems recessive traits that few, if any, close relatives may have had. -problems will not appear unless both parents carry harmful allele and child inherits particular gene from each parent. -exceptions : sex-linked defects that male child will display if recessive alleles appear on X chromosome inherited from his mother. -Some abnormalities caused by dominant alleles. -child will develop disorder by inheriting dominant allele from either parent. - parent contributing allele for disorder will also display defect - ex dominant genetic disorder Huntington's -may also result from mutations- changes in chemical structure of one or more genes produce a new phenotype -occur spontaneously and are harmful or even fatal. - possibly induced by environmental hazards ex toxic industrial waste, radiation, agricultural chemicals that enter food supply, even some additives and preservatives in processed foods -Might mutations ever be beneficial? -Evolutionary theorists think so. -Presumably, any mutation induced by stressors present in natural environment may provide an "adaptive" advantage to those who inherit mutant genes, enabling individuals to survive. - sickle-cell gene, mutation originated in Africa, Southeast Asia, other tropical areas where malaria is widespread. - Heterozygous children who inherit single sickle-cell allele well adapted to environments ; mutant gene makes them more resistant to malarial infection , likely to survive Predicting, Detecting, and Treating Hereditary Disorders -options for predicting whether couple at risk for hereditary disorder, prenatal detection of hereditary disorders, and medical treatment of hereditary disorders (both prenatally and after birth). Predicting Hereditary Disorders genetic counselling service designed to inform prospective parents about genetic diseases , help them determine likelihood they would transmit disorders to their children. -"genetic counselling" refers to prediction of both chromosomal abnormalities and genetic abnormalities -trained in genetics, interpretation of family histories, and counselling procedures. -helpful for couples who either have relatives w/ hereditary disorders or already borne child w/ disorder. -normally begin by obtaining complete family history; pedigree, from each parent to identify relatives affected by hereditary disorders. -used to estimate likelihood couple would bear child w/ chromosomal disorder - pedigrees only basis for determining whether children likely to be affected by certain disorders -( ex one type of diabetes and some forms of muscular dystrophy) -pedigree analysis cannot guarantee child will be healthy, -DNA analyses from parents' blood tests can now determine whether parents carry genes for many serious hereditary disorders -as well as Huntington's disease and fragile-X syndrome -fragile-X syndrome abnormality of X chromosome caused by defective gene assc w/mild to severe mental retardation, particularly when defective gene passed from mother to child. Detecting Hereditary Disorders -amniocentesis method of extracting amniotic fluid from pregnant woman so that fetal body cells within fluid can be tested for chromosomal abnormalities and genetic defects -chromosomal abnormalities dramatically increases after age 35, older mothers often undergo amniocentesis - hollow needle inserted into mother's abdomen to withdraw sample of amniotic fluid that surrounds fetus. - Fetal cells in fluid then tested to determine sex of fetus and presence of chromosomal abnormalities such as Down syndrome. - more than 100 genetic disorders-inc Tay-Sachs disease, cystic fibrosis, one type of diabetes, Duchenne muscular dystrophy, sickle-cell anemia, hemophilia- diagnosed by analyzing fetal cells in amniotic fluid - considered very safe procedure, triggers miscarriage in small percentage of cases (currently about 1 chance in 150 - amniocentesis less risky when performed after 13th week of pregnancy, amniotic fluid becomes sufficiently plentiful to withdraw for analysis - results of tests will not come back for two weeks, parents have little time to consider a second- trimester abortion if fetus has serious defect - alternative procedure : chorionic villus sampling (CVS) - chorionic villus sampling (CVS} alternative to amniocentesis fetal cells extracted from chorion for prenatal tests. performed earlier in pregnancy than possible w/ amniocentesis. - possible during eighth or ninth week of pregnancy - Either catheter inserted through vagina and cervix, or needle through abdomen, into membrane , the chorion, that surrounds fetus. - results typically available w/in 24 hours - allows parents to know whether fetus bears suspected abnormality early on, more time to consider pros and cons of continuing pregnancy - currently recommended only to parents at very high risk -entails greater chance of miscarriage (1 chance in 50) than does amniocentesis, its use has, in rare instances, linked to limb deformities in fetus - safer early screening technique may be widely available in near future - involves DNA analysis of fetal cells that begin to enter mother's bloodstream early pregnancy -when isolated can be tested to determine whether fetus carries abnormalities -very common and safe prenatal diagnostic technique : ultrasound - method of scanning womb w/ sound waves most useful after 14th week - particularly helpful for detecting multiple pregnancies and gross physical defects as well as age and sex of fetus. Treating Hereditary Disorders - detection of hereditary disorder leaves many in quandary, particularly if religious background or personal beliefs oppose abortion - if disease in question invariably fatal must decide violate moral principles, terminate pregnancy or have a baby who will appear normal , healthy but rapidly decline and die young. phenylketonuria (PKU} genetic disease which child unable to metabolize phenylalanine; if left untreated, soon causes hyperactivity and mental retardation. phenylketonuria (PKU)- Affected children that lack critical enzyme would allow them to metabolize phenylalanine, component of many foods, including milk. -As phenylalanine accumulates , converted into harmful substance, phenylpyruvic acid, attacks nervous system. - mid-1950s scientists developed diet low in phenylalanine ; 1961 developed simple blood test that could determine if child had PKU w/in few days after birth - infants now routinely screened for PKU (and other metabolic disorders); affected children immediately placed on low-phenylalanine diet for PKU - Children who remain on diet throughout middle childhood suffer few if any consequences - outcomes best for individuals who stay on diet for life - true of PKU women who hope to have children of their own; if abandon diet and their phenylalanine levels are high, great risk of either miscarrying or bearing mentally deficient child - new medical and surgical techniques, performed on fetuses in uterus, possible to treat some hereditary disorders by delivering drugs or hormones to developing fetus - or surgically repairing some genetically transmitted defects of heart, neural tube, urinary tract, and respiratory system children born
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