{"id":157,"date":"2014-09-18T15:36:33","date_gmt":"2014-09-18T15:36:33","guid":{"rendered":"https:\/\/courses.candelalearning.com\/lifespandevelopment1x1\/?post_type=chapter&p=157"},"modified":"2020-10-08T15:17:05","modified_gmt":"2020-10-08T15:17:05","slug":"lesson-3-heredity-prenatal-development-and-birth","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/suny-hccc-ss-152-1\/chapter\/lesson-3-heredity-prenatal-development-and-birth\/","title":{"raw":"Introduction to Heredity,\u00a0Prenatal Development, and Birth","rendered":"Introduction to Heredity,\u00a0Prenatal Development, and Birth"},"content":{"raw":"
\r\n

Learning Objectives<\/h3>\r\n

At the end of this lesson, you will be able to<\/p>\r\n\r\n

    \r\n \t
  1. Define gene.<\/li>\r\n \t
  2. Define chromosome.<\/li>\r\n \t
  3. Define gamete.<\/li>\r\n \t
  4. Explain what determines the chromosomal sex of the child.<\/li>\r\n \t
  5. Question the assertion that human traits are genetic.<\/li>\r\n \t
  6. Compare monozygotic and dizygotic twins.<\/li>\r\n \t
  7. Differentiate between genetic disorders and chromosomal abnormalities.<\/li>\r\n \t
  8. Describe Trisomy 21.<\/li>\r\n \t
  9. Differentiate between the germinal, embryonic, and fetal periods of development.<\/li>\r\n \t
  10. Describe\u00a0human development during the germinal, embryonic, and fetal periods.<\/li>\r\n \t
  11. Describe\u00a0a normal delivery and complications of pregnancy and delivery.<\/li>\r\n \t
  12. Predict\u00a0the risks to prenatal development\u00a0posed by exposure to teratogens.<\/li>\r\n \t
  13. Interpret\u00a0APGAR scores.<\/li>\r\n \t
  14. Discover\u00a0problems of newborns<\/li>\r\n<\/ol>\r\n<\/div>\r\n

    Heredity:\u00a0The Epigenetic Framework<\/h1>\r\n

    Nature or Nurture?<\/h2>\r\nIn this lesson, we will look at some of the ways in which heredity helps to shape the way we are. We will look at what happens genetically during conception and take a brief look some genetic abnormalities. Before going into these topics, however, it is important to emphasize the interplay between heredity and the environment. Why are you the way you are? As you consider some of your features (height, weight, personality, being diabetic, etc.), ask yourself whether these features are a result of heredity or environmental factors-or both. Chances are,\u00a0you can see the ways in which both heredity and environmental factors (such as lifestyle, diet, and so on) have contributed to these features.\u00a0 For decades, scholars have carried on the \"nature\/nurture\" debate. For any particular feature, those on the \"nature\" side would argue that heredity plays the most important role in bringing about that feature. Those on the \"nurture\" side would argue that one's environment is most significant in shaping the way we are. This debate continues in questions about what makes us masculine or feminine (Lippa, 2002), concerns about vision (Mutti,\u00a0Kadnik\u00a0and Adams, 1996), and many other developmental issues.\u00a0(Check out\u00a0www.googlescholar.com<\/a>\u00a0for over 20,000 entries for \u201ccurrent nature\/nurture debates\u201d!)\u00a0Yet most scholars agree that there is a constant interplay between the two forces. It is difficult to isolate the root of any single behavior as a result solely of nature or nurture and most scholars believe that even determining the extent to which nature or nurture impacts a human feature is difficult to answer. In fact, almost all human features are\u00a0polygenic\u00a0(a result of many genes) and\u00a0multifactorial\u00a0(a result of many factors, both genetic and environmental). It's as if one's genetic make-up sets up a range of possibilities, which may or may not be realized depending upon one's environmental experiences. For instance, a person might be genetically predisposed to develop diabetes, but the person's lifestyle may help bring about the disease.\r\n

    The Epigenetic\u00a0Framework<\/h2>\r\nGottlieb (1998, 2000,\u00a02002) suggests an analytic framework for the nature\/nurture debate that recognizes the interplay between the environment, behavior, and genetic expression. This bidirectional interplay suggests that the environment can\u00a0effect\u00a0the expression of genes just as genetic predispositions can impact a person\u2019s potentials.\u00a0And environmental circumstances can trigger symptoms of a genetic disorder.\u00a0 For example, a person who has sickle cell anemia, a recessive gene linked disorder, can experience a sickle cell crisis under conditions of oxygen deprivation.\u00a0 Someone predisposed genetically\u00a0for type two diabetes\u00a0can trigger the disease through poor diet and little exercise.\r\n

    The Human Genome Project<\/h2>\r\nThe\u00a0Human Genome Project is an internationally funded effort to map the locations of human genes and understand the role these genes play in development, health and illness.\u00a0(Check out recent developments at\u00a0www.genome.gov<\/a>)\u00a0\u00a0Genes\u00a0are segments of\u00a0chromosomes\u00a0(46 strands of a chemical substance called DNA that are contained in the nucleus of each normal human cell) that vary in length.\u00a0\u00a0 There are an estimated 25,000 to 30,000 genes on each chromosome; a\u00a0number far below the estimate of 100,000-150,000 held before the work of the Human Genome Project.\r\n\r\nUnderstanding the role of genes in health and illness can bring about both harm and good (Weitz, 2007).\u00a0A person who knows that they are at risk for developing a genetic disorder may be able to adopt lifestyle practices that minimize the risk and a person who discovers that they are not at risk may find comfort in knowing that they do not have to fear a particular disease.\u00a0However, a person who finds out that they are at risk and there is nothing that can be done about it may experience years of fear and anxiety.\u00a0And the availability of genetic testing may be more widespread than the availability of genetic counseling which can be very expensive.\u00a0\u00a0 The possible stigma and discrimination that those with illness or at risk for illness must also be considered.\u00a0\u00a0 In light of the high costs of health insurance, many companies are starting to offer benefits contingent on health assessments and lifestyle recommendations; and continued coverage depends on an employee following these recommendations.\u00a0So a smoker may have to pay a higher premium than a non-smoker or a person who is overweight may be required to engage in a program of exercise and be monitored for improvement.\u00a0What if a person finds out that they carry the gene for Huntington\u2019s disease (a neurological disorder that is ultimately fatal) which may surface when a person reaches their 40s?\u00a0The impact this knowledge will have on health care still remains unknown.\u00a0 Who should know what is on your genome?\u00a0 Do you think this information should be shared between mates?\u00a0 What about employers?\u00a0 What would be the advantages and disadvantages?\r\n

    Conception<\/h1>\r\n[caption id=\"attachment_577\" align=\"alignright\" width=\"240\"]\"female<\/a> The Female Reproductive System[\/caption]\r\n

    Gametes<\/h2>\r\nThere are two types of sex cells or gametes involved in reproduction: the male gametes or sperm and female gametes or ova. The male gametes are produced in the testes in a process called\u00a0spermatogenesis\u00a0which begin at about 12 years of age.\u00a0The female gametes or ova which are stored in the ovaries are present at birth but are immature. Each ovary contains about 250,000\u00a0(Rome 1998) but only about 400 of these will become mature eggs (Mackon\u00a0and\u00a0Fauser\u00a02000).\u00a0\u00a0 Beginning at puberty, one ovum ripens and is released about every 28 days, a process called\u00a0oogenesis.\r\n\r\nAfter the ovum or egg ripens and is released from the ovary, it is drawn into the fallopian tube and in 3 to 4 days, reaches the uterus. It is typically fertilized in the fallopian tube and continues its journey to the uterus. At ejaculation, millions of sperm are released into the vagina, but only a few reach the egg and typically, only one fertilizes the egg. Once a single sperm has entered the wall of the egg, the wall becomes hard and prevents other sperm from entering. After the sperm has entered the egg, the tail of the sperm breaks off and the head of the sperm, containing the genetic information from the father, unites with the nucleus of the egg. As a result, a new cell is formed. This cell, containing the combined genetic information from both parents, is referred to as a\u00a0zygote.\r\n\r\nChromosomes contain genetic information from each parent. While other normal human cells have 46 chromosomes (or 23 pair), gametes contain 23 chromosomes. In a process called\u00a0meiosis,\u00a0segments of the chromosomes from each parent form pairs and genetic segments are exchanged as determined by chance.\u00a0Because of the unpredictability of this exchange the likelihood of having offspring that are genetically identical (and not twins) is one in trillions (Gould and Keeton, 1997).\r\n

    Determining the Sex of the Child<\/h2>\r\nTwenty-two of those chromosomes from each parent are similar in length to a corresponding chromosome from the other parent. However, the remaining chromosome looks like an X or a Y. Half of the male's sperm contain a Y chromosome and half contain an X. All of the ova contain one X chromosome. If the child receives the combination of XY, the child will be genetically male. If it receives the XX combination, the child will be genetically female.\r\n\r\nMany potential parents have a clear preference for having a boy or a girl and would like to determine the sex of the child. Through the years, a number of tips have been offered for the potential parents to maximize their chances for having either a son or daughter as they prefer. For example, it has been suggested that sperm which carry a Y chromosome are more fragile than those carrying an X. So, if a couple desires a male child, they can take measures to maximize the chance that the Y sperm reaches the egg. This involves having intercourse 48 hours after ovulation, which helps the Y sperm have a shorter journey to reach the egg, douching to create a more alkaline environment in the vagina, and having the female reach orgasm first so that sperm are not pushed out of the vagina during orgasm. Today, however, there is new technology available that makes it possible to isolate sperm containing either an X or a Y, depending on the preference, and use that sperm to fertilize a mother's egg.\r\n

    Monozygotic and Dizygotic Twins<\/h2>\r\nMonozygotic twins occur when a single zygote or fertilized egg splits apart in the first two weeks of development. The result is the creation of two separate but genetically identical offspring. About one-third of twins are\u00a0monozygotic\u00a0twins.\u00a0Are you an identical twin?\r\n\r\nSometimes, however, two eggs or ova are released and fertilized by two separate sperm. The result is\u00a0dizygotic\u00a0or fraternal twins. About two-thirds of twins are\u00a0dizygotic. These two individuals share the same amount of genetic material as would any two children from the same mother and father.\u00a0 Older mothers are more likely to have\u00a0dizygotic\u00a0twins than are younger mothers and couples who use fertility drugs are also more likely to give birth to\u00a0dizygotic\u00a0twins.\u00a0Consequently, there has been in increase in the number of fraternal twins in recent years (Bortolus et. al., 1999).\r\n\r\nWhat are the other possibilities?\u00a0 Various degrees of sharing the placenta can occur depending on the timing of the separation and duplication of cells.\u00a0 This is known as\u00a0placentiation.\u00a0 Here is a diagram that illustrates various types of twins.\r\n\r\n[caption id=\"attachment_576\" align=\"aligncenter\" width=\"347\"]\"Diagram Author Kevin\u00a0Dufenbach[\/caption]\r\n

    Genotypes and Phenotypes (or why what you get is not always what you see)<\/h2>\r\nThe word\u00a0genotype\u00a0refers to the sum total of all the genes a person inherits. The word\u00a0phenotype\u00a0refers to the features that are actually expressed. Look in the mirror. What do you see, your genotype or your phenotype?\u00a0What determines whether or not genes are expressed? Actually, this is quite complicated (Berger, 2005). Some features follow the\u00a0additive pattern\u00a0which means that many different genes contribute to a final outcome. Height and skin tone are examples. In other cases, a gene might either be turned on or off depending on the gene with which it is paired. Some genes are considered dominant because they will be expressed. Others, termed recessive, are only expressed in the absence of a dominant gene. Some characteristics which were once thought of as\u00a0dominant-recessive, such as eye color, are now believed to be a result of the interaction between several genes (McKusick, 1998). Dominant traits include curly hair, facial dimples, normal vision, and dark hair.\u00a0Recessive characteristics include red hair, pattern baldness, and nearsightedness.\u00a0\u00a0 Sickle cell anemia is a recessive disease; Huntington\u00a0disease is a dominant disease.\u00a0Other traits are a result of\u00a0partial dominance\u00a0or\u00a0co-dominance\u00a0in which both genes are influential. For example, if a person inherits both recessive genes for sickle cell anemia, the disease will occur.\u00a0But if a person has only one recessive gene for the disease, the person may experience effects of the disease only under circumstances of oxygen deprivation such as high altitudes or physical exertion (Berk, 2004).\r\n

    Chromosomal Abnormalities and Genetic Disorders<\/h2>\r\nA\u00a0chromosomal abnormality\u00a0occurs when there a child inherits too many or two few chromosomes.\u00a0\u00a0The most common cause of chromosomal abnormalities is the age of the mother. A 20 year old woman has a 1 in 800 chance of having a child with a common chromosomal abnormality. A woman of 44, however, has a one in\u00a016 chance. It is believed that the problem occurs when the ovum is ripening prior to ovulation each month. As the mother ages, the ovum is more likely to suffer abnormalities at this time.\r\n\r\nSome gametes do not divide evenly when they are forming. Therefore, some cells have more than 46 chromosomes. In fact, it is believed that close to half of all zygotes have an odd number of chromosomes. Most of these zygotes fail to develop and are spontaneously aborted by the body. If the abnormal number occurs on pair #21 or # 23, however, the individual may have certain physical or other abnormalities.\r\n\r\nOne of the most common chromosomal abnormalities is on pair 21.\u00a0Trisomy 21\u00a0occurs when there are three rather than two chromosomes on #21.\u00a0\u00a0 A person with Down syndrome experiences problems such as mental retardation and certain physical features such as having short fingers and toes, having folds of skin over the eyes, and a protruding tongue.\u00a0Life expectancy of persons with Down syndrome has increased in recent years.\u00a0Keep in mind that there is as much variation in people with Down\u00a0Syndrome\u00a0as in most populations and those differences need to be recognized and appreciated.\u00a0Watch the following\u00a0video clip about Down\u00a0Syndrome\u00a0from the\u00a0National Down Syndrome Society:\r\n\r\nhttps:\/\/www.youtube.com\/watch?v=TIcbFrt4F_c\r\n\r\nWhen the abnormality is on pair #23, the result is a\u00a0sex-linked chromosomal abnormality. A person might have XXY, XYY, XXX, XO, or 45 or 47 chromosomes as a result.\u00a0Two of the more common sex-linked chromosomal disorders are\u00a0Turner\u2019s syndrome\u00a0and\u00a0Klinefelter\u2019s\u00a0syndrome.\u00a0Turner\u2019s syndrome occurs in 1 of every 2,500 live female births (Carroll, 2007) when an ovum which lacks a chromosome is fertilized by a sperm with an X chromosome.\u00a0The resulting zygote has an XO composition.\u00a0Fertilization by a Y sperm is not viable.\u00a0Turner syndrome affects cognitive functioning and sexual maturation.\u00a0The external genitalia appear normal, but breasts and ovaries do not develop fully and the woman does not menstruate.\u00a0Turner\u2019s syndrome also results in short stature and other physical characteristics.\u00a0 Learn more at\u00a0www.turnersyndrome.org\/<\/a>.\u00a0Klinefelter's\u00a0syndrome (XXY)\u00a0occurs in 1 out of 700 live male births and results when an ovum containing an extra X chromosome is fertilized by a Y sperm.\u00a0The Y chromosome stimulates the growth of male\u00a0genitalia, but the additional X chromosome inhibits this development.\u00a0An individual with\u00a0Klinefelter\u2019s\u00a0syndrome has some breast development, infertility (this is the most common cause of infertility in males), and has low levels of testosterone.\r\n\r\nMost of the known\u00a0genetic disorders\u00a0are dominant gene-linked; however, the vast majority of dominant gene linked disorders are not serious disorders, or if they are, they may still not be debilitating. For example, the majority of those with Tourette's\u00a0Syndrome\u00a0suffer only minor tics from time to time and can easily control or cover up their symptoms.\u00a0Huntington's\u00a0Disease\u00a0is a dominant gene linked disorder that affects the nervous system and is fatal but does not appear until midlife.\u00a0 Recessive gene disorders, such as cystic fibrosis and\u00a0sickel-cell anemia, are less common but may actually claim more lives because they are less likely to be detected as people are unaware that they are carriers of the disease.\u00a0If the genes inherited from each parent are the same, the child is\u00a0homozygous\u00a0for a particular trait and will inherit the trait.\u00a0If, however, the child inherits a gene from one parent but not the other, the child is\u00a0heterozygous, and interaction between the genes will in part determine whether or not that trait is expressed (Berk, 2004).\r\n

    REFERENCES<\/h2>\r\nBerger, K. S. (2005).\u00a0The developing person through the life span\u00a0(6th\u00a0ed.). New York: Worth.\r\n\r\nBerk, L. (2004).\u00a0Development through the life span\u00a0(3rd\u00a0ed.). Boston:\u00a0Allyn\u00a0and Bacon.\r\n\r\nBortolus, R.,\u00a0Parazzini, F.,\u00a0Chatenoud, L.,\u00a0Benzi, G., Bianchi, M. M., & Marini, A. (1999).\u00a0The epidemiology of multiple births.\u00a0Human Reproduction Update,\u00a05, 179-187.\r\n\r\nBrazelton, T. B., & Nugent, J. K. (1995).\u00a0Neonatal behavioral assessment scale.\u00a0London: Mac Keith Press.\r\n\r\nCarrell, D. T., Wilcox, A. L., Lowry, L., Peterson, C. M., Jones, K. P., & Erikson, L. (2003).\u00a0Elevated sperm chromosome aneuploidy and apoptosis in patients with unexplained recurrent pregnancy loss.\u00a0Obstetrics and Gynecology,\u00a0101(6), 1229-1235.\r\n\r\nCarroll, J. L. (2007).\u00a0Sexuality now: Embracing diversity\u00a0(2nd\u00a0ed.). Belmont, CA: Thomson.\r\n\r\nDietrich, K. N. (1999).\u00a0Environmental toxicants and child development.\u00a0In\u00a0Tager-Flusberg\u00a0(Ed.),\u00a0Neurodevelopmental disorders\u00a0(pp. 469-490). Boston: MIT Press.\r\n\r\nFASD, NCBDDD, CDC.\u00a0(2006, July\/August).\u00a0Centers for Disease Control and Prevention.\u00a0Retrieved May 03, 2011, from http:\/\/www.cdc.gov\/ncbddd\/fas\/fasask.htm\r\n\r\nGalinsky, E. (1987).\u00a0The six stages of parenthood.\u00a0Reading, MA: Addison-Wesley Pub.\r\n\r\nGottlieb, G. (1998). Normally occurring environmental and behavioral influences on gene activity: From central dogma to probabilistic epigenesis.\u00a0Psychological Review,\u00a0105, 792-802.\r\n\r\nGottlieb, G. (2000). Environmental and behavioral influences on gene activity.\u00a0Current Directions in Psychological Science,\u00a09, 93-97.\r\n\r\nGottlieb, G. (2002).\u00a0Individual development and evolution: The genesis of novel behavior. New York: Oxford University Press.\r\n\r\nGould, J. L. (1997).\u00a0Biological science.\u00a0New York: Norton.\r\n\r\nLippa, R. A. (2002).\u00a0Gender, nature, and nurture.\u00a0Mahwah, NJ: L. Erlbaum.\r\n\r\nMacDorman, M.,\u00a0Menacker, F., &\u00a0Declercq, E. (2010, August 30).\u00a0Trends and Characteristics of Home and Other out of Hospital Births in the United States, 1990-2006\u00a0(United States, Center for Disease Control). Retrieved December 22, 2010, from http:\/\/www.cdc.gov\/nchs\/data\/nvsr\/nvsr58;nvsr58_11.PDF\r\n\r\nMackon, N., &\u00a0Fauser, B. (2000).\u00a0Aspects of ovarian follicle development throughout life.\u00a0Hormone Research,\u00a052, 161-170.\r\n\r\nMcKusick, V. A. (1998).\u00a0Mendelian\u00a0inheritance in man: A catalog of human genes and genetic disorders.\u00a0Baltimore, MD: Johns Hopkins University Press.\r\n\r\nMoore, K. L., &\u00a0Persaud, T. V. (1998).\u00a0Before we are born\u00a0(5th\u00a0ed.). Philadelphia, PA: Saunders.\r\n\r\nMutti, D. O.,\u00a0Zadnik, K., & Adams, A. J. (n.d.).\u00a0Myopia.\u00a0The nature versus nurture debate goes on.\u00a0Investigative Ophthalmology & Visual Science.\u00a0Retrieved May 03, 2011, from http:\/\/www.iovs.org\/cgi\/reprint\/37\/6\/952\r\n\r\nNewell, M. (2005).\u00a0Current issues in the prevention of mother-to-child transmission of HIV-1 infection.\u00a0Transactions of the Royal Society of Tropical Medicine and Hygiene,\u00a0100(1), 1-5.\u00a0doi: 10.1016\/j.trstmh.2005.05.012\r\n\r\nRome, E. (1998).\u00a0Anatomy and physiology of sexuality and reproduction.\u00a0In\u00a0The New Our Bodies, Ourselves\u00a0(pp. 241-258). Carmichael, CA: Touchstone Books.\r\n\r\nUNAIDS, World Health Organization.\u00a0(2005).\u00a0Adults and Children Estimated to Be Living with HIV as of the End of 2005. Retrieved August 13, 2006, from http:\/\/www.unaids.org?NetTools\/Misc\/DocInfo.aspx?LANG=en&href http:\/\/GVA-DOC-OWL\/WEBcontent\/Documents\/pub\/Topics\/Epidemiology\/Slides02\/12-05\/EpiCoreDec05Slide004_en.ppt\r\n\r\nUnited States, Center for Disease Control.\u00a0(n.d.).\u00a0The Health Consequences of Smoking: 2004 Report of the Surgeon General. Retrieved August 14, 2004, from http:\/\/www.cdc.gov\/tobacco\/sqr\/sqr_2004\r\n\r\nUnited States, Center for Disease Control.\u00a0(2006, July\/August).\u00a0Sexually Transmitted Diseases Treatment Guidelines.\u00a0Retrieved August 14, 2006, from http:\/\/www.cdc.gov\/std\/treatment\/2006\/rr5511.pdf\r\n\r\nUnited States, Center for Disease Control, Health and Human Services. (2010, October 5).\u00a0Centers for Disease Control and Prevention.\u00a0Retrieved May 03, 2011, from http:\/\/www.cdc.gov\/nchs\/faststats\/birthwt.htm\r\n\r\nUnited States, Center for Disease Control, National Center on Birth Defects and Developmental Disabilities.\u00a0(2004, October 29).\u00a0Fast Facts about Medication Use during Pregnancy and While Breastfeeding.\u00a0Retrieved August 10, 2006, from\u00a0http:\/\/www.cdc.gov\/ncbddd\/fas\/fasask.htm\r\n\r\nWorld Health Organization.\u00a0(2010, September 15).\u00a0Maternal Deaths Worldwide Drop by a Third, WHO. Retrieved December 22, 2010, from\u00a0http:\/\/www.who.int\/mediacentre\/news\/releases\/2010\/maternal_mortality_20100915\/en\/index.html","rendered":"
    \n

    Learning Objectives<\/h3>\n

    At the end of this lesson, you will be able to<\/p>\n

      \n
    1. Define gene.<\/li>\n
    2. Define chromosome.<\/li>\n
    3. Define gamete.<\/li>\n
    4. Explain what determines the chromosomal sex of the child.<\/li>\n
    5. Question the assertion that human traits are genetic.<\/li>\n
    6. Compare monozygotic and dizygotic twins.<\/li>\n
    7. Differentiate between genetic disorders and chromosomal abnormalities.<\/li>\n
    8. Describe Trisomy 21.<\/li>\n
    9. Differentiate between the germinal, embryonic, and fetal periods of development.<\/li>\n
    10. Describe\u00a0human development during the germinal, embryonic, and fetal periods.<\/li>\n
    11. Describe\u00a0a normal delivery and complications of pregnancy and delivery.<\/li>\n
    12. Predict\u00a0the risks to prenatal development\u00a0posed by exposure to teratogens.<\/li>\n
    13. Interpret\u00a0APGAR scores.<\/li>\n
    14. Discover\u00a0problems of newborns<\/li>\n<\/ol>\n<\/div>\n

      Heredity:\u00a0The Epigenetic Framework<\/h1>\n

      Nature or Nurture?<\/h2>\n

      In this lesson, we will look at some of the ways in which heredity helps to shape the way we are. We will look at what happens genetically during conception and take a brief look some genetic abnormalities. Before going into these topics, however, it is important to emphasize the interplay between heredity and the environment. Why are you the way you are? As you consider some of your features (height, weight, personality, being diabetic, etc.), ask yourself whether these features are a result of heredity or environmental factors-or both. Chances are,\u00a0you can see the ways in which both heredity and environmental factors (such as lifestyle, diet, and so on) have contributed to these features.\u00a0 For decades, scholars have carried on the “nature\/nurture” debate. For any particular feature, those on the “nature” side would argue that heredity plays the most important role in bringing about that feature. Those on the “nurture” side would argue that one’s environment is most significant in shaping the way we are. This debate continues in questions about what makes us masculine or feminine (Lippa, 2002), concerns about vision (Mutti,\u00a0Kadnik\u00a0and Adams, 1996), and many other developmental issues.\u00a0(Check out\u00a0www.googlescholar.com<\/a>\u00a0for over 20,000 entries for \u201ccurrent nature\/nurture debates\u201d!)\u00a0Yet most scholars agree that there is a constant interplay between the two forces. It is difficult to isolate the root of any single behavior as a result solely of nature or nurture and most scholars believe that even determining the extent to which nature or nurture impacts a human feature is difficult to answer. In fact, almost all human features are\u00a0polygenic\u00a0(a result of many genes) and\u00a0multifactorial\u00a0(a result of many factors, both genetic and environmental). It’s as if one’s genetic make-up sets up a range of possibilities, which may or may not be realized depending upon one’s environmental experiences. For instance, a person might be genetically predisposed to develop diabetes, but the person’s lifestyle may help bring about the disease.<\/p>\n

      The Epigenetic\u00a0Framework<\/h2>\n

      Gottlieb (1998, 2000,\u00a02002) suggests an analytic framework for the nature\/nurture debate that recognizes the interplay between the environment, behavior, and genetic expression. This bidirectional interplay suggests that the environment can\u00a0effect\u00a0the expression of genes just as genetic predispositions can impact a person\u2019s potentials.\u00a0And environmental circumstances can trigger symptoms of a genetic disorder.\u00a0 For example, a person who has sickle cell anemia, a recessive gene linked disorder, can experience a sickle cell crisis under conditions of oxygen deprivation.\u00a0 Someone predisposed genetically\u00a0for type two diabetes\u00a0can trigger the disease through poor diet and little exercise.<\/p>\n

      The Human Genome Project<\/h2>\n

      The\u00a0Human Genome Project is an internationally funded effort to map the locations of human genes and understand the role these genes play in development, health and illness.\u00a0(Check out recent developments at\u00a0www.genome.gov<\/a>)\u00a0\u00a0Genes\u00a0are segments of\u00a0chromosomes\u00a0(46 strands of a chemical substance called DNA that are contained in the nucleus of each normal human cell) that vary in length.\u00a0\u00a0 There are an estimated 25,000 to 30,000 genes on each chromosome; a\u00a0number far below the estimate of 100,000-150,000 held before the work of the Human Genome Project.<\/p>\n

      Understanding the role of genes in health and illness can bring about both harm and good (Weitz, 2007).\u00a0A person who knows that they are at risk for developing a genetic disorder may be able to adopt lifestyle practices that minimize the risk and a person who discovers that they are not at risk may find comfort in knowing that they do not have to fear a particular disease.\u00a0However, a person who finds out that they are at risk and there is nothing that can be done about it may experience years of fear and anxiety.\u00a0And the availability of genetic testing may be more widespread than the availability of genetic counseling which can be very expensive.\u00a0\u00a0 The possible stigma and discrimination that those with illness or at risk for illness must also be considered.\u00a0\u00a0 In light of the high costs of health insurance, many companies are starting to offer benefits contingent on health assessments and lifestyle recommendations; and continued coverage depends on an employee following these recommendations.\u00a0So a smoker may have to pay a higher premium than a non-smoker or a person who is overweight may be required to engage in a program of exercise and be monitored for improvement.\u00a0What if a person finds out that they carry the gene for Huntington\u2019s disease (a neurological disorder that is ultimately fatal) which may surface when a person reaches their 40s?\u00a0The impact this knowledge will have on health care still remains unknown.\u00a0 Who should know what is on your genome?\u00a0 Do you think this information should be shared between mates?\u00a0 What about employers?\u00a0 What would be the advantages and disadvantages?<\/p>\n

      Conception<\/h1>\n
      \"female<\/a><\/p>\n

      The Female Reproductive System<\/p>\n<\/div>\n

      Gametes<\/h2>\n

      There are two types of sex cells or gametes involved in reproduction: the male gametes or sperm and female gametes or ova. The male gametes are produced in the testes in a process called\u00a0spermatogenesis\u00a0which begin at about 12 years of age.\u00a0The female gametes or ova which are stored in the ovaries are present at birth but are immature. Each ovary contains about 250,000\u00a0(Rome 1998) but only about 400 of these will become mature eggs (Mackon\u00a0and\u00a0Fauser\u00a02000).\u00a0\u00a0 Beginning at puberty, one ovum ripens and is released about every 28 days, a process called\u00a0oogenesis.<\/p>\n

      After the ovum or egg ripens and is released from the ovary, it is drawn into the fallopian tube and in 3 to 4 days, reaches the uterus. It is typically fertilized in the fallopian tube and continues its journey to the uterus. At ejaculation, millions of sperm are released into the vagina, but only a few reach the egg and typically, only one fertilizes the egg. Once a single sperm has entered the wall of the egg, the wall becomes hard and prevents other sperm from entering. After the sperm has entered the egg, the tail of the sperm breaks off and the head of the sperm, containing the genetic information from the father, unites with the nucleus of the egg. As a result, a new cell is formed. This cell, containing the combined genetic information from both parents, is referred to as a\u00a0zygote.<\/p>\n

      Chromosomes contain genetic information from each parent. While other normal human cells have 46 chromosomes (or 23 pair), gametes contain 23 chromosomes. In a process called\u00a0meiosis,\u00a0segments of the chromosomes from each parent form pairs and genetic segments are exchanged as determined by chance.\u00a0Because of the unpredictability of this exchange the likelihood of having offspring that are genetically identical (and not twins) is one in trillions (Gould and Keeton, 1997).<\/p>\n

      Determining the Sex of the Child<\/h2>\n

      Twenty-two of those chromosomes from each parent are similar in length to a corresponding chromosome from the other parent. However, the remaining chromosome looks like an X or a Y. Half of the male’s sperm contain a Y chromosome and half contain an X. All of the ova contain one X chromosome. If the child receives the combination of XY, the child will be genetically male. If it receives the XX combination, the child will be genetically female.<\/p>\n

      Many potential parents have a clear preference for having a boy or a girl and would like to determine the sex of the child. Through the years, a number of tips have been offered for the potential parents to maximize their chances for having either a son or daughter as they prefer. For example, it has been suggested that sperm which carry a Y chromosome are more fragile than those carrying an X. So, if a couple desires a male child, they can take measures to maximize the chance that the Y sperm reaches the egg. This involves having intercourse 48 hours after ovulation, which helps the Y sperm have a shorter journey to reach the egg, douching to create a more alkaline environment in the vagina, and having the female reach orgasm first so that sperm are not pushed out of the vagina during orgasm. Today, however, there is new technology available that makes it possible to isolate sperm containing either an X or a Y, depending on the preference, and use that sperm to fertilize a mother’s egg.<\/p>\n

      Monozygotic and Dizygotic Twins<\/h2>\n

      Monozygotic twins occur when a single zygote or fertilized egg splits apart in the first two weeks of development. The result is the creation of two separate but genetically identical offspring. About one-third of twins are\u00a0monozygotic\u00a0twins.\u00a0Are you an identical twin?<\/p>\n

      Sometimes, however, two eggs or ova are released and fertilized by two separate sperm. The result is\u00a0dizygotic\u00a0or fraternal twins. About two-thirds of twins are\u00a0dizygotic. These two individuals share the same amount of genetic material as would any two children from the same mother and father.\u00a0 Older mothers are more likely to have\u00a0dizygotic\u00a0twins than are younger mothers and couples who use fertility drugs are also more likely to give birth to\u00a0dizygotic\u00a0twins.\u00a0Consequently, there has been in increase in the number of fraternal twins in recent years (Bortolus et. al., 1999).<\/p>\n

      What are the other possibilities?\u00a0 Various degrees of sharing the placenta can occur depending on the timing of the separation and duplication of cells.\u00a0 This is known as\u00a0placentiation.\u00a0 Here is a diagram that illustrates various types of twins.<\/p>\n

      \"Diagram<\/p>\n

      Author Kevin\u00a0Dufenbach<\/p>\n<\/div>\n

      Genotypes and Phenotypes (or why what you get is not always what you see)<\/h2>\n

      The word\u00a0genotype\u00a0refers to the sum total of all the genes a person inherits. The word\u00a0phenotype\u00a0refers to the features that are actually expressed. Look in the mirror. What do you see, your genotype or your phenotype?\u00a0What determines whether or not genes are expressed? Actually, this is quite complicated (Berger, 2005). Some features follow the\u00a0additive pattern\u00a0which means that many different genes contribute to a final outcome. Height and skin tone are examples. In other cases, a gene might either be turned on or off depending on the gene with which it is paired. Some genes are considered dominant because they will be expressed. Others, termed recessive, are only expressed in the absence of a dominant gene. Some characteristics which were once thought of as\u00a0dominant-recessive, such as eye color, are now believed to be a result of the interaction between several genes (McKusick, 1998). Dominant traits include curly hair, facial dimples, normal vision, and dark hair.\u00a0Recessive characteristics include red hair, pattern baldness, and nearsightedness.\u00a0\u00a0 Sickle cell anemia is a recessive disease; Huntington\u00a0disease is a dominant disease.\u00a0Other traits are a result of\u00a0partial dominance\u00a0or\u00a0co-dominance\u00a0in which both genes are influential. For example, if a person inherits both recessive genes for sickle cell anemia, the disease will occur.\u00a0But if a person has only one recessive gene for the disease, the person may experience effects of the disease only under circumstances of oxygen deprivation such as high altitudes or physical exertion (Berk, 2004).<\/p>\n

      Chromosomal Abnormalities and Genetic Disorders<\/h2>\n

      A\u00a0chromosomal abnormality\u00a0occurs when there a child inherits too many or two few chromosomes.\u00a0\u00a0The most common cause of chromosomal abnormalities is the age of the mother. A 20 year old woman has a 1 in 800 chance of having a child with a common chromosomal abnormality. A woman of 44, however, has a one in\u00a016 chance. It is believed that the problem occurs when the ovum is ripening prior to ovulation each month. As the mother ages, the ovum is more likely to suffer abnormalities at this time.<\/p>\n

      Some gametes do not divide evenly when they are forming. Therefore, some cells have more than 46 chromosomes. In fact, it is believed that close to half of all zygotes have an odd number of chromosomes. Most of these zygotes fail to develop and are spontaneously aborted by the body. If the abnormal number occurs on pair #21 or # 23, however, the individual may have certain physical or other abnormalities.<\/p>\n

      One of the most common chromosomal abnormalities is on pair 21.\u00a0Trisomy 21\u00a0occurs when there are three rather than two chromosomes on #21.\u00a0\u00a0 A person with Down syndrome experiences problems such as mental retardation and certain physical features such as having short fingers and toes, having folds of skin over the eyes, and a protruding tongue.\u00a0Life expectancy of persons with Down syndrome has increased in recent years.\u00a0Keep in mind that there is as much variation in people with Down\u00a0Syndrome\u00a0as in most populations and those differences need to be recognized and appreciated.\u00a0Watch the following\u00a0video clip about Down\u00a0Syndrome\u00a0from the\u00a0National Down Syndrome Society:<\/p>\n