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CHAPTER 2

CHAPTER 2. Heredity and Prenatal Development. The Influence of Heredity on Development. Chromosomes and Genes. Genetics - the field of biology that studies heredity Fundamental in the transmission of physical traits Also plays a role in psychological traits

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CHAPTER 2

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  1. CHAPTER 2 Heredity and Prenatal Development

  2. The Influence of Heredity on Development

  3. Chromosomes and Genes • Genetics - the field of biology that studies heredity • Fundamental in the transmission of physical traits • Also plays a role in psychological traits • Chromosomes – Rod-shaped structures composed of genes that are found within the nuclei of cells • Gene – The basic unit of heredity. Composed of DNA • Polygenic – Resulting from many genes • Deoxyribonucleic Acid (DNA) – Genetic material that takes the form of a double helix composed of phosphates, sugars and bases.

  4. Mitosis • Life begins as a single cell or zygote that divides repeatedly. • Mitosis • Genetic code carried into new cells in our bodies • DNA breaks apart; double helix duplicates • DNA forms two camps on either side of the cell, and then the cell divides

  5. Fig. 2-1, p. 24

  6. Fig. 2-2, p. 25

  7. Meiosis • Sperm and ova are produced through meiosis or reduction division. • 46 chromosomes within the cell nucleus first line up into 23 pairs. • DNA ladders unzip, leaving unpaired halves of chromosome; when cell divides, one member of each pair goes to each newly formed cell. • Each new cell nucleus contains only 23 chromosomes, not 46.

  8. Fig. 2-3, p. 25

  9. Identical and Fraternal Twins • Monozygotic (MZ) - Zygote divides into two cells that separate so that each develops into an individual with the same genetic makeup. • Dizygotic (DZ) - Two ova are produced in the same month, each fertilized by a different sperm cell. • Ovulation - As women reach end of childbearing years, ovulation becomes less regular. -Results in months when more than one ovum is released

  10. Traits determined by pairs of genes each member of pairs of genes termed an allele Homozygous having two identical alleles Heterozygous having two different alleles Incomplete dominance/codominances when both alleles are shown Dominant trait trait that is expressed Recessive trait trait that is not expressed when genes have been paired with dominant genes Carriers people who bear one dominant gene and one recessive trait Dominant and Recessive Traits

  11. Fig. 2-4, p. 26

  12. Table 2-1, p. 27

  13. Chromosomal and Genetic Abnormalities • Down syndrome • Caused by an extra chromosome of the 21st pair, resulting in 47 chromosomes • Characteristic features -rounded face -protruding tongue -broad, flat nose -sloping fold of skin over the inner corners of the eyes • Show deficits in cognitive development and motor development

  14. Fig. 2-5, p. 28

  15. MALES 1 male in 700-1,000 has XYY leads to heightened male secondary sex characteristics most individuals with disorder are infertile 1 male in 500 has Klinefelter syndrome (XXY) leads to inadequate development of male primary and secondary sex characteristics FEMALES 1 female in 2,500 has a single X resulting in Turner Syndrome poorly developed ovaries, short, and infertile. 1 female in 1,000 has XXX sex chromosomal structure called Triple X syndrome normal in appearance but tend to show lower-than-average language skills and poorer memory for recent events Sex-Linked Chromosomal Abnormalities

  16. Klinefelter Syndrome (XXY) • 1 male in 500 • Caused by extra X on sex chromosome • Produces less testosterone than normal males • Characteristic features of testes, deepening of voice, musculature, and male pattern of body hair do not develop properly • Typically treated with testosterone replacement therapy

  17. Turner Syndrome (X) • 1 female in 2,500 • Single X chromosome • External genitals are normal, ovaries poorly developed, producing little estrogen • Shorter than average and infertile • Cognitive deficits with low estrogen: problems with visual-spatial skills, mathematics, and nonverbal memory

  18. Triple X Syndrome (XXX) • 1 female in 1,000 • Appears normal, but demonstrates lower-than-average language skills • Poorer memory for recent events • Development of external sex organs appears normal • Increased incidence of infertility

  19. Phenylketonuria (PKU) • Enzyme disorder transmitted by a recessive gene affecting 1 child in 8,000 • Cannot metabolize an amino acid called phenylalanine; builds up in body and impairs functioning of the central nervous system (CNS) • Results are mental retardation, psychological disorders, physical problems • No cure, but children can be placed on diets low in phenylalanine and develop normally

  20. Huntington’s Disease (HD) • Affects 1 in 18,000 Americans • Fatal, progressive degenerative disorder • Dominant trait • Physical symptoms include uncontrollable muscle movements • Psychological symptoms include loss of intellectual functioning and personality change • Onset of HD is delayed until middle adulthood • Possibly half their offspring will develop it • Medicines can deal with some symptoms

  21. Sickle-Cell Anemia • Carriers -1 in 10 African Americans -1 in 20 Latino or Latina Americans • Caused by a recessive gene • Red blood cells take on the shape of a sickle and clump together, obstructing small blood vessels and decreasing the oxygen supply • Lessened oxygen can impair cognitive skills • Physical problems -painful and swollen joints -jaundice -potentially fatal conditions such as pneumonia, stroke, and heart and kidney failure

  22. Tay-Sachs Disease • 1 in 30 Jewish Americans are carriers • Caused by recessive gene • Causes the central nervous system to degenerate, resulting in death • Commonly found among children in Jewish families of Eastern European background

  23. Cystic Fibrosis • Approx. 30,000 Americans have disorder, 10 million more are carriers (1 in 31 people) • Caused by recessive gene • Most common fatal hereditary disease among Caucasians • Children suffer from excessive production of thick mucus that clogs the pancreas and lungs • Most victims die of respiratory infections in their 20s

  24. Sex-Linked Genetic Abnormalities • Genetic defects only carried on the X sex chromosome • Hemophilia – genetic disorder in which blood does not clot properly • Duchenne – muscular dystrophy; sex linked • Involve recessive genes • Females with two X sex chromosomes are less likely than males to show sex-linked disorder • Sons of female carriers are more likely to be afflicted

  25. Genetic Counseling and Prenatal Testing • Genetic counselors compile information about a couple’s genetic heritage to explore if their children will have a genetic abnormality. • Couples with likelihood of passing on genetic abnormality tend to adopt or not have children of their own. • Prenatal testing can indicate if the embryo or fetus is carrying genetic abnormalities.

  26. Amniocentesis • Performed on mother 14-16 weeks after conception • Syringe withdraws fluid from the amniotic sac; contains cells sloughed off by fetus • Cells separated, grown in culture, and examined for genetic and chromosomal abnormalities • Routine for women over 35 to detect for Down syndrome; other chromosomal abnormalities increase dramatically as women approach 40 • Amniocentesis carries some risk of miscarriage.

  27. Fig. 2-6, p. 30

  28. Chorionic Villus Sampling • Carried out between 9th and 12th week of pregnancy • Syringe inserted through vagina into uterus to suck out threadlike projections (villi) from the outer membrane that covers the amniotic sac and fetus • Results available in days • CVS slightly greater risk than amniocentesis of miscarriage; both increase the risk of miscarriage

  29. Ultrasound • Sound waves that are too high in frequency to be heard by human ear obtain information about the fetus • Ultrasound waves are reflected by the fetus; computer uses the information to generate a picture of the fetus • Picture is termed a sonogram • Used to guide the syringe in amniocentesis and CVS by determining the position of the fetus • Used to track growth of fetus, detect multiple pregnancies, detect structural abnormalities

  30. Blood Tests • Used to identify sickle-cell anemia, Tay-Sachs disease, and cystic fibrosis • Alpha-fetoprotein (AFP) used to detect neural tube defects such as spina bifida and chromosomal abnormalities • Neural tube defects cause elevation in the AFP level in the mother’s blood • Elevated AFP levels related to increased risk of fetal death

  31. Heredity and the Environment

  32. Heredity and Environment • Inheritance, nutrition, learning, exercise, accident, and illness contribute to development of traits • Genotypes -Set of traits we inherit from our parents • Phenotypes -Actual set of traits -Both genetic and environmental influences

  33. Kinship Studies • The more closely people are related, more genes they have in common • Parents and children have 50% genetic overlap • Siblings have 50% genetic overlap • Aunts, uncles have 25% overlap with nieces and nephews; grandparents, 25% overlap with grandchildren • First cousins have 12.5% overlap

  34. Twin Studies • Monozygotic (MZ) twins share 100% of genes -MZ twins resemble each other more closely than DZ twins on a number of physical and psychological traits. -MZ twins more likely to look alike and be similar in height -MZ twins more likely to share autism, depression, schizophrenia, and vulnerability to alcoholism • Dizygotic (DZ) twins share 50% of genes; same as other siblings

  35. Adoption Studies • Children that are separated from their natural parents at an early age and reared by adoptive parents provide special opportunities for sorting out nature and nurture. • When children who are reared by adoptive parents are nonetheless more similar to their natural parents in a trait, a powerful argument is made for a genetic role in the appearance of that trait.

  36. Conception: Against All Odds

  37. Conception • Conception refers to the union of an ovum and a sperm cell. • Ova • Women born with all the ova they will ever have, about 400,000 • Released from ovarian follicle and enter fallopian tube; 3-4 days later, egg propelled by small, hairlike structures called cilia, and perhaps, by contractions in the wall of the tube • If egg not fertilized, discharged through the uterus and vagina, along with endometrium that had formed to support an embryo, in the menstrual flow; during reproductive years, about 400 ova will ripen and be released

  38. Conception (cont’d) • Ova are larger than sperm, barely visible to the eye • Sperm cells develop through stages; sperm with Y sex chromosomes swim faster than sperm with X sex chromosomes. • Male fetuses suffer a higher rate of miscarriage than females, often during the first month of pregnancy. • 150 million sperm ejaculated, only 1 in 1,000 can approach an ovum

  39. Conception (cont’d) • Sperm have to fight vaginal acidity, gravity, and swim against current fluid coming from the cervix. • If sperm survive, reach fallopian tubes 60-90 minutes after ejaculation; approximately 2,000 enter the correct tube • Sperm are “egged on”by a change in calcium ions that occurs when an ovum is released. • Only one enters; have to thin the gelatinous layer of egg; secrete an enzyme that briefly thins layer • Once sperm enters, layer thickens and locks out other sperm

  40. Conception (cont’d) • Chromosomes from the sperm cell line up across from corresponding chromosomes in the egg cell. • Form 23 new pairs with unique set of genetic instructions • The direction that sperm travel is guided by a change in calcium ions that occurs when an ovum is released.

  41. Fig. 2-8, p. 33

  42. Fig. 2-9, p. 34

  43. Infertility • Infertility problems with men - Low sperm count (most common) - Lack of sperm - Genetic factors - Environmental poisons - Diabetes - Sexually transmitted infections - Overheating of the testes - Pressure (e.g., bike seats) - Aging - Certain prescription and illicit drugs

  44. Infertility (cont’d) • Infertility problems with women - Irregular ovulation, lack of ovulation - Irregularities among the hormones that govern ovulation, stress, and malnutrition - Pelvic inflammatory disorder (PID) -infection that scars the fallopian tubes and other organs, impeding the passage of sperm -Endometriosis -can obstruct the fallopian tubes

  45. Infertility Options • Artificial insemination -Sperm is collected and quick-frozen -Sperm then injected into woman’s uterus at time of ovulation; addresses low sperm count as well as low sperm motility -Can be used for a woman to get pregnant who does not have a partner • In vitro fertilization -Ripened ova are surgically removed from the mother and placed in laboratory dish; father’s sperm also placed in dish -One or more ova fertilized and injected into mother’s uterus to become implanted -Used when fallopian tubes are blocked or father has low sperm motility -A donor can be used

  46. Infertility Options (cont’d) • Donor IVF -Mother does not produce ova -Ovum from another woman is fertilized and injected into the uterus of the mother-to-be • Surrogate mothers -Mothers who bring baby to term for other women who are infertile -Can be artificially inseminated by partner of infertile woman • Adoption -Another way for people to obtain children that results in the formation of loving new families

  47. Preimplantation Genetic Diagnosis (PGD) • Reliable method for selecting the sex of the child prior to implantation • Ova are fertilized in vitro • After a few days of cell division, cell is extracted from each • Sex chromosomal structure of the cell is examined microscopically to determine its sex • Embryos of desired sex are implanted in the woman’s uterus

  48. Prenatal Development

  49. Prenatal Development • Normal gestation period 280 days; from date of fertilization 266 days • Divided into three periods 1. Germinal stage (Weeks 0 - 2) 2. Embryonic stage (Weeks 3 - 8) 3. Fetal stage (Weeks 9 - Birth)

  50. The Germinal Stage • The period from conception to implantation • Blastocyst -Cells separate into groups that will become different structures -Inner part of blastocyst has two distinct layers that form a thickened mass of cells called embryonic disk -These cells eventually become the fetus • Trophoblast • Four membranes that will protect and nourish the embryo 1. One produces blood cells, then it disappears 2. One develops into umbilical cord and blood vessels of the placenta 3. One develops into the amniotic sac 4. One becomes the chorion, which will line the placenta

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