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Prenatal Development: Chromosomal and Hormonal Difficulties

Genetic Foundations

  • Chromosomes (23 pairs)
    • Rod-shaped structures that contain all basic hereditary information
    • Genes—smaller units through which genetic info is transmitted.
      • Composed of sequences of DNA
    • XX = female      XY = male


Multiple Births

  • Dizygotic twins (fraternal)—results from the release and fertilization of two ova
    • As genetically alike as ordinary siblings
    • Most common multiple birth type
    • Increased in industrialized nations because of:
      • Maternal age & fertility treatments
  • Monozygotic twins (identical)—results from a duplicating zygote separates into 2 clusters of cells
    • Same genetic makeup
    • 1 in every 330 births worldwide (Hall, 2003)
    • Likelihood increased by environmental and biological factors
      • Temperature changes & delayed fertilization


Best Age of Parenthood

  • Chromosomal disorders increase with age
    • Women’s ideal age for pregnancy is in their 20s
      • Down Syndrome risk significantly increases past 35
  • Male reproductive capacity
    • Amount of sperm and concentration per ejaculation gradually declines after 35
    • Conception achievement takes longer
  • Higher rate of problems for infants born to teens
    • Not directly linked to maternal age
      • Social, emotional, economic factors.


Conception to Birth

  • Germinal Period (first two weeks)
    • Zygote—new cell formed by the union of an egg and sperm
      • Develops 30 hours after conception
      • 60-70 cells by 4th day
    • Delicate Period
      • 30% of zygotes do not survive
    • Complex organism at end of period
  • Embryonic Period (weeks 2-8)
    • Embryo—developed zygote that has a heart, brain, and other organs.
      • Brain development begins at 3.5 weeks
      • Neuron development occurs at 250,000 per min
      • Discernable arms, legs, and face by week 8


  • Fetal Period
    • Fetus—a developing individual from week 8 until birth
      • External genitalia is well formed by week 12
      • Begins to be responsive to light and sound by week 20
      • Most of brain’s billions of neurons are in place
  • Age of viability—point at which it can survive if born prematurely
    • Between 22 and 26 weeks
  • Sensitive periods (critical periods)
    • Time when organisms are susceptible to certain kinds of stimuli
  • Preterm infants
    • Born before week 38



Common Genetic and Chromosomal Development

  • Phenylketonuria (PKU)–
    • Poisons lead to profound mental retardation.
    • Treatable if caught early
  • Sickle-cell anemia
    • 10 % African-Americans has possibility of passing it down.
    • Episodes of pain, yellowish eyes, vision problems, heart problems, etc.
    • Can lead to premature death in middle age
  • Tay-Sachs disease
    • Most often found in Jews of Eastern European ancestry
    • Usually die by age 3 or 4
    • Body is unable to break down fat
  • Down Syndrome
    • One of the causes of mental retardation
    • Occurs when zygote receives an extra chromosome at conception.
    • Higher risk when mother is younger than 18 or older than 35 years old.


Chromosomal Disorders

  • Over 70 sex chromosome abnormalities
  • Extra or missing sex chromosomes
  • 3 most common:
    • Klinefelter’s Syndrome
    • Turner’s Syndrome
    • XYY Syndrome and Triple X Syndrome


Klinefelter’s Syndrome

  • XXY—egg contained an extra X (47 chromosomes)
    • Y chromosome develops male genitalia, but not fully due to extra X chromosome
  • Tall, feminized body (small testes, breasts may develop)
  • Low testosterone levels
  • Abnormal male breast development
  • Infertile
  • Testosterone therapy can enhance secondary sexual characteristics
  • Often undiagnosed
    • 2/3 of men are never diagnosed


Turner Syndrome

  • XO—egg has no sex chromosome (45 chromosomes)
  • Prevalence rate: 1/2500 live female births
  • Ovaries aren’t fully developed
    • Amenorrhea—absence of menstruation
    • Infertile
  • Short stature
  • Immature breast development (widely spaced nipples)
  • Mental retardation
  • Estrogen and progesterone therapy can enhance secondary sexual characteristics
  • Health problems in adulthood
    • Hypertension, bone thinning, and/or thyroid problems


Hormonal Disorders

  • Congenital Adrenal Hyperplasia (CAH)
  • Androgen-Insensitivity Syndrome (AIS)


Congenital Adrenal Hyperplasia (CAH)

  • XX girl exposed to excess androgen from adrenal glands prenatally
  • Prevalence rate: 1/15,000 girls
  • Female internal organs; sometimes masculinized external genitalia
  • Similar syndrome develops from mother taking male hormones
  • Corrective surgery, drugs to control adrenal output
  • Pregnancy possible in many CAH females
  • Higher rates of bisexuality and homosexuality


Androgen-Insensitivity Syndrome (AIS)

  • XY—the body doesn’t respond to testosterone that is produced by the testes
  • Prevalence rate: 1/20,000 boys per year
  • No internal reproductive structure except two undescended testes
  • Shallow “vagina”
  • Breasts develop
  • Do not menstruate; infertile
  • Surgery can lengthen vagina
  • Genetically male, but fully feminized as female



Genetic Counseling

  • Genetic Counseling—is a communication process designed to help couples assess their chances of giving birth to a baby with a hereditary disorder and choose the best course of action in view of risks and family goals
    • Typically done through blood tests
    • Common candidates
      • Repeated miscarriages
      • Post 35 years of age
  • Gene Therapy—correcting genetic abnormalities by delivering DNA carrying a functional gene to the cells.
    • Immune system dysfunction, asthma, etc.




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