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.