Chapter 2 – Genes and Prenatal Development

1. Chapter 2 – Genes and Prenatal Development

2. The Beginning of Life • DNA (deoxyribonucleic acid): • Molecule that contains the chemical instructions for cells to manufacture various proteins • Promotes growth and sustains life • Chromosomes: • Molecules of DNA • DNA consists of 46 chromosomes arranged in 23 pairs

3. The Beginning of Life • Gametes: • Reproductive cells (sperm and ova) • Each gamete consists of 23 chromosomes. • Zygote: • Two gametes (sperm and ovum) combine and produce a new individual with 23 chromosomes from each parent. • Genes: • Specific sections of a chromosome that carry instructions via four chemicals. • Chemicals are organized in four pairs (AT, TA, CG, GC). • Each person has about 3 billion pairs.

4. The Beginning of Life

5. Genes are passed down from generation to generation Genotype: An organism’s genetic inheritance, or genetic potential Unique for each organism Phenotype: The observable characteristics of an organism, including appearance, personality, intelligence, and all other traits. Genetic Variations and Similarities

6. Genetic Variations and Similarities • Many genes are identical for every human being. • Some genes vary slightly in their codes from one person to another. • Allele: • Variation of a gene or any of the possible forms in which a gene for a particular trait can occur. • Effects of variations vary greatly from causing life-threatening conditions to having no detectable effect at all.

7. The Human Genome • Genetic Diversity • Distinguishes each person • Allows the human species to adapt to pressures of the environment (e.g. climate changes, illnesses). • Genome: • The full set of genes that are the instructions to make an individual member of a certain species • Similarity between two people: 99.5% • Similarity between humans and chimpanzees: 98% • Similarity between humans and every other mammal: 90+%

8. Human Genome Project and HapMap • Human Genome Project: • Found “only” about 20,000 genes in humans • Exact number is unknown • HapMap: • International project trying to spot all variations in the human genome • Found 11 million variations among the 3 billion chemical pairs

9. Siblings and Twins • Sibling Similarities • Each sibling gets abut 10,000 genes from each parent. • Millions of variations and billions of combinations are possible. • Full siblings are not necessarily alike. • Monozygotic (identical) twins: • Originate from one zygote that splits apart very early in development. • Incomplete split results in conjoined twins. • Same genotype but slight variations in phenotype are possible due to environmental influences.

10. Siblings and Twins • Dizygotic (fraternal) twins • Result from fertilization of two separate ova by two separate sperm. • Dizygotic twins have half their genes in common and occur twice as often as monozygotic twins. • Incidence is genetic and varies by ethnicity and age.

11. Male and Female • Humans usually possess 46 chromosomes. • 44 autosomes and 2 sex chromosomes • Females: XX • Males: XY • Sex of offspring depends on whether the father’s Y sperm or X sperm fertilizes the ovum.

12. Male and Female

13. Genetic Interactions • Almost every trait is: • polygenic (affected by many genes) • multifactorial (influenced by many factors) • Regulator genes • Direct the interactions of other genes, controlling their genetic expression, duplication, and transcription • Responsible for differences between species

14. Additive Heredity • Additive genes • Genes that add something to some aspect of the phenotype • Effects of additive genes add up to make the phenotype • Example: Height is affected by the contributions of about 100 genes

15. Dominant-Recessive Heredity • Dominant gene is far more influential than the recessive gene (non-additive). • Dominant gene can completely control the phenotype with no noticeable effect of recessive gene. • Genes for blood type B and Rh-positive blood are dominant. • Effect of recessive genes can sometimes be noticed. • Dominant brown eye gene and recessive blue eye gene can result in hazel eyes.

16. Carrier • A person whose genotype includes a gene that is not expressed in the phenotype • Unexpressed gene occurs in half of the carrier’s gametes and is passed on to half of the carrier’s offspring • Offspring can be carrier or express the gene in the phenotype (e.g. when unexpressed gene is inherited by both parents)

17. Heritability • Statistical term that indicates what portion of the variation in a particular trait within a particular population is inherited. • Example: 90% of the height differences among children of the same age is genetic. • Environment can affect the expression of inherited genes.

18. Chromosomal Abnormalities Down Syndrome (Trisomy-21) Three copies of chromosome 21 Specific facial characteristics (thick tongue, round face, slanted eyes) Hearing losses, heart abnormalities, muscle weakness, short stature Slow to develop language Accelerated aging (cataracts, dementia, certain forms of cancer common at age 40) Genetic Problems

19. Genetic Problems • Abnormalities of the Sex Chromosomes • Turner Syndrome (X only): • Unusually short, underdeveloped female organs, infertile • Slow to develop • Problems in spatial understanding • Klinefelter Syndrome (XXY): • Symptoms include learning problems and infertility • Boy’s penis does not grow during puberty and fat accumulates around the breast area • Sometimes not recognized until adulthood

20. Genetic Problems • Gene Disorders • Dominant Disorders • Half of the offspring of parents with a dominant disorder will have the disorder. • Most dominant disorders begin in adulthood (fatal dominant childhood conditions cannot be passed on). • Many dominant disorders have relatively mild or variable symptoms. • Tourette syndrome • Some who inherit the dominant gene exhibit uncontrollable tics and explosive outbursts • Most have milder, barely noticeable symptoms

21. Genetic Problems • Recessive Disorders • Millions of different types, lethal conditions are rare • Sickle-cell trait • Offers some protection against malaria • African carriers are more likely than non-carriers to survive • Cystic fibrosis • More common among people with northern European ancestors • Carriers may have been protected against cholera

22. Genetic Problems

23. Sex-Linked Conditions All X-linked conditions are more common in boys; girls are likely to be protected by their second X • Fragile X Syndrome • Most common form of inherited mental retardation • Additional symptoms include muscle weakness, shyness, and poor social skills • Hemophilia • Blood clotting is inhibited • Blood transfusions can save children’s lives • Color blindness • Most common sex-linked condition • Often passed on because it is not debilitating

24. Sex-Linked Conditions

25. Advising Prospective Parents • Genetic Counseling • Consultation and testing by trained professionals • Enables prospective parents to learn about their genetic heritage, including harmful conditions that may be passed on to their offspring • Individuals with a parent, sibling, or child with a serious genetic condition known to be dominant or recessive • Couples with history of early spontaneous abortions, stillbirths, or infertility • Couples from the same ethnic group or subgroup—especially if closely related • Women over 35 and men over 40

26. Advising Prospective Parents • Ethical Guidelines • Test results are kept confidential • Decisions regarding sterilization, adoption, abortion, or carrying a pregnancy to term are made by the clients • Counselor constructs couples’ family history • charts patterns of health and illness over generations • Some tests provide information before conception; Other tests are prenatal • Nuchal fold screening

27. Advising Prospective Parents

28. Potential genetic counseling issues Prospective parents must base some decisions on odds Tests often reveal that more tests are needed False positives and false negatives True positives may cause additional stress Presenting the Facts

29. Alternatives If both partners are carriers of a serious condition or are at high risk because of age or family characteristics, they may turn to in-vitro fertilization (IVF) gamete intrafallopian transfer (GIFT) zygote intrafallopian transfer (ZIF) artificial insemination donor (AID) postponement of pregnancy until promising treatments are further developed

30. PKU: A Success Story Phenylketonuria (PKU) • Recessive condition • Results in inability to metabolize phenylalanine (amino acid found in many foods) • Buildup of phenylalanine causes brain damage, progressive mental retardation, and other symptoms • Early testing and a special diet usually results in normal development

31. From Zygote to Newborn Three main periods of prenatal development • Germinal Period (first two weeks after conception): rapid cell division and beginning of cell differentiation • Embryonic Period (3rd through 8th week): basic forms of all body structures develop • Fetal Period (9th week until birth): fetus grows in size and matures in functioning

32. The Germinal Period • Zygote begins duplication and division within hours of conception (when does that happen?) • Development of the placenta • Organ that surrounds the developing embryo and is attached to the wall of the uterus • Sustains life via the umbilical chord (nourishes organism and carries waste products away) • Implantation (about 10 days after conception) • Developing organism burrows into the placenta that lines the uterus • Organism grows rapidly • Pregnancy can be detected due to new chemicals in the mother’s urine

33. The Germinal Period

34. The Germinal Period

35. The Embryonic Period Embryo • Developing human organism from about the 3rd through the 8th week after conception • Begins when the primitive streak appears down the middle of the cell mass • Primitive streak becomes the neural tube and later forms the brain and spine of the CNS • Head takes shape • Eyes, ears, nose, and mouth form • Heart begins to pulsate • Extremities develop and webbed fingers and toes separate

36. The Embryonic Period

37. Fetus Developing human organism from the start of the 9th week after conception until birth Genitals form and sex hormones cause differences in fetal brain organization Cephalocaudal and proximodistal growth Heartbeat detectable via stethoscope All body parts and systems complete prenatal development Cortex is not fully mature at birth (e.g. prefrontal lobes) Brain at birth is far bigger than any other part of the baby The Fetal Period

38. Age of viability Age at which a preterm newborn may survive outside the mother’s uterus if medical care is available About 22 weeks after conception Brain is able to regulate basic body functions (e.g. breathing) Chances of survival increase with each day after the 22-week mark The Fetal Period

39. The Fetal Period

40. Birth • Fetal brain signals the release of hormones to trigger the female’s uterine muscles • Labor begins • Average duration for firstborn babies: 12 hours • Quicker labor for later-born babies • Apgar scale: Appearance (color), Pulse (HR), Grimace (reflex), Activity (muscle), Respiration (breathing) • Quick assessment of newborn’s heart rate, breathing, muscle tone, color, and reflexes • Completed twice (1 minute and 5 minutes after birth) • Score of 0, 1, or 2 in each category • Desired score: 7 or above

41. Birth

42. Traditional and Modern Birthing Practices • Home births • Hospital births • Doula • Woman who helps with labor, delivery, breast-feeding, and newborn care • May be related to lower rate of cesarean sections

43. Cesarean Section (C-Section) • Surgical birth • Fetus can be removed quickly • Rates and reasons for c-sections vary greatly • Lower rates in poorer countries (emergencies) • Higher rates in richer countries (planned for the convenience of mother and/or obstetrician) • 1/3 of births in the United States • Less trauma for the newborn but slower recovery for the mother • Subsequent cesarean deliveries may be necessary

45. Medical Intervention • Infant mortality has decreased due to better medical care • 1900: 5% • Today: <1 in 200 • Childbirth has become safer for mothers • Death rate in poorest nations: 1 in 20 women • Excessive medical care also has disadvantages • Increase in unnecessary c-sections is associated with higher rate of low-birth weight babies

46. Low Birthweight • Low birthweight (LBW) • Less than 2,500 grams (5½ pounds) at birth • United States • Steady increase in LBW over the past 25 years • 8% of newborns are seriously underweight • More susceptible to teratogens, higher birth risks, lower survival rate • Very low birthweight (VLBW) • Under 1,500 grams (3 pounds, 5 ounces) at birth • Extremely low birthweight • Under 1,000 grams (2 pounds, 3 ounces) at birth

48. Preterm or Slow Growing? • Preterm • Birth that occurs at 35 or fewer weeks after conception • Usually associated with low birthweight • Small for gestational age (SGA) • Birthweight is significantly lower than expected, given the time since conception • Suggests impairment throughout prenatal development and serious problems

49. Genetic factors Maternal illness Exhaustion Infection Malnutrition Drug use Multiple births Causes of Low Birthweight

50. Mothers, Fathers, and a Good Start The Father’s Role • Supportive father helps mother stay healthy • Father can decrease or increase mother’s stress (affects fetus) • Most fathers are helpful to their pregnant wives • Two way street: Pregnant mothers should support, involve, and encourage fathers