Genetics and Prenatal Development

1. Genetics and Prenatal Development Child Development

2. A. Genetics • We each have 46 chromosomes (23 pairs from each parent). • Chromosomes-threadlike structures--carry genetic information that directs development. • Chromosomes made up of DNA molecules (double helix)

3. What is DNA (deoxyribonucleic acid): • A long, double-stranded molecule that looks like a twisted ladder. • Each of the latter consists of a specific pair of chemical substances called bases, joined together between the two sides. • Humans have 4 base pairs.

4. What is a gene? • A segment of DNA along the length of the chromosome. • DNA can replicate itself, leading to the development of a human being from 1 cell. • This process is mitosis.

5. Sex Cells (Gametes) • Gametes- sperm and ova • Contain only 23 Chromosomes. Formed through meiosis-which halves the # of chromosomes present in body. • In males- sperm produced throughout life, in a female-she is born will all ova she will have (350-450 during maturity).

6. Multiple offspring • Monozygotic twins- a fertilized egg (zygote) separates into two distinct cell clusters that form into two genetically identical humans. • Odds (3 out of every 1,000 births). • Dizyogotic twins- two separate ova are fertilized by two different sperm cells.

7. Is it a boy or girl? • 22 of our 23 chromosome pairs can be distinguished from one another. These are called autosomes. The 23rd pair consists of sex chromosomes. • Females-XX and males-XY. The X is long, the Y short and carries less genetic material.

8. Who determines the sex? • Males- the X and Y-chromosomes separate into different sperm cells. • X- sperm cells; Y-sperm cells. • Females-gametes carry X chromosomes only. • Male’s sperm determines baby’s sex.

9. What is the default? • To be female!!! • A genetic male will only develop into a male infant if testosterone is presented during prenatal development. • Deficient male hormones will lead to development of female infant.

10. Becoming male • Testes in male fetuses: Wolffian system to develop & Mullerian inhibiting hormone (MIH) is released. • Hormones—must be present during 3rd and 4th months of pregnancy.

11. Sex Chromosome anomalies • Turner’s Syndrome: When a male’s sperm fails to have an X or Y sex chromosome, the child is an XO. She only has one X from her mother. These children will be short, have webbed necks, mouth/facial anomalies, and cognitive impairments. • Klinefelter’s syndrome: Occurs when a male child has an extra X chromosome (XXY), and displays female secondary sex characteristics and some cognitive impairments. • XXY males—appear to be significantly taller than normal males and may have cognitive impairments. • Fragile X- occurs in male children only in which the X is fragmented or broken. Leads to facial anomalies and mental retardation which gets progressively worse with age.

12. Genetic inheritance • 2 or more forms of each gene occur at the same place on the chromosomes. • Each different form of a gene is called an allele (1-mother, 1-father).

13. Alleles • If alleles from both parents are alike the child will be homozygous (AA, aa) for that characteristic. • If alleles are different, the child will be heterozygous (Aa, aA) for that characteristic. • Here, relationships between alleles determine if trait will appear.

14. What happens if a child is heterozygous? e.g., eye color, Bb (B-brown, b-blue) • Oneallele will be dominant for a trait, whereas the other will be recessive. • This child will have Brown eyes!

15. Possible Outcomes with heterozygous Allele patterns: • 1. Dominant allele will be expressed, while the recessive allele will not (e.g., brown eyes winning out over blue.) • 2. The trait expressed may be in between the dominant and recessive alleles (a dominant dark skin allele and recessive light skin allele, may yield a child with skin color in between the two). • 3. Both alleles may be expressed simultaneously at full intensity (called codominance). A child with an allele for A blood and an allele for B blood, may have both AB antigens expressed in their blood.

16. Carriers of recessive genes Heterozygous individuals with just one recessive allele (Bb) can pass that trait to their children. These are carriers (blue eyes, blond hair, cystic fibrosis, PKU)

17. Dominant and Recessive Characteristics • DominantRecessive • Dark hair Blond hair • Normal hair Pattern baldness • Curly hair Straight hair • Nonred hair Red hair • Facial dimples No dimples • Normal hearing deafness • Normal vision myopia • Normally pigmented skin Albinism • Type A blood Type O blood • Type B blood Type O blood • Rh-positive blood Rh-negative blood

18. PKU-a recessive disease • Phenylketonuria – lack an enzyme that converts one of the basic amino acids that make up proteins (phenylalanine). • phenylalanine quickly builds to toxic levels in brain • Will lead to mental retardation, but if caught early can be treated with diet restrictions.

19. What if a harmful recessive gene occurs on the X-chromosome? • Females may have an extra allele that will cancel-out the effects of the harmful allele. • Males only have 1 X on 23rd pair, so they don’t have any extra alleles to cancel-out effects. • (E.g, color blindness)

20. Chromosomal abnormalities • Damage to the chromosomes may result in birth defects/disease. • Most common—Down’s Syndrome • Results when an extra chromosome is present on the 21st pair.

21. Down’s Syndromeresults in: • mental retardation, speech difficulties, limited vocabulary, & slow motor development. • Down’s babies have more problems (breathing, feeding) than healthy infants.

22. Predisposing factors • Maternal age (35 +) • Paternal- Marijuana smoking increases likelihood of Down’s syndrome.

23. B. Prenatal Diagnosis • Good News!!! • 95 % of fetuses examined through prenatal diagnosis are normal.

24. Prenatal Diagnostic Tests • 1. Amniocentesis- A hollow needed is inserted through the abdominal wall to obtain a sample of fluid in the uterus. • May be performed 11-14 • wks following conception. • 1-2 weeks for results.

25. 2. Chorionic Villi sampling • A hollow probe is inserted through the vagina. Sample collects chorionic villi, hairlike projections surrounding organism. • Performed (6 to 8 weeks following conception), results known within 24 hours.

27. 3. Fetoscopy • A small tube with light source at one end is inserted into womb to inspect fetus for defects of the limbs & face. Blood may be taken. Diagnoses- hemophilia, sickle-cell anemia, & neural tube defects. • Performed between 15 & 18 weeks post conception

28. 4. Ultrasound • High-frequency sound waves beamed at the uterus & their reflection is recorded. • Provides picture of fetus. • Detects fetal age, multiple pregnancies, & identification of gross physical defects.

29. 5. Maternal Blood Analysis • A blood test done at 2nd month of pregnancy. • Looks for elevated levels of alpha-fetoprotein --may detect neural tube defects & Down’s Syndrome

30. C. Prenatal Development • Female releases a mature egg (ovum) once a month. The egg travels from the ovaries to the fallopian tubes where it awaits a sperm cell to fertilize it. • When sperm meets egg, the egg is fertilized and travels down to the uterus.

31. Period of zygote: from fertilization to Implantation (two week period). • 7th -9th day post conception. • Zygote becomes a blastocyst, a hollow, fluid-filled ball. • Cells inside, form embryonic disk (will become baby).

32. Period of the embryo • Implantation -8th week of pregnancy. • Period marks most rapid prenatal changes (organ development). Ectoderm-NS & skin • Mesoderm-muscle, skeletal • Endoderm-digestive tract, • (6 weeks)

33. First Month • Nervous system develops first -- neural tube or primitive spinal cord. • At 3-4 weeks (brain & other organs form • The Heart pumps blood!!! • (4weeks)

34. The Second Month • Eyes, ears, nose, jaw, & neck form. • Tiny buds become arms, legs, fingers, • & toes. • Organs become more distinct • (heart) • Is 1 inch long, can move. • (8 weeks)

35. Third Month • Fetus can kick, bend its arms, • forms a fist, curls its toes, & • opens it mouth. • By the 12th week, the • external genitals are • well formed. • (12 weeks)

36. The Second Trimester: • By end of 2nd trimester (6 months) all major organs are formed. • Baby continues growing, • Is felt moving by mother.

37. Third Trimester (6-9 mos.) • Babies born during this time have a chance of survival (24 weeks-50% survival rate). • Fetus gains weight (about 5 lbs.) • And continues growing. • Baby prepares for birth. • (24 weeks)

38. Third trimester • 30 weeks

39. D. Childbirth: Stage 1: Dilation and effacement of the cervix (avg 12-14 hrs.) • Stage 2: Delivery of the baby. • Stage 3: Birth of the placenta

40. Complications • Failure to progress • Fetal distress • Placental abruption Often results in C-section delivery, where baby is surgically removed from the mother.