Genetics and Prenatal Development

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Genetics and Prenatal Development

• Child Development

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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)

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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.

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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.

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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).

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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.

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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.

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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.
• Males sperm determines babys sex.

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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.

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Becoming male

• Testes in male fetuses Wolffian system to
  develop Mullerian inhibiting hormone (MIH) is
  released.
• Hormonesmust be present during 3rd and 4th
  months of pregnancy.

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Sex Chromosome anomalies

• Turners Syndrome When a males 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.
• Klinefelters syndrome Occurs when a male child
  has an extra X chromosome (XXY), and displays
  female secondary sex characteristics and some
  cognitive impairments.
• XXY malesappear 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.

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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).

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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.

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What happens if a child is heterozygous?

• e.g., eye color, Bb (B-brown, b-blue)
• One allele will be dominant for a trait, whereas
  the other will be recessive.
• This child will have Brown eyes!

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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.

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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)

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Dominant and Recessive Characteristics

• Dominant Recessive
• 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

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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.

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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 dont
  have any extra alleles to cancel-out effects.
• (E.g, color blindness)

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Chromosomal abnormalities

• Damage to the chromosomes may result in birth
  defects/disease.
• Most commonDowns Syndrome
• Results when an extra chromosome is present on
  the 21st pair.

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Downs Syndrome results in

• mental retardation, speech difficulties, limited
  vocabulary, slow motor development.
• Downs babies have more problems (breathing,
  feeding) than healthy infants.

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Predisposing factors

• Maternal age (35 )
• Paternal- Marijuana smoking increases likelihood
  of Downs syndrome.

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B. Prenatal Diagnosis

• Good News!!!
• 95 of fetuses examined through prenatal
  diagnosis are normal.

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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.

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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.

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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

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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.

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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 Downs
  Syndrome

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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.

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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).

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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)

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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)

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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)

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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)

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The Second Trimester

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

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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)

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Third trimester

• 30 weeks

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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

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Complications

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