Transcript Genetics and Prenatal Development

Genetics and Prenatal
Development
Child Development
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)
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.
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.
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).
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.
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.
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.
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.
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.
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.
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).
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.
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!
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.
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)
Dominant and Recessive Characteristics

Dominant
 Dark hair
 Normal hair
 Curly hair
 Nonred hair
 Facial dimples
 Normal hearing
 Normal vision
 Normally pigmented skin
 Type A blood
 Type B blood
 Rh-positive blood
Recessive
Blond hair
Pattern baldness
Straight hair
Red hair
No dimples
deafness
myopia
Albinism
Type O blood
Type O blood
Rh-negative blood
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.
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)
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.
Down’s Syndrome results in:

mental retardation, speech difficulties, limited
vocabulary, & slow motor development.

Down’s babies have more problems (breathing,
feeding) than healthy infants.
Predisposing factors

Maternal age (35 +)

Paternal- Marijuana smoking increases
likelihood of Down’s syndrome.
B. Prenatal Diagnosis

Good News!!!

95 % of fetuses examined through prenatal
diagnosis are normal.
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.
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.
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
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.
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
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.
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).
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)
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)
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)
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)
The Second Trimester:


By end of 2nd trimester (6 months) all major
organs are formed.
Baby continues growing,
 Is felt moving by mother.
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)
Third trimester

30 weeks
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
Complications

Failure to progress
 Fetal distress
 Placental abruption
Often results in C-section delivery, where baby
is surgically removed from the mother.