Genetics and Prenatal Development Child Development

Genetics and Prenatal Genetics and Prenatal DevelopmentDevelopment

Child Development

A. GeneticsA. GeneticsWe 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):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?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)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 offspringMultiple 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?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?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?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 maleBecoming 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 anomaliesSex 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 inheritanceGenetic 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).

AllelesAllelesIf 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 What happens if a child is heterozygous?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 Possible Outcomes with heterozygous Allele patterns: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 genesCarriers 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 CharacteristicsDominant 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

PKU-a recessive diseasePKU-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 What if a harmful recessive gene occurs on the X-chromosome?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 abnormalitiesChromosomal 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 SyndromeDown’s Syndrome rresults in:esults in:

mental retardation, speech difficulties, limited vocabulary, & slow motor development.

Down’s babies have more problems (breathing, feeding) than healthy infants.

Predisposing factorsPredisposing factors

Maternal age (35 +)

Paternal- Marijuana smoking increases likelihood of Down’s syndrome.

B. Prenatal DiagnosisB. Prenatal Diagnosis Good News!!!

95 % of fetuses examined through prenatal diagnosis are normal.

Prenatal Diagnostic TestsPrenatal 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. 2. Chorionic Villi samplingChorionic 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. 3. FetoscopyFetoscopy 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. 4. UltrasoundUltrasoundHigh-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. 5. Maternal Blood AnalysisMaternal 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 DevelopmentC. Prenatal DevelopmentFemale 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 Period of zygote: from fertilization to Implantation (two week period).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 embryoPeriod of the embryoImplantation -8th week of pregnancy.

Period marks most rapid prenatal changes (organ development).

Ectoderm-NS & skinMesoderm-muscle, skeletalEndoderm-digestive tract, (6 weeks)

First MonthFirst 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 MonthThe 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 MonthThird MonthFetus 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: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.)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 trimesterThird trimester

30 weeks

D. Childbirth: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

ComplicationsComplicationsFailure to progressFetal distressPlacental abruption

Often results in C-section delivery, where baby is surgically removed from the mother.