Genetics and Prenatal

Screening

Genetics

• Each human cell contains 46 chromosomes made up of DNA.

• Short segments of DNA are called “genes”

• Dominant and Recessive Genes– When one is “dominant” then that

expression “trumps” the recessive gene.• Carriers?• Table 3.2- Dark Hair/Blond, Curly/Straight,

Dimples/no dimples, double-jointed/normal joints

Chromosome and Gene-linked Abnormalities

• Chromosome Abnormalities– Down syndrome- trisomy 21

• round face, flattened skull, extra skin over the eyelids, protruding tongue, short limbs, and motor and mental retardation.

• 1/1900 @ 20 years; 1/45 @ 42 years

Chromosome and Gene-linked Abnormalities

• Chromosome Abnormalities– Down syndrome- trisomy 21

• round face, flattened skull, extra skin over the eyelids, protruding tongue, short limbs, and motor and mental retardation.

• 1/1900 @ 20 years; 1/45 @ 42 years

– Fragile X- Problem with the X-chromosome: often breaks

• typically mental retardation• more frequent in males. Why?• 1/250 females are carriers vs. 1/800 males

Chromosome and Gene-linked Abnormalities• Genetic Abnormalities

– PKU- Phenylketonuria- Cannot properly metabolize the substance (phenylalanine).

• mental retardation and hyperactivity if untreated.• “Curable” -Nature/nurture

– Sickle-cell anemia- Deforms the body’s red blood cells

• cells die causing anemia• Early death if left untreated.- Penicillin• 1/400 Af-Am babies born with• <1/10 Af-Am are carriers

• 1/5 Africans

Prenatal Screening

• Dangerous- Why get it? – <3% of preg genetic abnormalities

• 20% of infant deaths

– Parents have family history of disease? Is mom/dad carrier? (Dominant”)

– Does mother have any diseases? CMV-Cytomegalovirus

– Are parents carries of a recessive sickle cell allele

Prenatal Screening

• Family History– Blood relatives with disorders– History of previous miscarriages– Children with specific genetic

disorders– Members of at-risk group– Mother’s age and Father’s age– Behavior- Do parents use

drugs/alcohol

Prenatal detection

• HCG-Human Chorionic Gonadotropin- Blood test- similar to a typical pregnancy test

– Can determine an ectopic pregnancy– Sometimes combined with….

Prenatal Detection

• AFP- Alpha-fetoprotein- (also a Blood test)

– looking for AFP being produced amniotic fluid and mothers blood.

– Done at 15-20 weeks– Can determine if child has Down Syndrome

(Decreased Levels)- though only in about 60% of cases

– Can “help” determine neural tube defects such as spina bifida (Increased levels)

– Fetal distress

Prenatal Detection

• Ultrasound- Standard of Care– Done at about 20-22 weeks– 2-D image using sound waves– Considered low-risk as long as non-

repeated exposure- possible low birth weight

– Doppler Ultrasound- 1 hour session- Color- Can look for signs of pre-eclampsia, poor blood flow.

Other ultrasounds

• http://www.youtube.com/watch?v=uo5gOWzgoaY

• 4-D ultrasound

Prenatal Detection

• Amniocentesis– Done between 16th and 18th week though

used to be done at 20th-22nd week. • Why the change????

– Takes two weeks to month to get results.– Needle through stomach, uterine wall, and

amniotic sac, remove small amt of fluid– Avg. range=.3-.5%, but as high as 3-8%

fetal loss– Risk of loss is greater than risk of birth

defects in older moms– Can detect several hundred diff birth defects

(out of 4000)

Prenatal Detection

• CVS- Chorionic Villi Sampling– Done at 10-12 weeks– Results take less than 10 days– Using ultrasound, take fetal cells from

the chorion (placenta)– between a 0.5% and 1% risk of

miscarriage – Slightly higher risk than amnio– Possible birth defects (1/3000)