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Transcript
Chapter 6 – Pedigree Analysis and Applications Pedigree • Visual representation of family tree with history of studied trait – Proband – person originally studied • Oldest generation at the top; youngest generation at the bottom – Roman numerals used for generations (I being the oldest) • Numbered from left to right within a single generation Autosomal recessive traits • Trait seen in roughly equal amounts of males and females • Seem to skip generations – Affected individual can have unaffected parents Autosomal dominant • Equal frequency of males and females • No skipping of generations • All affected individuals have an affected parent • (affected individuals tend to be heterozygous) – Some traits are lethal in homozygous form • Achondroplasia X linked recessive • Affected phenotype seen more commonly in males • Tend to skip generations • Affected males do not pass trait to sons • If woman is affected, 100% of sons will be affected X linked dominant • Do not skip generations • Seen in both males and females • Females may be more numerous – Females can get disease from either parent while males can only get from mother • Affected female will have 100% sons affected • Affected male will have 100% daughters affected Y linked • Only males affected • Affected males will have 100% affected sons • Do not skip generations Twin studies • Dizygotic – Non-identical twins; fraternal – 2 separate eggs fertilized – 50% average relatedness; same as any sibling pair • Monozygotic – Identical – One zygote that splits very early in embryonic development Concordance studies • % of twin pairs that have the same trait • Monozygotic twins are 100% genetically identical; dizygotic approx 50% • Used to evaluate genetic vs environmental factors • Genetic influenced traits will show higher concordance in monozygotic twins Adoption studies • Examines effects of genes (biological parental traits) vs environment (adoptive parental traits) • Adoption parents have 0% relatedness to adopted child, but share same environment • Adoptees tend to resemble biological parents (obesity, alcoholism) Genetic counseling • Provide education regarding genetic diseases – risks, testing options – Provides NON-DIRECTIONAL information – Informed consent • Reasons for seeing a genetic counselor – – – – – Positive family history Advanced maternal age Abnormal prenatal test results Infertility Ethnic background Prenatal testing • Ultrasound – Can be performed as early as several weeks after fertilization – Noninvasive – Gives image of fetus • Anatomical abnormalities, neural tube defects, nuchal translucency, amount of amniotic fluid, fetal size Prenatal testing • Amniocentesis – 15-18 weeks – Trans-abdominally or trans-vaginally, depending on placental location – Ultrasound guided – Needle inserted and ~15ml of fluid extracted • Fluid can be tested directly (biochemical) or fetal cells cultured prior to testing (biochemical, molecular, cytogenetics) • Each ml of fluid contains only ~10-15 cells Prenatal testing • Chorionic villi sampling (CVS) – Ultrasound guided – Small section of chorion is suctioned off (1015mg) – Large number of fetal cells reduces time/need for culturing – Increased risk for limb reduction of performed at earlier gestation • Eliminates proper blood supply to developing limb Prenatal testing • Fetal cell sorting – in development – Isolation of fetal cells from maternal bloodstream – Minimally invasive • Pre-implantation – IVF procedure – One cell is removed from 8-16 cell embryo and tested • Only “healthy” embryos are implanted Postnatal testing • Newborn screening – Panel varies from state to state • Heterozygote/carrier testing – Positive family history or particular ethnic background – Biochemical or molecular testing • Depends on specific disease involved Postnatal testing • Pre-symptomatic testing – Inherited cancer alleles – increased risk for cancer – Late-onset diseases • Huntington disease • Chromosome analysis/cytogenetic testing – Diagnostic and prognostic value in cancer – Infertility – Child with structural chromosomal abnormality • Inherited or de novo mutation