Genetics Ch. 6

• Pedigree


  Pictorial representation of a family history; a family tree that outlines the inheritance of one or more characteristics
• Proband

  The person from whom the pedigree is initiated
• Types of traits shown in pedigrees

  • Autosomal recessive traits
  • Autosomal dominant traits
  • X-linked recessive traits
  • X-linked dominant traits
  • Y-linked traits
• Familial hypercholesterolemia

  Autosomal dominant trait with defect in LDL receptor leading to greatly elevated blood cholesterol
• Dizygotic twins

  Nonidentical twins
• Monozygotic twins

  Identical twins
• Concordant trait

  The trait shared by both members of a twin pair
• Concordance

  The percentage of twin pairs that are concordant for a trait
• Monozygotic twins develop from a single egg fertilized by a single sperm, whereas dizygotic twins develop from two eggs fertilized by two different sperm.
• Monozygotic twins

  Develop from a single egg fertilized by a single sperm
• Dizygotic twins

  Develop from two eggs fertilized by two different sperm
• Monozygotic twins tend to look more similar
• Genetic counseling

  Provides information related to hereditary conditions
• Common reasons for seeking genetic counseling

  • A person knows of a genetic disease in the family
  • A couple has given birth to a child with a genetic disease, birth defect, or chromosome abnormality
  • A couple has a child who is intellectually disabled or has a close relative who is intellectually disabled
  • An older woman becomes pregnant or wants to become pregnant
  • Husband and wife are closely related
  • A couple experiences difficulties achieving a successful pregnancy
  • A pregnant woman is concerned about exposure to an environmental substance
  • A couple needs assistance in interpreting the results of a prenatal or other test
  • Both prospective parents are known carriers for a recessive genetic disease or both belong to an ethnic group with a high frequency of a genetic disease
• Autosomal Recessive Traits

  • usually appears equally in males and females
  • recessive means it can skip generations
  • more likely it appear among progeny of related parents
• Autosomal Dominant Traits

  • equally in males and females
  • don't skip generations
  • affected people have at least one affected parent
  • unaffected don't transmit the trait
• X-Linked Recessive Traits

  • appear more frequently in males
  • not passed from father to son
  • carrier female (mother) to son
• X-Linked Dominant Traits

  • affected males pass to all their daughters
  • affected females pass half to son and half to daughter
• For adoption studies, it is important that:

  • adoptees have no contact with w/ their biological parents after birth
  • adoptive parents and biological parents aren't related
  • the environments of the adoptive and biological parents are independent
• Ultrasonography

  • can be used to detect some genetic disorders in a fetus and locate the fetus
  • types: Amniocentesis & Chorionic Villus Sampling
• Amniocentesis 


  a procedure for obtaining fetal cells for genetic testing; captures fluid; risky as the needle could rupture the sack
• Amniocentesis Steps

  1. Under the guidance of ultrasound, a sterile needle is inserted through the abdominal wall into the amniotic sac
  2. A small amount of amniotic fluid is withdrawn through the needle
  3. The amniotic fluid contains fetal cells, which are separated from the amniotic fluid
  4. Removed fetal cells are cultured
  5. Test are the performed on the cultured cells
• Chorionic Villus Sampling

  1. Guidance of ultrasound, a catheter is inserted through the vagina and cervix and into the uterus
  2. Catheter is placed in contact with the chorion - the outer layer of the placenta
  3. Suction removes a small piece of the chorion
  4. Cells of the chorion are used directly for many genetic testing and culturing is not required
• Interpreting Genetic Testing

  More than a thousand genetic tests available that look at: diseases caused by numerous mutations and incomplete penetrance & environmental factors