GEN3000 Chapter 6: Pedigree Analysis, Applications, & Genetic Testing

Pedigree Analysis and Standardization Rules

• Pedigree Definition: A pedigree is a pictorial representation of a family history that outlines the inheritance of one or more phenotypic traits across multiple generations.
• Utility in Genetics:
  • Pedigree analysis reveals underlying patterns of inheritance for human genetic traits (e.g., determining whether an allele operates in an autosomal dominant, autosomal recessive, X-linked, or Y-linked manner).
  • It is vital in human genetics because controlled experimental breeding crosses cannot ethically or practically be performed on human subjects.
  • It can also be applied to assess line breeding and trait propagation in other organisms.
• Standardized Pedigree Symbols and Conventions:

*   **Biological Sex and Gender Identification:**
    *   **Male:** Represented by a square symbol.
    *   **Female:** Represented by a circle symbol.
    *   **Sex Unknown or Unspecified:** Represented by a diamond symbol.
    *   **Transgender Man:** Represented by a square symbol marked with AFAB (assigned female at birth).
    *   **Transgender Woman:** Represented by a circle symbol marked with AMAB (assigned male at birth).
    *   **Non-binary Person:** Represented by a diamond symbol marked with AFAB or AMAB.
*   **Phenotypic Status:**
    *   **Unaffected Person:** Represented by an open (unfilled) symbol.
    *   **Person Affected with Trait:** Represented by a fully filled (shaded) symbol.
    *   **Obligate Carrier:** Represented by a symbol containing a central filled dot (carries the mutant gene but does not display the phenotypic trait).
    *   **Asymptomatic Carrier:** Represented by a symbol split vertically with one half filled (currently unaffected but may exhibit the phenotype later in life).
*   **Groupings, Status, and Proband:**
    *   **Multiple People:** Indicated by placing an Arabic numeral inside the symbol (e.g., a square, circle, or diamond with a $\text{5}$ inside denotes five individuals of that designation).
    *   **Deceased Person:** Represented by a symbol with a diagonal strike-through slash line.
    *   **Proband:** The first affected family member coming to the attention of a geneticist. Indicated by an arrow pointing directly to the symbol, accompanied by the letter $P$
*   **Family Structure and Relationships:**
    *   **Family History Unknown:** Indicated by a question mark $?$ above the symbol.
    *   **Parents and Offspring:** A horizontal mating line connects parents; a vertical lineage line descends to a horizontal sibling line connecting offspring. Siblings are depicted in order of birth from left to right.
    *   **Generational and Individual Numbering:** Generations are designated by sequential Roman numerals ($I, II, III, IV$). Within each generation, individuals are numbered sequentially from left to right using Arabic numerals ($1, 2, 3...$).
    *   **Adoption:** Square brackets enclose the adopted individual; a dashed line leads to adoptive parents, while a solid line connects to biological parents.
    *   **Twins:**
        *   *Monozygotic (Identical) Twins:* Represented by diagonal branches originating from a single point, connected by a horizontal crossbar.
        *   *Dizygotic (Nonidentical) Twins:* Represented by diagonal branches originating from a single point without a connecting horizontal crossbar.
        *   *Twin Type Unknown:* Represented by diagonal branches originating from a single point with a question mark $?$ between them.
    *   **Consanguinity:** Mating between biologically related individuals (e.g., first cousins). Indicated by a double horizontal mating line between the parents.

Autosomal Dominant Inheritance

• Key Characteristics of Autosomal Dominant Traits:
  1. Usually appears in both sexes with equal frequency ($\text{Males} = \text{Females}$).
  2. Both sexes transmit the trait to their offspring.
  3. Does not skip generations; present in every generation of a pedigree unless non-penetrance or a new de novo mutation occurs.
  4. Affected offspring must have an affected parent unless the individual possesses a new mutation.
  5. When one parent is affected (heterozygous $Aa$) and the other parent is unaffected ($aa$), approximately half ($50\%$) of the offspring will be affected ($Aa \times aa \rightarrow \frac{1}{2}\,Aa, \frac{1}{2}\,aa$).
  6. Unaffected parents do not transmit the trait to their offspring ($aa \times aa \rightarrow 100\%\,aa$).

• Case Example: Waardenburg Syndrome:

*   Waardenburg syndrome is inherited as an autosomal dominant trait.
*   **Phenotypic Manifestations:** Physical features include a prominent white forelock of hair, fair skin pigmentation, juvenile deafness, and light-colored eyes (or heterochromia iridum).
*   **Pedigree Analysis Rules for Waardenburg Syndrome:**
    *   Generations are labeled with Roman numerals ($I, II, III, IV$).
    *   Individuals within each generation are identified by Arabic numerals ($1, 2, 3...$).
    *   Filled red symbols represent family members exhibiting Waardenburg syndrome; open symbols represent unaffected family members.
    *   Children in each nuclear family unit are listed from left to right in exact birth order.

Autosomal Recessive Inheritance

• Key Characteristics of Autosomal Recessive Traits:
  1. Usually appears in both sexes with equal frequency ($\text{Males} = \text{Females}$).
  2. Tends to skip generations, remaining unexpressed in heterozygous carriers ($Aa$).
  3. Affected offspring ($aa$) are usually born to unaffected carrier parents ($Aa \times Aa$).
  4. When both parents are heterozygous carriers ($Aa$), approximately one-fourth ($25\%$) of their offspring will be affected ($Aa \times Aa \rightarrow \frac{1}{4}\,aa$).
  5. Appears with significantly higher frequency among the progeny of consanguineous marriages (mating between related individuals, such as first cousins), because relatives are more likely to carry the identical recessive allele inherited from a shared ancestor.

• Comparative Assessment of Autosomal Patterns:

*   **Autosomal Recessive Diagnostic Cues:** In a pedigree exhibiting autosomal recessive inheritance, affected individuals (such as $III-3$ or $III-4$) are born to unaffected parents ($II-3$ and $II-4$), proving that either parent $I-3$ or $I-4$ must be heterozygous carriers. Recessive autosomal traits appear equally in both sexes.
*   **Autosomal Dominant Diagnostic Cues:** Individual $I-1$ is heterozygous for a dominant allele ($Aa$). Dominant traits almost always appear in every single generation, and every affected individual has at least one affected parent.

X-Linked Recessive Inheritance and Case Studies

• Key Characteristics of X-Linked Recessive Traits:
  1. Usually affects significantly more males than females (males are hemizygous $X^rY$ and require only a single copy of the mutant allele, whereas females require two copies $X^rX^r$).
  2. Affected sons are usually born to unaffected carrier mothers ($X^RX^r$); thus, the trait frequently skips generations.
  3. Approximately half ($50\%$) of a carrier mother's sons will be affected ($X^RX^r \times X^RY \rightarrow X^rY$ with a probability of $0.50$).
  4. The trait is never passed directly from father to son (fathers pass their $Y$ chromosome to male offspring).
  5. All daughters of an affected father ($X^rY$) are obligate carriers ($X^RX^r$) if the mother is homozygous normal ($X^RX^R$).

• Case Study 1: Classic Hemophilia in European Royalty:

*   Classic hemophilia is an X-linked recessive blood-clotting disorder.
*   Queen Victoria of England was an asymptomatic heterozygous carrier ($X^HX^h$) who introduced the allele into the royal lineages of Europe.
*   **Transmission Details Across Royal Lines:**
    *   Her son Prince Leopold was affected ($X^hY$).
    *   Her daughters Princess Alice and Princess Beatrice were carriers ($X^HX^h$).
    *   Tsarevich Alexei of Russia (son of Alexandra and Czar Nicholas II) inherited the disease via Alexandra from Queen Victoria.
    *   Affected males in the Prussian Royal Family: Frederick, Waldemar, Prince Henry.
    *   Affected males in the Spanish Royal Family: Alfonso, Gonzalo, Leopold, Maurice, Rupert.

• Case Study 2: Red-Green Color Blindness:

*   **Diagnostic Testing:** Evaluated using Ishihara color-blindness charts. Individuals with normal color vision perceive the number $15$, whereas individuals with red-green color blindness perceive the number $17$
*   **Genotypic Rules for Red-Green Color Blindness ($X^R$ = normal, $X^r$ = color-blind):**
    *   **If Mother is Affected ($X^rX^r$) and Father is Unaffected ($X^RY$):** All sons will be affected ($100\%\,X^rY$). None of the daughters will be affected ($0\%\,X^rX^r$), but $100\%$ of the daughters will be obligate carriers ($X^RX^r$).
    *   **If Mother is a Carrier ($X^RX^r$) and Father is Unaffected ($X^RY$):** Sons have a $50/50$ ($50\%$) chance of being affected ($X^rY$) or normal ($X^RY$). Daughters have a $50\%$ chance of being homozygous normal ($X^RX^R$) and a $50\%$ chance of being carriers ($X^RX^r$).
    *   **If Father is Affected ($X^rY$) and Mother is Homozygous Normal ($X^RX^R$):** No children exhibit the phenotype. All daughters inherit the father's $X^r$ and become obligate carriers ($X^RX^r$); sons receive the father's $Y$ chromosome and are completely unaffected ($X^RY$).

X-Linked Dominant Inheritance

• Key Characteristics of X-Linked Dominant Traits:
  1. Both males and females are affected; frequently, more females than males are affected because females have two X chromosomes ($X^AX^A$ or $X^AX^a$), giving them two chances to inherit the dominant allele.
  2. Does not skip generations.
  3. Affected sons must have an affected mother ($X^AY$ receives $X^A$ from the female parent).
  4. Affected daughters must have either an affected mother ($X^AX^a$) or an affected father ($X^AY$).
  5. Affected fathers ($X^AY$) pass the trait to all ($100\%$) of their daughters and none ($0\%$) of their sons.
  6. Heterozygous affected mothers ($X^AX^a$) pass the trait to half ($50\%$) of their sons and half ($50\%$) of their daughters.

Y-Linked Inheritance

• Key Characteristics of Y-Linked (Holandric) Traits:
  1. Only males are affected.
  2. Passed directly from an affected father to all ($100\%$) of his male offspring.
  3. Does not skip generations within the paternal lineage.
  4. Females never inherit, express, or transmit the trait.

Twin Studies and Adoption Studies

• Importance of Twin and Adoption Studies: These approaches help isolate and quantify the relative contributions of genetic variance versus environmental variance in determining complex human traits.
• Twin Terminology and Biological Origins:

*   **Dizygotic (DZ) Twins:** Nonidentical twins resulting from two independent oocytes fertilized by two distinct sperm cells. They share approximately $50\%$ of their genetic material on average (identical to standard full siblings). Placentation is dichorionic and diamniotic.
*   **Monozygotic (MZ) Twins:** Identical twins resulting from a single fertilized zygote that splits into two separate embryos during early cleavage. They share $100\%$ of their genetic material.
    *   *Morula splitting (Days $0-3$):* Produces dichorionic diamniotic twins.
    *   *Split at hatching (Days $4-8$):* Produces monochorionic diamniotic twins.
    *   *Blastocyst splitting up to 1 week after implantation (Days $8-13$):* Produces monochorionic monoamniotic twins.
*   **Concordant Trait:** A phenotypic trait shared by both individual members of a twin pair.
*   **Concordance:** The percentage of twin pairs in which both individuals express the specified trait. A higher concordance rate in MZ twins compared to DZ twins indicates a strong genetic etiology.

• Concordance Percentages in Monozygotic vs. Dizygotic Twins:

*   *Heart attack (males):* Monozygotic = $39\%$, Dizygotic = $26\%$
*   *Heart attack (females):* Monozygotic = $44\%$, Dizygotic = $14\%$
*   *Bronchial asthma:* Monozygotic = $47\%$, Dizygotic = $24\%$
*   *Cancer (all sites):* Monozygotic = $12\%$, Dizygotic = $15\%$
*   *Epilepsy:* Monozygotic = $59\%$, Dizygotic = $19\%$
*   *Death from acute infection:* Monozygotic = $7.9\%$, Dizygotic = $8.8\%$
*   *Rheumatoid arthritis:* Monozygotic = $32\%$, Dizygotic = $6\%$
*   *Multiple sclerosis:* Monozygotic = $28\%$, Dizygotic = $5\%$
*   *Interpretation:* Traits like epilepsy, rheumatoid arthritis, and multiple sclerosis demonstrate markedly higher concordance in MZ twins, confirming a major genetic component. Traits like death from acute infection display nearly identical low concordance rates ($7.9\%$ vs. $8.8\%$), demonstrating environmental factors predominant over genetics.

• Adoption Studies and Body Mass Index (BMI):

*   **Experimental Question:** Is Body Mass Index (BMI) influenced by genetic factors?
*   **Method:** Compare the BMIs of adopted children against the BMIs of their biological parents (shared genes, separate environment) and adoptive parents (shared environment, separate genes).
*   **Results:**
    *   *Biological Parents:* Overweight biological parents tend to have overweight children; a direct positive association exists between adoptee weight class and biological parental BMI.
    *   *Adoptive Parents:* No consistent association exists between the weight class of adopted children and the BMI of their adoptive parents.
*   **Conclusion:** Genetic factors play a significant role in influencing human Body Mass Index.

Genetic Counseling

• Definition and Purpose: Genetic counseling is an educational and supportive process that provides information and support to individuals and families dealing with genetic disorders or risks.
• Common Clinical Indications for Seeking Genetic Counseling:
  • A person knows of a specific genetic condition present in their family history.
  • A couple has previously given birth to a child with a genetic condition, birth defect, or chromosomal abnormality.
  • A couple has a child who is intellectually disabled or has a close relative with an intellectual disability.
  • Advanced maternal age ($\ge 35$ years of age at the time of delivery), which increases the risk of chromosomal non-disjunction events.
  • Biological parents are closely related by blood (consanguinity, e.g., first cousins).
  • A couple experiences recurrent pregnancy loss, miscarriages, or difficulty achieving a successful pregnancy.
  • A pregnant woman is concerned about potential exposure to environmental teratogens (such as drugs, industrial chemicals, radiation, or viral infections).
  • A couple requires assistance interpreting technical results from prenatal or diagnostic genetic tests.
  • Both prospective parents are confirmed carriers for an autosomal recessive genetic disease, or belong to a specific ethnic demographic associated with high carrier frequencies for targeted conditions.

Prenatal Genetic Diagnosis Techniques

• Methods of Prenatal Detection for Specific Genetic Disorders:

*   *Chromosome abnormalities:* Examination of a karyotype from cells obtained via amniocentesis or chorionic villus sampling (CVS). Some forms are detectable via non-invasive DNA analysis of cell-free fetal DNA in maternal blood.
*   *Cleft lip and palate:* Diagnostic ultrasound imaging.
*   *Cystic fibrosis:* Direct DNA analysis of fetal cells obtained via amniocentesis or CVS.
*   *Dwarfism:* Ultrasound imaging or skeletal X-ray; select genetic forms detected via DNA analysis of fetal cells obtained via amniocentesis or CVS.
*   *Hemophilia:* Fetal blood sampling or DNA analysis of fetal cells obtained via amniocentesis or CVS.
*   *Lesch-Nyhan syndrome:* Enzymatic and biochemical assays on fetal cells obtained via amniocentesis or CVS.
*   *Neural-tube defects:* Initial screening via maternal blood test (elevated serum $\alpha$-fetoprotein), followed by biochemical testing on amniotic fluid obtained via amniocentesis or structural confirmation via ultrasound imaging.
*   *Osteogenesis imperfecta:* Diagnostic ultrasound imaging or skeletal X-ray (identifying bone fractures and deformities).
*   *Phenylketonuria (PKU):* Direct DNA analysis of fetal cells obtained via amniocentesis or CVS.
*   *Sickle-cell anemia:* Fetal blood sampling or direct DNA analysis of fetal cells obtained via amniocentesis or CVS.
*   *Tay-Sachs disease:* Biochemical enzyme assays (hexosaminidase A activity) on fetal cells obtained via amniocentesis or CVS.

• Amniocentesis Procedure:

1.  Under continuous guidance of ultrasound imaging, a sterile needle is inserted through the mother's abdominal wall and uterine wall into the amniotic sac.
2.  A small volume of amniotic fluid is aspirated into a syringe.
3.  The amniotic fluid contains suspended fetal skin/mucosal cells, which are separated from the fluid via centrifugation.
4.  Fetal cells are placed into culture media and cultured in vitro for several weeks.
5.  Diagnostic testing is performed on cultured cells: Chemical analysis, DNA sequencing analysis, and Chromosomal analysis (karyotyping).
*   *Note:* Non-invasive prenatal testing (NIPT) can isolate cell-free fetal DNA directly from maternal blood, bypassing the invasive risks of amniocentesis.

• Chorionic Villus Sampling (CVS) Procedure:

1.  CVS can be performed significantly earlier in gestation ($10-11 weeks fetus) than amniocentesis.\n    2.  Under continuous guidance of ultrasound imaging, a flexible catheter is inserted through the vagina and cervix into the uterus.\n    3.  The catheter is placed into direct contact with the chorion, the outer fetal membrane layer of the placenta.\n    4.  Gentle suction is applied to remove a small tissue biopsy of chorionic villi.\n    5.  Cells of the chorion are dividing rapidly and can be used directly for genetic, chemical, DNA, and chromosomal analysis without requiring weeks of cell culture.\n\n\n# Newborn Screening Procedures\n\n*   **Purpose:** Postnatal screening conducted shortly after birth to identify treatable metabolic, enzymatic, and structural conditions before irreversible damage occurs.\n*   **Recommended Panel of Genetic Conditions for Newborn Screening:**\n\n![Newborn Screening Table](https://assets.knowt.com/pdf-flow-prod/59a79356-2bd9-4355-9e3c-8504f9f34c6d-figures/18.png)\n\n    *   Propionic acidemia\n    *   Methylmalonic acidemia (methylmalonyl-CoA mutase deficiency)\n    *   Methylmalonic acidemia (cobalamin disorders)\n    *   Isovaleric acidemia\n    *   \text{3-Methylcrotonyl-CoA} carboxylase deficiency\n    *   \text{3-Hydroxy-3-methylglutaric} aciduria\n    *   Holocarboxylase synthase deficiency\n    *   \beta-Ketothiolase deficiency\n    *   Glutaric acidemia type I\n    *   Carnitine uptake defect / carnitine transport defect\n    *   Medium-chain acyl-CoA dehydrogenase deficiency (MCADD)\n    *   Very long-chain acyl-CoA dehydrogenase deficiency (VLCADD)\n    *   Long-chain L-3-hydroxyacyl-CoA dehydrogenase deficiency (LCHADD)\n    *   Trifunctional protein deficiency\n    *   Argininosuccinic aciduria\n    *   Citrullinemia, type I\n    *   Maple syrup urine disease (MSUD)\n    *   Homocystinuria\n    *   Classic phenylketonuria (PKU)\n    *   Tyrosinemia, type I\n    *   Primary congenital hypothyroidism\n    *   Congenital adrenal hyperplasia\n    *   S,S disease (sickle cell anemia)\n    *   S, \beta-thalassemia\n    *   S,C disease\n    *   Biotinidase deficiency\n    *   Critical congenital heart disease\n    *   Cystic fibrosis\n    *   Classic galactosemia\n    *   Glycogen storage disease type II (Pompe disease)\n    *   Hearing loss\n    *   Severe combined immunodeficiencies (SCID)\n    *   Mucopolysaccharidosis type 1 (MPS I)\n    *   X-linked adrenoleukodystrophy (X-ALD)\n    *   Spinal muscular atrophy (SMA) due to homozygous deletion of exon 7 in SMN1$$