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Author: Laurie Ann Demmer, M.D.
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Genetic Counseling

Reading

Jorde, Carey, Bamshad & White: Medical Genetics, 3rd edition, C.V. Mosby Publishing, 2005.

  • Chapter 4, 58-62
  • Chapter 14, 305-316

Objectives

  1. To be able to calculate risk frequency for inheritance of autosomal recessive, autosomal dominant, and X-linked disorders.
  2. To be able to do a simple Hardy-Weinberg equation to determine carrier risk.
  3. To recognize what occurs in a genetic counseling session.
  4. To recognize which patients should be referred for genetic counseling.

Hardy-Weinberg Principle

  • Allows calculation of recurrence risk for Mendelian disorders based on the incidence of the condition in the population.
  • Requires that the population in question be in Hardy-Weinberg equilibrium and thus must have:
    • random mating (selection of a partner regardless of the partner's genotype)
    • no new mutations
    • no selection for or against any particular genotype
    • no gain or loss of genes from migration
    • no genetic drift (population must be large enough that an allele could not be lost by chance alone)

Hardy-Weinberg Equation

p2 + 2pq + q2 = 1

This refers to a two-allele system which is true for most conditions of interest.

  • p stands for the frequency of one allele and q stands for the frequency of the other allele. Thus p + q = 1.
  • p2 are the homozygotes for one of the alleles, q2 are the homozygotes for the other allele, and 2pq are the heterozygote individuals (carriers).
  • If we know one of the frequencies (usually p2 or q2) than we can find out the other 2 frequencies.

Example

What is the risk that Ann, who has no family history of CF, could be a CF carrier?

  • Let q2 stand for the incidence of CF in the general population, which is 1 in 2,500. If q2 = 1/2500 then q is the square root of 1/2500, which is 1 in 50. Since p + q = 1, p must equal 49/50.
  • 2pq will give us the incidence of carriers of CF in the general population. Typically we ignore p since it is so close to 1 so: 2 x 1/50 = 1/25.
  • Therefore, Ann's risk of being a CF carrier is 1 in 25.

About Genetic Counseling

Genetic counseling is a communication process that deals with the human problems associated with the occurrence or risk of occurrence of a genetic disorder in a family. This process involves an attempt by one or more appropriately trained persons to help the individual or family to:

  1. Comprehend the medical facts including diagnosis, probable course of the disorder, and the available management.
  2. Appreciate the way heredity contributes to the disorder and the risk of recurrence in specified relatives.
  3. Understand the alternatives for dealing with the risk of recurrence.
  4. Choose a course of action that seems to them appropriate in their view of their risk, their family goals, and their ethical and religious standards and act in accordance with that decision.
  5. To make the best possible adjustment to the disorder in an affected family member and/or to the risk of recurrence of that disorder.

(American Society of Human Genetics, 1975)

Major Goal of Genetic Counseling

To help patients and families understand and cope with genetic disease, not to reduce the incidence of genetic disease.

Questions Answered in a Genetic Counseling Session

What is the condition? Diagnosis
What caused it to happen? Etiology
What does it mean for the affected individual? Prognosis
Will it happen again? Recurrence risk
Is there anything I can do about it? Prevention/Treatment

The Genetic Counseling Process

Genetic counseling follows the principle of nondirectiveness. The goal is to help an individual or family understand the genetic condition present, or at risk for occurring, and then to discuss all the possible options in a non-judgmental way.

Who Needs Genetic Counseling?

  1. Individuals with a known genetic disease (e.g. Marfan syndrome, achondroplasia).
  2. Persons with a family history of a known genetic disease.
  3. Parent(s) of a child diagnosed with a genetic condition (e.g., hemophilia, cystic fibrosis).
  4. Persons with symptoms that may suggest a genetic disorder (e.g. marked joint hyperextensibility).
  5. Pregnant women 35 years or older at time of delivery (increased risk for a chromosomal anomaly such as trisomy 13, 18, 21)
  6. Pregnant women with abnormal testing (fetal ultrasound examination, abnormal maternal serum screening tests i.e., triple or quad screen)
  7. Parent(s) of children identified through newborn screening with an abnormality (hypothyroidism, PKU, tyrosinemia, maple syrup urine disease, homocystinuria, galactosemia, congenital adrenal hyperplasia, sickle cell anemia, etc).
  8. Teratogenic exposure during pregnancy (anti-epileptic medications, alcohol, street drugs).
  9. Couples with recurrent miscarriages or infertility (possible risk for chromosomal abnormalities).
  10. Individuals of certain ethnic groups:
    1. Southeast Asian, Italian, Greek - Thalassemia
    2. French Canadian - Tyrosinemia, Tay-Sachs, Usher syndrome
    3. Ashkenazi Jewish - Tay-Sachs, Gaucher, Canavan disease
    4. African - Thalassemia, sickle cell anemia
  11. Couples who are related (consanguineous).
    1. First cousins have a 4-5% chance of having a child with a genetic condition.
    2. This is in addition to the 3-4% risk that all couples have of having a child with a birth defect.
    3. The closer the blood relationship between a couple, the higher the risk that a genetic condition will be present in any offspring.

Exercises

  1. A mother is a known carrier for X-linked hemophilia. She has 4 boys. What is the chance that at least one boy will have hemophilia?
  2. A woman is a known carrier for a rare autosomal recessive disease (incidence 1/10,000). What is the probability of having an affected child if she marries her first cousin?
  3. Joseph's brother died of a rare autosomal recessive disease. Joseph is unaffected. (Incidence 1/10,000.)
    1. What is the risk to his offspring if he marries his second cousin?
    2. What is the risk to his offspring if he marries an unrelated individual who has no family history of the disease?
  4. Assume the frequencies of the ABO alleles in a certain population are as follows: freq(A) = .2 freq(B) = .2 freq(O) = .6 What is the expected frequency of each blood type listed below?
    1. A
    2. B
    3. AB
    4. O