26.1 Counseling for Chromosomal Disorders

• The human karyotype contains 22 pairs of autosomes and one pair of sex chromosomes for a total of 46 chromosomes.
• Amniocentesis and chorionic villi sampling provide fetal cells for karyotyping purposes.
• Inheritance of an abnormal chromosome number is most likely due to nondisjunction.
• Down syndrome is due to the inheritance of three copies of chromosome 21.
• Several syndromes are the result of inheriting an incorrect number of sex chromosomes.
• Chromosome mutations include deletions, duplications, translocations, and inversions.
• Chromosome mutations are associated with various syndromes.

1. What is a karyotype?
   Answer
   
   
2. What is a syndrome?
   Answer
   
   
3. How can an abnormal number of chromsosomes be detected in a fetus?
   Answer
   
   
4. What is the most common cause of an abnormal number of chromosomes?
   Answer
   
   
5. What is a Barr body?
   Answer
   
   
6. What possible causes are associated with changes in chromosome structure?
   Answer
   
   
7. Why do chromosomal inversions cause problems when the same genetic material is still present?
   Answer
   
   

Essential Study Partner
    Introduction
    Chromosomes
    Karyotype
    Sex Chromosomes
    Abnormal Chromosomes

Art Quiz
    Nondisjunction and Sex Chromosomes
    Barr Bodies

Exploration
    Exploring Meiosis: Down Syndrome

26.2 Counseling for Chromosomal Disorders: The Present

• Pedigree charts are used to determine the pattern of inheritance for a condition that runs in the family.
• The pattern of inheritance indicates whether the disorder is a recessive or a dominant disorder.
• It is possible to determine the chances of an offspring inheriting a particular parental gene, and therefore a particular condition.
• Many genetic disorders are inherited according to Mendel's laws
• Certain traits, unrelated to the gender of the individual, are controlled by genes located on the sex chromosomes.
• Males who inherit an allele for an X-linked recessive disorder exhibit the disorder because they inherit only one X chromosome.
• Protein tests, genetic markers and DNA markers are three ways of testing for genetic disorders.
• Some genetic disorders can be identified prenataly by testing the egg, the embryo, or the fetus.

1. How is it possible to recognize an autosomal dominant or recessive trait from the pattern of inheritance?
   Answer
   
   
2. When considering genetic diseases, what is a "carrier?"
   Answer
   
   
3. What are some examples of inherited genetic disorders?
   Answer
   
   
4. What are some examples of X-Linked disorders?
   Answer
   
   
5. Why are sex-linked disorders, such as red-green colorblindness, much more common in males than in females?
   Answer
   
   

Exploration
    Cystic Fibrosis

Explorations
    Constructing a Genetic Map
    Heredity in Families
    Gene Segregation within Families

26.3 Counseling for Chromosomal Disorders: The Future

• The Human Genome Project has two goals: to genetically map each chromosome and to sequence the DNA bases of each chromosome.
• Genomics, Proteomics, and Bioinformatics will help researchers find new ways to identify, prevent, and treat disease.
• Gene therapy is now being used to replace defective genes with healthy genes and to help cure various human ills.

1. What purposes will the Human Genome Project serve?
   Answer
   
   
2. How many human genes code for proteins?
   Answer
   
   
3. What are the potential benefits of a genetic profile?
   Answer
   
   
4. How do ex vivo and in vivo gene therapies differ?
   Answer
   
   

Case Study
    The New York Times: Iceland Sells Genes for Profit