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Chapter 4 Genetic and Congenital Disorders Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins Genes Are Found on Chromosomes • X or Y chromosome: sex-linked • Other chromosomes: autosomal Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins Alleles • Alleles are copies of a gene • If all your copies of a gene are alike, you are homozygous • If they differ, you’re heterozygous – If you are heterozygous for a recessive trait and do not show it, you are a carrier • If you have only one copy of a gene, you are hemizygous Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins Question Which of the following statements is true about an individual who is a carrier for the cystic fibrosis (CF) gene? a. Homozygous; suffers from CF b. Homozygous; does not suffer from CF c. Heterozygous; suffers from CF d. Heterozygous; does not suffer from CF Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins Answer d. Heterozygous; does not suffer from CF Rationale: If an individual is a carrier, they are heterozygous for the recessive trait (in this case, CF), but they do not display the trait (because the trait is recessive). Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins Discussion: If you have the allele for a trait, do you display the trait? • No: it is recessive • Yes: it is dominant • Sometimes: it has intermediate penetrance Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins Scenario: A young man’s father died of Brugada syndrome… • Unexplained sudden death • Due to mutation in the gene from the Na+ channel in cardiac muscle • Autosomal dominant trait • His mother is alive and healthy • Both grandmothers are alive and healthy Question: What is the son’s chance of having the disease? Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins Punnett Square: Single-gene Autosomal Mom’s alleles b b b bb bb B Bb Bb Dad’s alleles Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins Penetrance • Brugada syndrome has only a 12.5% penetrance • What does this mean for the young man whose father died of it? Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins Sex-linked Traits • The gene is usually on the X chromosome • If Brugada syndrome were sex-linked, what would the young man’s father’s genotype have been? • What would his chances of inheriting the disease be? Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins Question If an unaffected mother carries one normal and one mutant allele on her X chromosome, what are the chances that she will transmit the defective gene to her sons? a. 25% b. 50% c. 75% d. 100% Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins Answer b. 50% Rationale: Because a son receives the X sex chromosome from his mother (who has one normal and one mutant allele), there is a 50% chance that the mutant allele will be inherited. Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins Punnett Square: Single-gene Sex-linked Mom’s alleles Xb Xb Y Xb Y Xb Y XB Xb XB Xb XB Dad’s alleles Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins Mitochondrial DNA • Codes for the proteins needed for aerobic metabolism • Mitochondria all come from the mother • She has many more than two mitochondria, so she may pass several different mitochondrial alleles to the child (Mendelian Inheritance in Man. [2005]. Mitochondrial deafness modifier gene. Retrieved May 15, 2005, from http://www.ncbi.nlm.nih.gov/entrez/ dispomim.cgi?cmd=entry&id=221745.) Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins Chromosomal Damage • Breakage and rearrangement – Deletion – Inversion – Isochromosome formation – Ring formation – Translocation Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins Question Which type of chromosome alteration generally results in normal offspring? a. Deletion b. Inversion c. Translocation d. Ring formation Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins Answer c. Translocation Rationale: Translocation is the only chromosomal alteration where no genetic material is actually lost (chromosome parts are exchanged), so the offspring are usually normal. Offspring are translocation carriers, so their children may or may not be normal. Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins Alterations in Chromosome Duplication • Mosaicism • Aneuploidy – Monosomy – Polysomy – Trisomy º Down syndrome Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins Environmental Causes for Congenital Disorders Scenario: Mrs. K is a diabetic and is having trouble keeping her blood glucose constant during pregnancy • She works in a dry cleaner’s store next to a gas station and drinks heavily • She is diagnosed with toxoplasmosis Question: What kinds of stresses is her fetus experiencing? Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins