Chapter 4 Genetic and Congenital Disorders

1. Chapter 4Genetic and Congenital Disorders

2. Genes Are Found on Chromosomes • X or Y chromosome: sex-linked • Other chromosomes: autosomal

3. 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

4. Question Which of the following statements is true about an individual who is a carrier for the cystic fibrosis (CF) gene? • Homozygous; suffers from CF • Homozygous; does not suffer from CF • Heterozygous; suffers from CF • Heterozygous; does not suffer from CF

5. Answer • 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).

6. 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

7. 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?

8. Punnett Square: Single-gene Autosomal

9. Penetrance • Brugada syndrome has only a 12.5% penetrance • What does this mean for the young man whose father died of it?

10. 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?

11. 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? • 25% • 50% • 75% • 100%

12. Answer • 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.

13. Punnett Square: Single-gene Sex-linked

14. 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.)

15. Chromosomal Damage • Breakage and rearrangement • Deletion • Inversion • Isochromosome formation • Ring formation • Translocation

16. Question Which type of chromosome alteration generally results in normal offspring? • Deletion • Inversion • Translocation • Ring formation

17. Answer • 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.

18. Alterations in Chromosome Duplication • Mosaicism • Aneuploidy • Monosomy • Polysomy • Trisomy • Down syndrome

19. 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?