Question

Report
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
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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).
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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
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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
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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
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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.
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Alterations in Chromosome Duplication
• Mosaicism
• Aneuploidy
– Monosomy
– Polysomy
– Trisomy
º Down syndrome
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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

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