Genetics Exam 2

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Genetics Exam 2
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2015-10-04 21:50:27
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Genetics Exam 2
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  1. Why would studying genetics with human subjects be considered very difficult?
    Controlled matings are not possible and are unethical (human mating is determined by social determinations)

    Humans have a long generation time

    Human family size is very small
  2. Define: Pedigree
    A pedigree is a family tree that outlines the inheritance of one or more characteristics
  3. Use of pedigrees can give valuable clues to ___________
    Use of pedigrees can give valuable clues to inheritance pattern
  4. Define: Proband
    A Proband is a person from whom the pedigree is initiated
  5. Is Pedigree Analysis always a viable tool to determine Mendelian ratios for humans?
    No, the limited number of offspring in most human families means that clear Mendelian ratios in a single pedigree are usually impossible to discern
  6. What are the 5 basic types of inheritance patterns?
    • Autosomal recessive
    • Autosomal dominant
    • Sex-linked recessive
    • Sex-linked dominant
    • Y-linked (Holandric) inheritance
  7. Describe what is observed in Autosomal Recessive Traits
    • Usually appears in both sexes with equal frequency
    • Tends to skip generations
    • Affected offspring are born to unaffected parents
    • When both parents are heterozygous, approximately 1/4 of the offspring will be affected
    • Appears more frequently among families with consanguinity (mating between closely related persons)
    • When both parents are affected (rare), all offspring will be affected
  8. Autosomal Recessive Traits are observed when an offspring inherits _______.
    Autosomal Recessive Traits are observed when an offspring inherits two alleles for the trait, one from each parent.
  9. Define: Consanguinity
    Mating between closely related people
  10. Describe what is observed in Autosomal Dominant Traits
    • Usually appears in both sexes with equal frequency
    • Both sexes transmit the trait to their offspring 
    • Does not skip generations
    • Affected offspring must have an affected parent unless they possess a new mutation
    • When one parent is affected (heterozygous) and the other parent is unaffected, approximately 1/2 of the offspring will be affected
    • Unaffected parents do not transmit the trait
  11. Describe what is observed in X-linked Recessive Traits
    • Usually more males than females are affected.
    • Affected sons are usually born to unaffected mothers; thus, the trait skips generations
    • Approximately half of a carrier (heterozygous) mother’s sons are affected
    • Never passed from father to son
    • When a mother is affected and heterozygous, 1/2 of the sons will be affected and 1/2 of the daughters will be unaffected carriers
    • All daughters of affected fathers are carriers
  12. Describe what is observed in X-linked Dominant Traits
    • Both males and females are usually affected; often more females than males are affected 
    • Does not skip generations
    • Affected sons must have an affected mother; affected daughters must have either an affected mother or an affected father
    • Affected fathers will pass the trait on to all their daughters
    • Affected mothers (if heterozygous) will pass the trait on to half of their sons and half of their daughters
  13. Describe what is observed in Y-linked Traits
    • Only males are affected
    • Passed from father to all sons
    • Does not skip generations.
  14. Autosomal recessive traits often appear in pedigrees in which there have been consanguine matings, because these traits...

    A. tend to skip generations.
    B. usually arise in children born to parents who are unaffected.
    C. appear equally in males and females.
    D. appear only when both parents carry a copy of the gene for the trait, which is more likely when the parents are related.
    D. appear only when both parents carry a copy of the gene for the trait, which is more likely
    (this multiple choice question has been scrambled)
  15. When might you see autosomal dominant traits skip generations?
    It might skip generations when a new mutation arises or the trait has reduced penetrance.
  16. How can you distinguish between an autosomal recessive trait with higher penetrance in males and an X-linked recessive trait?
    If X-linked recessive, the trait will not be passed from father to son
  17. A male affected with an X-linked dominant trait will have what proportion of offspring affected with the trait?

    A. 1/2 sons and 1/2 daughters
    B. All sons and no daughters
    C. All daughters and no sons
    D. 3/4 daughters and 1/4 sons
    C. All daughters and no sons
    (this multiple choice question has been scrambled)
  18. What features of a pedigree would distinguish between a Y-linked trait and a trait that is rare, autosomal dominant, and sex-limited to males?
    If the trait were Y-linked, an affected male would pass it on to all his sons, whereas if the trait were autosomal and sex-limited, affected heterozygous males would pass it on to only half of their sons on average.
  19. In pedigree analysis, the proband is ...

    A. The medical geneticist who analyzes the pedigree to find the mode of inheritance for the disorder.
    B. The most common software package that geneticists use to analyze pedigrees.
    C. One of the grandparents or great grandparents who are in the first generation of the pedigree.
    D. The parents of the first child in the family to show the trait or disorder.
    E. The individual having the trait or disorder from whom the pedigree is initiated
    E. The individual having the trait or disorder from whom the pedigree is initiated
    (this multiple choice question has been scrambled)
  20. A pedigree that demonstrates an autosomal recessive trait, such as Tay-Sachs disease, will likely show all of the following characteristics EXCEPT:

    A. Equal numbers of males and females affected.
    B. Affected individuals born to unaffected parents.
    C. On average, half the offspring of carriers showing the trait.
    D. Actually, all of these are expected.
    E. Skipping of a generation by the trait.
    C. On average, half the offspring of carriers showing the trait.
    (this multiple choice question has been scrambled)
  21. The ability to roll the tongue is caused by a dominant allele. A woman is a “roller,” but one of her parents is not. The woman is expecting a child with a man who is a “nonroller.” What is the probability that their first child will be a “roller”?

    A. 3/4
    B. 0
    C. 1/4
    D. 1/2
    D. 1/2
    (this multiple choice question has been scrambled)
  22. Could the characteristic followed in the pedigree be caused by an autosomal dominant disease? Why or why not?

    No, the offspring of II-5 and II-6 contradict an autosomal dominant inheritance.
  23. For each of the following modes of inheritance, describe the features that will be exhibited in a pedigree in which the trait is present: autosomal recessive, autosomal dominant, X-linked recessive, X-linked dominant, and Y- linked inheritance.
    • Autosomal recessive trait: affected males and females arise with equal frequency from unaffected parents; often appears to skip generations; unaffected child of an affected parent is a carrier.
    • Autosomal dominant trait: affected males and females arise with equal frequency from a single affected parent; does not usually skip generations.
    • X-linked recessive trait: affects males predominantly and is passed from an affected male through his unaffected daughter to his grandson; not passed from father to son.
    • X-linked dominant trait: affects males and females; is passed from an affected male to all his daughters but not to his sons; is passed from an affected woman (usually heterozygous for a rare dominant trait) equally to half her daughters and half her sons.
    • Y-linked trait: affects males exclusively; is passed from father to all sons.
  24. Joe is color blind. Both his mother and his father have normal vision, but his mother’s father (Joe’s maternal grandfather) is color blind. All Joe’s other grandparents have normal color vision. Joe has three sisters—Patty, Betsy, and Lora—all with normal color vision. Joe’s oldest sister, Patty, is married to a man with normal color vision; they have two children, a 9-year-old color-blind boy and a 4-year-old girl with normal color vision.

    b. What is the most likely mode of inheritance for color blindness in Joe’s family? 
    c. If Joe marries a woman who has no family history of color blindness, what is the probability that their first child will be a color-blind boy?
    d. If Joe marries a woman who is a carrier of the color-blind allele, what is the probability that their first child will be a color-blind boy?
    e. If Patty and her husband have another child, what is the probability that the child will be a color-blind boy?
    • (b) X-linked recessive
    • (c) zero
    • (d) 1/4
    • (e) 1/4.
  25. For the following pedigree, give the most likely mode of inheritance, assuming that the trait is rare. Carefully explain your reasoning.

    Autosomal Dominant: The trait must be autosomal because affected males pass the trait to both sons and daughters. It is dominant because it does not skip generations, all affected individuals have affected parents, and it is extremely unlikely that multiple unrelated individuals mating into the pedigree are carriers of a rare trait.
  26. For the following pedigree, give the most likely mode of inheritance, assuming that the trait is rare. Carefully explain your reasoning.

    X-linked dominant: Superficially, this pedigree appears to be similar to the pedigree in part (a) in that both males and females are affected, and it appears to be a dominant trait. However, closer inspection reveals that, whereas affected females can pass the trait to either sons or daughters, affected males pass the trait only to all daughters.
  27. For the following pedigree, give the most likely mode of inheritance, assuming that the trait is rare. Carefully explain your reasoning.

    Y-linked: The trait affects only males and is passed from father to son. All sons of an affected male are affected.
  28. For the following pedigree, give the most likely mode of inheritance, assuming that the trait is rare. Carefully explain your reasoning.

    X-linked recessive or sex-limited autosomal dominant: Because only males show the trait, the trait could be X-linked recessive, Y-linked, or sex-limited. We can eliminate Y-linkage because af- fected males do not pass the trait to their sons. X-linked recessive inheritance is consistent with the pattern of unaffected female carriers producing both affected and unaffected sons and affected males producing unaffected female carriers, but no affected sons. Sex-limited autosomal dominant inheritance is also consistent with unaffected heterozygous females producing affected heterozygous sons, unaffected homozygous recessive sons, and unaffected heterozygous or homozygous recessive daughters. The two remaining possibilities of X-linked recessive versus sex-limited autosomal dominant could be distinguished if we had enough data to determine whether affected males have both affected and unaffected sons, as expected from autosomal dominant inheritance, or whether affected males have only unaffected sons, as expected from X-linked recessive inheritance. Unfortunately, this pedigree shows only two sons from affected males. In both cases, the sons are unaffected, consistent with X-linked recessive inheritance, but two male progeny are not enough to conclude that affected males cannot produce affected sons.
  29. For the following pedigree, give the most likely mode of inheritance, assuming that the trait is rare. Carefully explain your reasoning.

    Autosomal recessive: Unaffected parents produce affected progeny, and so the trait is recessive. The affected daughter must have inherited recessive alleles from both unaffected parents, and so the trait must be autosomal. If it were X-linked, her father would show the trait.
  30. The following pedigree illustrates the inheritance of Nance–Horan syndrome, a rare genetic condition in which affected persons have cataracts and abnormally shaped teeth.



    a. On the basis of this pedigree, what do you think is the most likely mode of inheritance for Nance–Horan syndrome?
    b. If couple III-7 and III-8 have another child, what is the probability that the child will have Nance–Horan syndrome? 
    c. If III-2 and III-7 were to mate, what is the probability that one of their children would have Nance–Horan syndrome??
    • a) X-linked recessive
    • b) 1/4
    • c) 1/2
  31. When solving pedigree analysis, in the case of a rare mutation, what can you assume?
    Assume that an unrelated individual marrying into the family is homozygous for the wild-type (non-disease allele), unless there is evidence suggesting otherwise
  32. Dominant mutations are usually lethal when ______. Assume that individuals carrying dominant mutations are _______.
    • Dominant mutations are usually lethal when homozygous
    • Assume that individuals carrying dominant mutations are heterozygotes (exception, X-linked dominant in males)
  33. If crossing over occurs in some but not all meiotic cells, the majority of offspring will be _________.
    If crossing over occurs in some but not all meiotic cells, the majority of offspring will be nonrecombinant and resemble the parents.
  34. If a crossover between A and B occurs in every meiotic cell, then the frequency of recombinants (recombination frequency) will be _____.
    If a crossover between A and B occurs in every meiotic cell, then the frequency of recombinants (recombination frequency) will be 50%.
  35. Recombination frequencies between two genes cannot exceed _____.
    Recombination frequencies between two genes cannot exceed 50%.

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