Women (and all female mammals) have one active X chromosome per cell instead of two. What causes this?
A) modification of the XIST gene so that it is active only on one X chromosome, which then becomes inactive
B) activation of the Barr gene on one of the two X chromosomes that then inactivates
C) crossover between the XIST gene on one X chromosome and a related gene on an autosome
D) inactivation of the XIST gene on the X chromosome derived from the male parent
E) the removal of methyl (CH3) groups from the X chromosome that will remain active
Answer: A
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Biology
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- Down syndrome has a frequency in the U.S. population of ~1/700 live births. In which of the following groups would you expect this frequency to be significantly higher?
- What is the source of the extra chromosome 21 in an individual with Down syndrome?
- An inversion in a human chromosome often results in no demonstrable phenotypic effect in the individual. What else may occur?
- Abnormal chromosomes are frequently found in malignant tumors. Errors such as translocations may place a gene in close proximity to different control regions. Which of the following might then occur to make the cancer worse?
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- In humans, male-pattern baldness is controlled by an autosomal gene that occurs in two allelic forms. Allele Hn determines nonbaldness, and allele Hb determines pattern baldness. In males, because of the presence of testosterone, allele Hb is dominant over Hn. If a man and woman both with genotype HnHb have a son, what is the chance that he will eventually be bald?
- A nonreciprocal crossover causes which of the following products?
- One possible result of chromosomal breakage is for a fragment to join a nonhomologous chromosome. What is this alteration called?
- If nondisjunction occurs in meiosis II during gametogenesis, what will be the result at the completion of meiosis?
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- Recombination between linked genes comes about for what reason?
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- What does a frequency of recombination of 50% indicate?
- How would one explain a testcross involving F1 dihybrid flies in which more parental-type offspring than recombinant-type offspring are produced?
- Which of the following statements is true of linkage?
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