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Chapter 12. Atypical Modes of Inheritance

Suppose that there is an important growth factor during fetal development (GF). Like most polypeptide hormones it has a receptor (GFR). It appears that during fetal life only the paternal GF gene is active and only the maternal GFR gene is active. Mutations that cause both copies of either gene to become active typically result in fetal overgrowth. This is an example of:

A. Uniparental disomy.

B. Genomic imprinting.

C. Unstable DNA.

D. Genetic anticipation.

E. Partial expression.

B is the correct answer.

  • A. Uniparental disomy is inheriting two copies of the same chromosome (or locus) from the same parent.

  • C. This does not describe expanding repeats or transposable elements.

  • D. Anticipation is the progressive worsening of a condition through the generations.

  • E. Partial expression would refer to how the trait were expressed, not the selective silencing of one allele.

In regards to imprinting, which of the following statements is most likely true?

A. Imprinting represents a pathologic mechanism of gene expression.

B. Imprinting changes the DNA code.

C. Imprinting is erased during meiosis.

D. The most common mechanism of imprinting is by DNA glycosylation.

E. All human gene loci are normally imprinted.

C is the correct answer.

  • A. Imprinting is a normal phenomenon. It is the way certain genes are supposed to be expressed.

  • B. Imprinting is an epigenetic mechanism. The gene function, but not the code, is changed.

  • D. The most common mechanism of imprinting is methylation.

  • E. Only a small number of loci (maybe 70 or so) are imprinted. The majority of loci have typical di-allelic expression.

In regards to X-inactivation:

A. Males will usually have one Barr body in their cells.

B. Because it is random and thus there is almost always a distribution of very close to a 50:50 proportion of one or the other X being inactivated.

C. A structurally abnormal X chromosome is preferentially inactivated, leaving the normal X active.

D. It is permanent in germ cells

E. Its clonal distribution suggests that it occurs late in embryonic development.

C is the correct answer.

  • A. Females, with two X chromosomes have Barr ...

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