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Which is characteristic of X-linked recessive inheritance?
- A. There are no carriers.
- B. Affected individuals are principally males.
- C. Affected individuals are principally females.
- D. Affected individuals will always have affected parents.
Correct Answer: B
Rationale: In X-linked recessive disorders, the affected individuals are usually male. With recessive traits, usually two copies of the gene are needed to produce the effect. Because the male only has one X chromosome, the effect is visible with only one copy of the gene. Females are usually only carriers of X-linked recessive disorders. The X chromosome that does not have the recessive gene will produce the normal protein, so the woman will not show evidence of the disorder. The transmission is from mother to son. Usually the mother and father are unaffected.
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Turner syndrome is suspected in an adolescent girl with short stature. What causes this?
- A. Absence of one of the X chromosomes
- B. Presence of an incomplete Y chromosome
- C. Precocious puberty in an otherwise healthy child
- D. Excess production of both androgens and estrogens
Correct Answer: A
Rationale: Turner syndrome is caused by an absence of one of the X chromosomes. Most girls who have this disorder have one X chromosome missing from all cells. No Y chromosome is present in individuals with Turner syndrome. These young women have 45 rather than 46 chromosomes.
A couple has given birth to their first child, a boy with a recessive disorder. The genetic counselor tells them that the risk of recurrence is one in four. Which statement is a correct interpretation of this information?
- A. The risk factor remains the same for each pregnancy.
- B. The risk factor will change when they have a second child.
- C. Because the parents have one affected child, the next three children should be unaffected.
- D. Because the parents have one affected child, the next child is four times more likely to be affected.
Correct Answer: A
Rationale: Each pregnancy has the same risks for an affected child. Because an odds ratio reflects the risk, this does not change over time. The statement by the genetic counselor refers to a probability. This does not change over time. The statement Because the parents have one affected child, the next child is four times more likely to be affected does not reflect autosomal recessive inheritance.
A couple asks the nurse about the optimal time for genetic counseling. They do not plan to have children for several years. When should the nurse recommend they begin genetic counseling?
- A. As soon as the woman suspects that she may be pregnant
- B. Whenever they are ready to start their family
- C. Now, if one of them has a family history of congenital heart disease
- D. Now, if they are members of a population at risk for certain diseases
Correct Answer: D
Rationale: Persons who seek genetic evaluation and counseling must first be aware if there is a genetic or potential problem in their families. Genetic testing should be done now if the couple is part of a population at risk. It is not feasible at this time to test for all genetic diseases. The optimal time for genetic counseling is before pregnancy occurs. During the pregnancy, genetic counseling may be indicated if a genetic disorder is suspected. Congenital heart disease is not a single-gene disorder.
Which is a birth defect or disorder that occurs as a new case in a family and is not inherited?
- A. Sporadic
- B. Polygenic
- C. Monosomy
- D. Association
Correct Answer: A
Rationale: Sporadic describes a birth defect previously unidentified in a family. It is not inherited. Polygenic inheritance involves the inheritance of many genes at separate loci whose combined effects produce a given phenotype. Monosomy is an abnormal number of chromosomes whereby the chromosome is represented by a single copy in a somatic cell. A nonrandom cluster of malformations without a specific cause is an association.
The nurse is teaching parents of a child with cri du chat syndrome about this disorder. The nurse understands parents understand the teaching if they make which statement?
- A. This disorder is very common.
- B. This is an autosomal recessive disorder.
- C. The crying pattern is abnormal and catlike.
- D. The child will always have a moon-shaped face.
Correct Answer: C
Rationale: Typical of this disease is a crying pattern that is abnormal and catlike. Cri du chat, or cats cry, syndrome is a rare (one in 50,000 live births) chromosome deletion syndrome, not autosomal recessive, resulting from loss of the small arm of chromosome 5. In early infancy this syndrome manifests with a typical but nondistinctive facial appearance, often a moon-shaped face with wide-spaced eyes (hypertelorism). As the child grows, this feature is progressively diluted, and by age 2 years, the child is indistinguishable from age-matched control participants.