1. You send a blood sample from a dysmorphic infant to the chromosome laboratory for analysis. The laboratory’s report states that the child’s karyotype is 46,XY,del(18)(q12).

a. What does this karyotype mean?

b. The laboratory asks for blood samples from the clinically normal parents for analysis. Why?

c. The laboratory reports the mother’s karyotype as 46,XX and the father’s karyotype as 46,XY,t(7;18)(q35;q12). What does the latter karyotype mean? Referring to the normal chromosome ideograms in Figure 5-2, sketch the translocation chromosome or chromosomes in the father and in his son. Sketch these chromosomes in meiosis in the father. What kinds of gametes can he produce?

d. In light of this new information, what does the child’s karyotype mean now? What regions are monosomic? trisomic? Given information from Chapters 2 and 3, estimate the number of genes present in the trisomic or monosomic regions.

2. A spontaneously aborted fetus is found to have trisomy 18.

a. What proportion of fetuses with trisomy 18 are lost by spontaneous abortion?

b. What is the risk that the parents will have a liveborn child with trisomy 18 in a future pregnancy?

3. A newborn child with Down syndrome, when karyotyped, is found to have two cell lines: 70% of her cells have the typical 47,XX,+21 karyotype, and 30% are normal 46,XX. When did the nondisjunctional event probably occur? What is the prognosis for this child?

4. Which of the following persons is or is expected to be phenotypically normal?

a. A female with 47 chromosomes, including a small supernumerary chromosome derived from the centromeric region of chromosome 15

b. A female with the karyotype 47,XX,+13

c. A male with deletion of a band on chromosome 4

d. A person with a balanced reciprocal translocation

e. A person with a pericentric inversion of chromosome 6

What kinds of gametes can each of these individuals produce? What kinds of offspring might result, assuming that the other parent is chromosomally normal?

5. For each of the following, state whether chromosome or genome analysis is indicated or not. For which family members, if any? For what kind of chromosome abnormality might the family in each case be at risk?

a. A pregnant 29-year-old woman and her 41-year-old husband, with no history of genetic defects

b. A pregnant 41-year-old woman and her 29-year-old husband, with no history of genetic defects

c. A couple whose only child has Down syndrome

d. A couple whose only child has cystic fibrosis

e. A couple who has two boys with severe intellectual disability

6. Explain the nature of the chromosome abnormality and the method of detection indicated by the following nomenclature.

a. inv(X)(q21q26)

b. 46,XX,del(1)(1qter → p36.2:)

c. 46,XX.ish del(15)(q11.2q11.2)(SNRPN−,D15S10−)

d. 46,XX,del(15)(q11q13).ishdel(15)(q11.2q11.2)(SNRPN−,D15S10−)

e. 46,XX.arrcgh1p36.3(RP11-319A11,RP11-58A11,RP11-92O17) × 1

f. 47,XY,+mar.ish r(8)(D8Z1+)

g. 46,XX,rob(13;21)(q10;q10),+21

h. 45,XY,rob(13;21)(q10;q10)

7. Using the nomenclature system in Table 5-1, describe the “molecular karyotypes” that correspond to the microarray data in Figures 5-6C and 5-9C.

a. Referring to Figure 5-6C, is the individual whose array result is shown a male or a female? How do you know?

b. Referring to Figure 5-9C, is the individual whose array result is shown a male or a female? How do you know?