Chapter Overview

A young couple is planning to have children. The husband knows that his grandmother had a child with Down syndrome by a second marriage. Down syndrome is a set of physical and mental disorders caused by the presence of an extra chromosome 21 (Figure 15-1). No records of the birth, which occurred early in the twentieth century, are available, but the couple knows of no other cases of Down syndrome in their families.

The couple has heard that Down syndrome results from a rare chance mistake in egg production and therefore decide that they stand only a low chance of having such a child. They decide to have children. Their first child is unaffected, but the next conception aborts spontaneously (a miscarriage), and their second child is born with Down syndrome. Was this a coincidence, or is it possible that there is a connection between the genetic makeup of the man and that of his grandmother that led to their both having Down syndrome children? Was the spontaneous abortion significant? What tests might be necessary to investigate this situation? The analysis of such questions is the topic of this chapter.

We saw in Chapter 10 that gene mutations are one source of genomic change. However, the genome can also be remodeled on a larger scale by alterations to chromosome structure or by changes in the number of copies of chromosomes in a cell. These large-scale variations are termed chromosome mutations to distinguish them from gene mutations. Broadly speaking, gene mutations are defined as changes that take place within a gene, whereas

Figure 15-1 Children with Down syndrome.

[Bob Daemmrich/The Image Works.]

Figure 15-1 Children with Down syndrome.

[Bob Daemmrich/The Image Works.]

chromosome mutations are changes to a chromosome region encompassing multiple genes. Chromosome mutations can be detected by microscopic examination, genetic analysis, or both. In contrast, gene mutations are never detectable microscopically; a chromosome bearing a gene mutation looks the same under the microscope as one carrying the wild-type allele. Chromosome mutations

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