Every human pregnancy that goes to full term in North America and other modern industrialized nations has a small risk (about 2-3%) of serious birth defects. This is the case even when the mother is young and healthy. However, some pregnancies are at a higher risk due to a variety of factors. Fortunately, it is now possible to connect some of the inherited genetic defects to specific chromosome irregularities. This can be done by examining small tissue samples from adults, children, or even unborn babies. The samples are cultured to induce mitosis so that the chromosomes become visible. In this state, the chromosomes can be photographed. The images are then converted into a karyotype of the individual from whom the sample was taken. This involves the precise measurements of chromatid length ratios and other morphological features so that they can be placed into homologous pairs.
Cells cultured to produce a karyotype
(Much of this process is now facilitated with computers.)
On comparison with a normal karyotype, major abnormalities can be discovered in a suspect sample. Irregular numbers of chromosomes and missing or deformed chromatids are easily observed. Many types of gross chromosomal abnormalities have been connected to specific medical syndromes , including mental retardation.
By sampling embryonic or fetal cells within the uterus of a mother, it is possible to determine whether or not her child will be born with serious genetic problems. More than 80 common types of chromosomal abnormalities are now identifiable by this method. The unborn child's gender can also be determined by noting whether its karyotype includes X and Y sex chromosomes or only X's. Gender is important because of the higher probability of males expressing X-linked traits in their phenotypes.
By this simple examination of karyotypes, it is not possible to discover mutations that occur at the gene level. That is to say, mutations in the DNA molecules that make up chromosomes are not usually detected by this process. Since it is likely that most of the more than 5,000 known genetic disorders are at this lower level, karyotype analysis is limited. However, those disorders that are due to gross chromosomal irregularities are comparatively common and often serious. As a result, karyotype analysis and other associated tests have become important medical tools.
NOTE: There are now DNA tests for more than 1500 genetically inherited human diseases and problem conditions. They are based on the discovery of specific telltale genetic markers in DNA samples rather than the examination of karyotypes. These tests generally are not done unless there is an above average risk of inheriting such conditions.
NOTE: A mother's risk of having a baby with serious birth defects such as neural tube defects and sickle-cell anemia is roughly double in the poor nations of Africa, Asia, and Latin America where prenatal screening and care are not readily available. About 7.9 million babies around the world are born with such defects every year and roughly 3.3 million children under the age of five die from them annually (Lauran Neergaard, AP medical writer 1/31/06).
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