Karyotype

Geneticists are able to photograph the nuclei of human cells and enlarge them enough to see the 46 chromosomes, or karyotype. These chromosomes are cut from the picture, matched in pairs, and grouped from large to small. The result is called a karyogram (karyo, nucleus; gram, chart). The karyogram of a normal individual shows 22 pairs of chromosomes called autosomes. These chromosomes, which are alike in males and females, direct the development of the individual. An example of an autosomal defect is Down syndrome, also known as trisomy 21 because these individuals have a third, or extra, number 21 chromosome.

The remaining pair of chromosomes are sex chromosomes. These differ in males and females, determining gender and secondary sexual characteristics. Defects in sex chromosomes are more prevalent than those in autosomes and account for a greater variety of abnormal conditions. The Y chromosome is small and apparently carries only the genes for masculinity. The X chromosome is much larger and carries the female genes plus many traits essential to life, such as those that direct various aspects of metabolism, blood formation, color blindness, and defense against bacteria. When the genes that cause a specific condition are known to be carried on the sex chromosomes, the disorder is sex-linked (Box 4-1).

Omissions and duplications of chromosomes can occur during meiosis (the cell division seen only in sex cells in which the chromosomes divide in half before the cell divides in two). When either a piece of a chromosome or an entire chromosome becomes joined to another chromosome, or when broken segments exchange places, the abnormality is termed a translocation.

Mutations, or accidental errors in duplication, rearrangement, or loss of parts of the DNA genetic code, are not completely understood. Once a gene becomes abnormal, the defect is repeated whenever the chromosome on which it appears reproduces itself during normal cell division. Radiation in the form of radiographs, radium, atomic energy, and isotopes can cause mutations. Because a defect may not appear for generations, the amount of radiation to which a person can safely be exposed is difficult to determine. New gene mutations may also occur. A mutation of a gene that directs the production of an enzyme can result in a disruption of the orderly process of metabolism. These biochemical disorders are termed inborn errors of metabolism. Without proper direction of the enzymes, harmful chemical products accumulate in the system. If a genetic mistake affects only an unimportant link in the metabolism chain or if the body otherwise compensates, no abnormal symptoms may occur, even though a gene is defective. Some examples of inherited pathologic conditions discussed in this text are cystic fibrosis, sickle cell anemia, and hemophilia.

Genetic Counseling and Research

As researchers gather more information concerning the mysteries of the gene, they are able to discover more ways to prevent and treat genetic mishaps. The role of the genetic counselor has broadened and taken on greater importance in recent years. Genetic counseling is the process whereby parents and families are counseled regarding the pattern of a gene’s transmission and the probability of occurrence or recurrence. Patterns of inheritance are known for hundreds of specific birth defects. Counselors often can suggest laboratory tests to determine whether prospective parents are carriers. A list of genetic counseling services can be obtained from the National Foundation/March of Dimes.

In specific genetic disorders in which a precise enzyme or protein is missing, it is often possible to supply the necessary factor. For example, clotting agents may be given to persons with hemophilia and pancreatic enzymes to those with cystic fibrosis. In other disorders, elimination of an offending substance can correct the problem. This is seen in phenylketonuria, in which the elimination of foods high in phenylalanine can prevent brain damage.

Medical research is on the threshold of important breakthroughs in the understanding and treatment of genetic disorders. The U.S. Human Genome Project, funded by the federal government, is locating DNA on genes. Biochemists can at this time create many of these genes in laboratories. As the genetic code is further deciphered, sending messages to the nuclei of cells and supplying the minute amount of DNA needed to correct a mistake will become possible. Gene therapy replacement is a highly experimental project that holds great potential for diseases that are the result of a defective gene, such as cystic fibrosis.

With the development of new technology, obstetric and pediatric providers are faced with questions about pediatric genetic disorders. The ethical and moral obligations for care of such disorders are a constant issue for providers. Definition of standards of care for those who receive genetic testing and counseling is crucial to this area.

Clarification of Terms

The stages of growth and development referred to in this chapter are as follows:

As growth refers to an increase in physical size, measured in inches/centimeters or pounds/kilograms, development refers to a progressive increase in the function of the body. The two are inseparable. Maturation (maturus, ripe) refers to the total way in which a person grows and develops, as dictated by inheritance. Although maturation is independent of environment, the timing of maturation may be affected by the environment.

Directional patterns are fundamental to the growth of all humans. Cephalocaudal development proceeds from head to toe (Figure 4-2). The infant is able to raise the head before he or she can sit and gains control of the trunk before walking. The second pattern is proximodistal development, or inner to outer. Development proceeds from the center of the body to the periphery. These patterns occur bilaterally. Development also proceeds from the general to the specific. The infant grasps with the hands before pinching with the fingers.

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