Extra or missing chromosomes constitute aneuploidy. Unequal division of chromosome pairs into sperm and egg cells can occur at either the first or the second meiotic division. (a) A single pair of chromosomes is unevenly partitioned into the two cells arising from the first division of meiosis in a male. The result: two sperm cells that have two copies of the chromosome and two sperm cells that have no copies of that chromosome. When a sperm cell with two copies of the chromosome fertilizes a normal egg cell, the zygote produced is trisomic for that chromosome; when a sperm cell lacking the chromosome fertilizes a normal egg cell, the zygote is monosomic for that chromosome. Symptoms depend upon which chromosome is involved. (b) This nondisjunction occurs at the second meiotic division. Because the two products of the first division are unaffected, two of the mature sperm are normal, and two are aneuploid. Egg cells can undergo nondisjunction as well, leading to zygotes with extra or missing chromosomes when they are fertilized by normal sperm cells.

funding. However, researchers perform such pervasive changes in other species to create transgenic organisms. For example, mice engineered to harbor a human disease-causing gene in each cell are commonly used to study the early stages of human diseases and to test treatments.

In contrast is nonheritable gene therapy, also called somatic gene therapy, which targets only affected cells and therefore cannot be transmitted to the next generation. Nonheritable gene therapy for hemophilia, for example, provides genes that encode the needed clotting factors. One type of nonheritable gene therapy for cystic fibrosis is an aerosol containing a virus that has had its pathogenic genes removed and the normal CFTR gene added. When the person inhales the aerosol, the needed gene enters airway epithelium, providing instructions to replace the nonfunctional ion channel protein that causes the symptoms.

Experiments to develop gene therapies have been ongoing since 1990, with thousands of patients participating, to varying degrees of success. For example, gene therapy for hemophilia has been successful in a few

Shier-Butler-Lewis: I VI. The Human Life Cycle I 24. Genetics and Genomics I I © The McGraw-Hill

Human Anatomy and Companies, 2001

Physiology, Ninth Edition


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