Cells missing a chromosome or having an extra one are aneuploid. A normal chromosome number is termed eu-ploid. Symptoms that result from aneuploidy depend upon which chromosome is missing or extra. Autosomal aneuploidy often results in mental retardation, possibly because so many genes affect brain function. Sex chromosome aneuploidy is less severe.

Extra genetic material is apparently less dangerous than missing material, and this is why most children born with the wrong number of chromosomes have an extra one, called a trisomy, rather than a missing one, called a monosomy. Aneuploid conditions have historically been named for the researchers or clinicians who identified them, but today chromosome designations are preferred because they are more precise. Down syndrome, for example, refers to a distinct set of symptoms that are usually caused by trisomy 21. However, the syndrome may also arise from one copy of chromosome 21 exchanging parts with a different chromosome, which is a type of aberration called a translocation. Knowing whether a child with these symptoms has trisomy 21 or translocation Down syndrome is very important in a practical sense, because the probability of trisomy 21 recurring in a sibling is about 1 in 100, but the chance of translocation Down syndrome recurring is considerably greater. Clinical Application 24.2 takes a closer look at trisomy 21.

Aneuploidy results from a meiotic error called nondisjunction (non"dis-jungk'shun) (fig. 24.14). In normal meiosis, pairs of homologous chromosomes separate, and each of the resulting gametes contains only one member of each pair. In nondisjunction, a chromosome pair fails to separate, either at the first or at the second meiotic division, producing a sperm or egg that has two copies of a particular chromosome or none, rather than the normal one copy. When such a gamete fuses with its mate at fertilization, the resulting zygote has either 47 or 45 chromosomes, instead of the normal 46.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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