Anatomy & Physiology

Anatomy & Physiology Study Guide

Sex-Linked Inheritance Unlike the other 22 chromosomal pairs, the sex chromosomes are never identical in appearance and gene content. There are two types of sex chromosomes: an X chromosome and a Y chromosome. X chromosomes have more genes and are larger than Y chromosomes . The normal pair of sex chromosomes in males is XY. Females do not have a Y chromosome; their sex chromosome pair is XX. All oocytes carry an X chromosome because the only sex chromosomes females have are X chromosomes. Each sperm carries either an X or a Y chromosome because males have one of each and can pass along either one. Since a Y-chromosome sperm does not have to carry the extra weight of the larger X chromosome, it has been suggested that they reach the oocyte first and, thus, more eggs are fertilized with Y chromosomes. A number of clinical disorders are X-linked traits, including certain forms of muscular dystrophy, hemophilia, and diabetes insipidus. In several instances, advances in molecular genetics techniques have enabled geneticists to localize the specific genes on the X chromosome. Before signs and symptoms appear, these techniques provide a relatively direct method of screening for the presence of a particular condition. Chromosomal Abnormalities and Genetic Analysis Embryos that have abnormal autosomal chromosomes rarely survive. However, translocation defects and trisomy are two types of autosomal chromosome abnormalities that do not invariably result in prenatal death. In a translocation defect , crossing over occurs between different chromosome pairs such that, for example, a piece of chromosome 8 may become attached to chromosome 14. The genes moved to their new position may function abnormally, becoming inactive or overactive. Embryos may survive in a balanced translocation, where there is no net loss or gain of chromosomal material. In trisomy , the issue occurs during meiosis. One of the gametes involved in fertilization carries an extra copy of one chromosome; thus, the zygote then has three copies of this chromosome rather than two. (The nature of the trisomy is indicated by the number of the chromosome involved. Thus, individuals with trisomy 13 have three copies of chromosome 13.) Zygotes with extra copies of chromosomes seldom survive. Individuals with trisomy 13 and trisomy 18 may survive until delivery but rarely live longer than a year. The notable exception is trisomy 21. Trisomy 21 , or Down syndrome, is the most common viable chromosomal abnormality. Estimates of its incidence in the U.S. population range from 1.5 to 1.9 per 1000 births. Affected individuals exhibit mental retardation and characteristic physical malformations, including a facial appearance that gave rise to the term mongolism, once used to describe this condition. The degree of mental retardation ranges from moderate to severe. Yet, some individuals with this condition lead somewhat independent lives. Anatomical problems affecting the cardiovascular system often prove fatal during childhood or early adulthood. Although some individuals survive to moderate old age, many develop Alzheimer disease while still relatively young (before age 40).

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