Читать книгу Child Development From Infancy to Adolescence - Laura E. Levine - Страница 182

Some genetically based disorders result from a single gene. You have already learned about sickle-cell disease and Tay-Sachs disease. Two other such diseases are phenylketonuria (PKU) and cystic fibrosis. Phenylketonuria is a condition in which the child cannot digest a common protein in the human diet. This condition can result in intellectual disability (U.S National Library of Medicine, 2017b). In cystic fibrosis the child’s body produces a thick, sticky mucus that clogs the lungs, making the child vulnerable to pulmonary infections. It is also associated with nutritional deficiencies (Elborn, 2016).

Single-gene disorders can occur in two ways: (1) An individual inherits a pair of recessive genes that carry the instructions for that disorder, or (2) mutations occur as cells divide so that some of the bases that give the instructions to create proteins are out of order or missing. In the case of cystic fibrosis, the genetic cause of the disorder is a missing sequence (or what we called a deletion earlier in this chapter) in a specific gene called the CFTR gene. The normal sequence is ATCATCTTTGGTGTT. If the three bases highlighted here are missing on one chromosome, the child will not develop cystic fibrosis. However, the child will develop the disease if he inherits this mutation from both parents (U.S. National Library of Medicine, 2017a).

Single-gene disorders: Genetic disorders caused by a single recessive gene or mutation.

Many genetic disorders are based on recessive genes, but most of the time the recessive gene is paired with a dominant gene that does not carry the disorder so the information in the dominant gene protects the individual from developing the disorder. One student put this succinctly: “If one gene is screwed up, you have a backup.” As long as the dominant gene is doing its job, the dysfunctional gene will likely not be noticed. However, in one situation a single recessive gene will be expressed because there is no second gene to create a pair. As you can see in the photo on this page, the Y chromosome is much smaller than the X chromosome and contains only 50 to 60 genes, the fewest of all the chromosomes (U.S. National Library of Medicine, 2017e). By comparison, the X chromosome contains 800 to 900 genes (U.S. National Library of Medicine, 2017d). In addition, only some of the genes on the Y chromosome are active. Thus, when an X chromosome pairs with a Y chromosome to create a boy, some of the X chromosome’s active genes will not find a partner on the Y chromosome. These genes will be expressed whether they are normally recessive or dominant. The outcome is increased vulnerability in boys to the effects of recessive genes on the X chromosome that cause such problems as red-green color blindness, hemophilia, and Duchenne muscular dystrophy (Jorde, Carey, Bamshad, & White, 2006).

T/F #5

Males are more likely to have a genetic disorder than females. True

X and Y chromosomes. Do you see the potential problem when the X chromosome and the Y chromosome pair up? Large portions of the X chromosome (the one on the right) do not have a partner on the Y chromosome (the one on the left), and therefore any recessive gene on the X chromosome without a partner will appear in the male’s phenotype. If the recessive gene is the source of a genetic problem, the man is vulnerable to it.

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