Child Development From Infancy to Adolescence. Laura E. Levine
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.
BIOPHOTO ASSOCIATES via Getty Images
Chromosome Disorders
Genetic disorders can also occur at the level of the chromosome. Chromosome disorders occur when one of the 23 pairs of chromosomes contains a number of chromosomes other than two. For example, Figure 3.5 shows the chromosomal configuration of an individual with Down syndrome. In the 21st position, there are three chromosomes rather than two. In another condition, Turner syndrome, there is only a single chromosome in the 23rd position in a female. Any deviation from the 46 chromosomes that are normally contained in 23 pairs will result in a chromosome disorder. Because a single chromosome contains a large number of genes, any deviation—whether an addition or a loss—results in a number of characteristics being affected.
Chromosome disorders: Disorders that result when too many or too few chromosomes are formed or when there is a change in the structure of the chromosome caused by breakage.
Figure 3.5 Chromosomes for Down syndrome.
Source: Biophoto Associates/Photo Researchers, Inc.
The second way a chromosome disorder occurs is when there is a change in the structure of the chromosome caused by breakage (NHGRI, 2016b). When sections of chromosomes break apart, they may not come back together in their original form. Some sections may be backward or may even link to a different chromosome. Both these types of abnormality may occur by chance, but the second type also can be passed along to a child by a parent who has such a damaged chromosome. Table 3.1 describes a number of conditions caused by chromosome abnormalities.
Table 3.1
Source: NHGRI (2013b, 2013c, 2016d, 2017).
As with all disorders, there is a range of effects resulting from chromosome disorders. For instance, children with Down syndrome have intellectual impairment, but some are profoundly impaired while others are better able to lead productive lives. It is impossible to tell at birth what level a child will reach. Therefore, it is important that these infants and their parents receive early intervention to help them achieve as much as they can.
Down syndrome. This boy has facial features that are typically associated with Down syndrome, including small upturned eyes, small ears, and a flat facial profile.
JSCook via Getty Images
Multifactorial Inheritance Disorders
Multifactorial inheritance disorders result from the interaction of many genes that also interact with environmental influences. For instance, disorders such as depression, schizophrenia, and attention deficit/hyperactivity disorder (ADHD) are believed to result from the interaction of many genes with environmental influences. To discover which genes are implicated in these disorders, genome-wide association tests are being carried out comparing the genomes of individuals who have each disorder with similar individuals who do not, but this research is in its very early stages (NHGRI, 2015b). However individuals can take preventative steps when they know from their family history that they are at higher risk of developing a disorder. For example, an individual who has many family members with major depression can learn to identify those symptoms early to get appropriate treatment if needed.
Multifactorial inheritance disorders: Disorders that result from the interaction of many genes and environmental influences.
Genetic Counseling and Testing
People become particularly concerned about genetics when they are about to start a family. In each pregnancy, any couple statistically has a 3% chance of having a child with a genetically based disorder (CDC, 2017b). Based on these low odds, there is usually no reason for genetic counseling. However, in some cases individual risk is higher, and individuals may want to seek out a genetic counselor to help them assess the type and amount of risk. See Table 3.2 for additional information about couples who might consider having genetic counseling because they are at increased risk of conceiving a child with a genetic disorder or birth defect.
Table 3.2
Source: March of Dimes (2017). Used with permission.
Genetic counseling and testing can occur before or during a pregnancy. Counselors ask about the couple’s own medical histories and their families’ history of diseases and genetic disorders and may recommend certain tests. There are tests that can identify whether an individual is a carrier of one recessive gene for a disorder. As you learned in Active Learning: Understanding the Inheritance of Tay-Sachs Disease, a child will only have the disease if both partners