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A special instance of the dominant–recessive pattern occurs with genes that are located on the X chromosome (Shah, DeRemigis, Hageman, Sriram, & Waggoner, 2017). Recall that males (XY) have both an X and a Y chromosome. Some recessive genetic disorders, like the gene for red-green colorblindness, are carried on the X chromosome. Males are more likely to be affected by X-linked genetic disorders because they have only one X chromosome and therefore any genetic marks on their X chromosome are displayed. Females (XX) have two X chromosomes; a recessive gene located on one X chromosome will be masked by a dominant gene on the other X chromosome. Females are thereby less likely to display X-linked genetic disorders because both of their X chromosomes must carry the recessive genetic disorder for it to be displayed.

Hemophilia, a condition in which the blood does not clot normally, is another example of a recessive disease inherited through genes on the X chromosome (Shah et al., 2017). Daughters who inherit the gene for hemophilia typically do not show the disorder because the gene on their second X chromosome promotes normal blood clotting and is a dominant gene. Females, therefore, can carry the gene for hemophilia without exhibiting the disorder. A female carrier has a 50/50 chance of transmitting the gene to each child. Sons who inherit the gene will display the disorder because the Y chromosome does not have the corresponding genetic information to counter the gene. Daughters who inherit the gene, again, will be carriers (unless their second X chromosome also carries the gene). Table 2.4 illustrates diseases acquired through X-linked inheritance.

Table 2.4

Source: McKusick-Nathans Institute of Genetic Medicine (2019).

In contrast, fragile X syndrome is an example of a dominant disorder carried on the X chromosome (Hagerman et al., 2017). Because the gene is dominant, it need appear on only one X chromosome to be displayed. That means that fragile X syndrome occurs in both males and females, although females tend to experience more mild symptoms. Males with fragile X syndrome typically have large ears, large testes, and a long, narrow face. Fragile X syndrome is the most common known inherited form of intellectual disability (Doherty & Scerif, 2017), and children with fragile X syndrome tend to show moderate to severe intellectual disability (Raspa, Wheeler, & Riley, 2017). Cardiac defects are common as well as several behavioral mannerisms, including poor eye contact and repetitive behaviors such as hand flapping, hand biting, and mimicking others, behaviors common in individuals with autistic spectrum disorders (Hagerman et al., 2017). Fragile X syndrome is often codiagnosed with autism, with estimates of 30% to 54% of boys and 16% to 20% of girls with fragile X syndrome meeting the diagnostic criteria for autism (Kaufmann et al., 2017).