Glossary

X

X-linked dominant trait:

Human traits, such as an individual’s specific blood group, eye color, or expression of certain diseases, result from the interaction of one gene inherited from the mother and one from the father. In X-linked dominant disorders, the gene mutation for the disease trait is transmitted as a dominant gene on the X chromosome and therefore may “override” the instructions of the normal gene on the other chromosome, resulting in expression of the disease. Because females have two X chromosomes yet only one functioning copy is necessary, a percentage of X chromosomes with the mutated gene may not be expressed (random X chromosome inactivation). As a result, females often have less severe symptoms than affected males. In contrast, because males have one X chromosome from the mother and one Y chromosome from the father, those who inherit an X-linked dominant disease trait typically fully express the mutated gene on the X chromosome, causing a more severe form of the disorder that may result in lethality before or shortly after birth. Fathers with an X-linked dominant trait transmit the gene to their daughters but not to their sons. Mothers with a single copy of an X-linked dominant gene have a 50 percent risk of transmitting the gene to their daughters as well as to their sons.

Human traits, such as a person’s eye color, specific blood group, or expression of certain diseases, result from the interaction of one gene inherited from the mother and one from the father. In X-linked recessive disorders, the gene mutation for the disease trait is located on the X chromosome. Because females have two X chromosomes yet only one functioning copy is necessary, a percentage of X chromosomes with the mutated gene may not be expressed (random X chromosome inactivation). However, because males have one X chromosome from the mother and one Y chromosome from the father, those who inherit an X-linked recessive disease trait typically fully express the mutated gene on the X chromosome. Some females who carry a single copy of the disease gene (heterozygous carriers) may have certain symptoms associated with the disorder; however, such findings are usually more variable and less severe than those seen in affected males. Fathers with an X-linked recessive trait may transmit the gene to their daughters but not to their sons. Mothers with an X-linked recessive gene have a 50 percent risk of transmitting the gene to their daughters and their sons.

X-linked recessive trait: 

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