Sex linked is a trait in which a gene is located on a sex chromosome. In humans, the term generally refers to traits that are influenced by genes on the X chromosome. This is because the X chromosome is large and contains many more genes than the smaller Y chromosome. In a sex-linked disease, it is usually males who are affected because they have a single copy of X chromosome that carries the mutation. In females, the effect of the mutation may be masked by the second healthy copy of the X chromosome.
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Sex linked... These are traits that are found on either one of the chromosomes that determine sex, or the sex chromosomes. And in humans this is the X or the Y chromosomes. And so some of the more familiar sex-linked traits are hemophilia, red-green color blindness, congenital night blindness, some high blood pressure genes, Duchenne muscular dystrophy, and also Fragile X syndrome. So what's also very interesting is that you can imagine that for individuals who are XY or males, having these different mutations on the genes, on the X chromosome, is particularly problematic, because unlike females, there are not two X chromosomes that give you the potential of carrying a normal gene on the X chromosome. Which is why in many cases you'll see that males are more often afflicted with these sex-linked disorders.
Carla Easter, Ph.D.
Deputy Chief, Education and Community Involvement Branch
Dr. Easter is the education specialist with the Education and Community Involvement Branch of the National Human Genome Research Institute. From 2003 to 2006, she was director of outreach for Washington University School of Medicine's Genome Sequencing Center in St. Louis. Before assuming her position at the NIH, Dr. Easter was a research associate in the Department of Education at Washington University, where she explored the notions of science among secondary students, educators and administrators. She served as project associate for the Quality Education for Minorities Network and the Pre-College Coordinator for the NASA Summer High School Apprenticeship Research Plus program.