Sex-linked diseases are passed down through families through one of the X or Y chromosomes. X and Y are sex chromosomes.
Dominant inheritance occurs when an abnormal gene from one parent causes disease, even though the matching gene from the other parent is normal. The abnormal gene dominates.
But in recessive inheritance, both matching genes must be abnormal to cause disease. If only one gene in the pair is abnormal, the disease does not occur or it is mild. Someone who has one abnormal gene (but no symptoms) is called a carrier. Carriers can pass abnormal genes to their children.
The term "sex-linked recessive" most often refers to X-linked recessive.
Inheritance - sex-linked recessive; Genetics - sex-linked recessive; X-linked recessive
X-linked recessive diseases most often occur in males. Males have only one X chromosome. A single recessive gene on that X chromosome will cause the disease.
The Y chromosome is the other half of the XY gene pair in the male. However, the Y chromosome doesn't contain most of the genes of the X chromosome. Because of that, it doesn't protect the male. Diseases such as hemophilia and Duchenne muscular dystrophy occur from a recessive gene on the X chromosome.
In each pregnancy, if the mother is a carrier of a certain disease (she has only one abnormal X chromosome) and the father is not a carrier for the disease, the expected outcome is:
- 25% chance of a healthy boy
- 25% chance of a boy with disease
- 25% chance of a healthy girl
- 25% chance of a carrier girl without disease
If the father has the disease and the mother is not a carrier, the expected outcomes are:
- 100% chance of a healthy boy
- 100% chance of a carrier girl without disease
X-LINKED RECESSIVE DISORDERS IN FEMALES
Females can get an X-linked recessive disorder, but this is very rare. An abnormal gene on the X chromosome from each parent would be required, since a female has two X chromosomes. This could occur in the two scenarios below.
In each pregnancy, if the mother is a carrier and the father has the disease, the expected outcomes are:
- 25% chance of a healthy boy
- 25% chance of a boy with the disease
- 25% chance of a carrier girl
- 25% chance of a girl with the disease
If both the mother and the father have the disease, the expected outcomes are:
- 100% chance of the child having the disease, whether boy or girl
The odds of either of these two scenarios are so low that X-linked recessive diseases are sometimes referred to as male only diseases. However, this is not technically correct.
Female carriers can have a normal X chromosome that is abnormally inactivated. This is called "skewed X-inactivation." These females may have symptoms similar to those of males, or they may have only mild symptoms.
Feero WG, Zazove P, Chen F. Clinical genomics. In: Rakel RE, Rakel DP, eds. Textbook of Family Medicine. 9th ed. Philadelphia, PA: Elsevier Saunders; 2016:chap 43.
Groden J, Gocha AS, Croce CM. Human basic genetics and patterns of inheritance. In: Creasy RK, Resnik R, Iams JD, Lockwood CJ, Moore TR, Greene MF, eds. Creasy and Resnik's Maternal-Fetal Medicine: Principles and Practice. 7th ed. Philadelphia, PA: Elsevier Saunders; 2014:chap 1.
Jorde LB, Carey JC, Bamshad MJ. Sex-linked and nontraditional modes of inheritance. In: Jorde LB, Carey JC, Bamshad MJ, eds. Medical Genetics. 5th ed. Philadelphia, PA: Elsevier; 2016:chap 5.
Korf BR. Principles of genetics. In: Goldman L, Schafer AI, eds. Goldman-Cecil Medicine. 25th ed. Philadelphia, PA: Elsevier Saunders; 2016:chap 40.