SEX-LINKED RECESSIVE
Alternative names:
genetics (sex-linked recessive); inheritance (sex-linked recessive); X-linked recessive
Information:
Definition:
An abnormal gene on the X chromosome from each parent is required to cause the disease in females since the female has 2 X chromosomes. In males there is only one X chromosome, therefore, a single recessive gene on the X chromosome will cause the disease. (Note: Although the Y chromosome is the other half of the XY gene pair in the male, the Y chromosome is only a portion of the X chromosome and doesn't protect the male. Therefore recessive genes on the X chromosome of the male will be expressed. This is seen in diseases such as hemophilia and muscular dystrophy.)
BACKGROUND:
The inheritance of genetic diseases, abnormalities, or traits is described by both the type of chromosome the abnormal gene resides on (autosomal or sex chromosome) and by whether the gene itself is dominant or recessive.
Autosomally inherited diseases are inherited through the non-sex chromosomes, pairs 1 through 22. Sex-linked diseases are inherited through one of the "sex chromosomes", the X chromosome (diseases are not inherited through the Y chromosome).
Dominant inheritance occurs when an abnormal gene from ONE parent is capable of causing disease even though the matching gene from the other parent is normal. The abnormal gene dominates the outcome of the gene pair.
Recessive inheritance occurs when BOTH matching genes must be abnormal to produce disease. If only one gene in the pair is abnormal the disease is not manifest or is only mildly manifest; however the disease can be passed on to the children.
STATISTICAL CHANCES OF INHERITING A TRAIT:
For an X-linked recessive disorder:- If only the mother carries the gene and the father is normal, all of the female children will be normal (50% with 2 normal chromosomes and 50% carriers), one half of all the male children will exhibit the disease, and one half will be normal. The recessive gene is expressed in the male because there is not another X to counteract it, only the Y (which determines for maleness).
- If only the father caries the recessive gene, all of his daughters will be carriers and all of his sons will be normal
- If both the mother and the father carry the abnormal gene, then STATISTICALLY out of 4 children 1 daughter will have the disease (two recessive genes on the X chromosome), 1 daughter will be a carrier, 1 son will have the disease (one recessive gene on the X and a Y chromosome) and the other son will be normal. In other words, 50% of the children (boys and girls) will be affected and 50% normal.
In other words, if it is assumed that 4 children are produced (2 boys and 2 girls), the mother is a carrier (one abnormal X but no disease), and the father is normal, the STATISTICAL expectation is for:- 1 boy normal
- 1 boy with disease
- 1 girl normal
- 1 girl carrier without disease
If it is assumed that 4 children are produced (2 boys and 2 girls), the father is a carrier (1 abnormal X, he has the disease), and the mother is normal, the STATISTICAL expectation is for:- 2 boys normal
- 2 girls carriers without disease
If it is assumed that 4 children are produced (2 boys and 2 girls), the father is a carrier (1 abnormal X, he has the disease), and the mother is a carrier (one abnormal X but no disease), the STATISTICAL expectation is for:- 1 girl with disease
- 1 girl carrier without disease
- 1 boy (abnormal X) with disease
- 1 boy normal
This does not mean that children WILL necessarily be affected; it does mean that EACH child has a chance of inheriting the disorder or of being a carrier.
RELATED TOPICS:
autosomal dominant
autosomal recessive
genetic counseling and prenatal diagnosis
sex-linked dominant
For detailed information, see heredity and disease (genetics).
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