What is the inheritance pattern of factor IX deficiency?

What is the inheritance pattern of factor IX deficiency?

• A. Autosomal dominant
• B. Autosomal recessive
• C. X-linked recessive
• D. Mitochondrial

Answer: (C)

Factor IX deficiency (hemophilia B) is inherited in an X-linked recessive pattern. The gene for factor IX is on the X chromosome, so males have one X and one Y. A single mutated X leads to disease in males because there is no second X to compensate. Females have two X chromosomes, so a single mutated allele usually makes them carriers rather than affected. A carrier mother has a 50% chance with each pregnancy to pass the mutated X to a son (who would be affected) and a 50% chance to pass it to a daughter (who would be a carrier). Since fathers pass their Y chromosome to sons, there is no father-to-son transmission of the defect. Many cases arise from new mutations, so a family history may be absent. Other inheritance patterns don’t fit because autosomal dominant would affect both sexes across generations, autosomal recessive would require both parents to be carriers, and mitochondrial inheritance would come from the mother and affect all her children, which doesn’t describe X-linked recessive hemophilia.

Factor IX deficiency (hemophilia B) is inherited in an X-linked recessive pattern. The gene for factor IX is on the X chromosome, so males have one X and one Y. A single mutated X leads to disease in males because there is no second X to compensate. Females have two X chromosomes, so a single mutated allele usually makes them carriers rather than affected. A carrier mother has a 50% chance with each pregnancy to pass the mutated X to a son (who would be affected) and a 50% chance to pass it to a daughter (who would be a carrier). Since fathers pass their Y chromosome to sons, there is no father-to-son transmission of the defect. Many cases arise from new mutations, so a family history may be absent. Other inheritance patterns don’t fit because autosomal dominant would affect both sexes across generations, autosomal recessive would require both parents to be carriers, and mitochondrial inheritance would come from the mother and affect all her children, which doesn’t describe X-linked recessive hemophilia.