A. Cystic Fibrosis
B. Marfan’s Syndrome
C. Hemophilia A
D. Leber hereditary optic neuropathy
Ans:C. Hemophilia A
Determine if the pedigree chart shows an autosomal or X linked disease.
- If most of the males in the pedigree are affected, then the disorder is X-linked
- If it is a 50/50 ratio between men and women the disorder is autosomal.
Determine whether the disorder is dominant or recessive.
- If the disorder is dominant, one of the parents must have the disorder.
- If the disorder is recessive, neither parent has to have the disorder because they can be heterozygous.
X-Linked Recessive Inheritance is shown in the image.
X-LINKED RECESSIVE DISEASE
- As in autosomal recessive inheritance, two copies of a disease allele on the X chromosome are required for an individual with two X chromosomes (a female) to be affected with an X-linked recessive disease.
- Since males are hemizygous for X-linked genes (they have only one X chromosome), any male with one copy of an X-linked recessive disease allele is affected.
- Females are usually carriers because they only have one copy of the disease allele. Affected males are related through carrier females.
- For a carrier female, with each pregnancy there is a one in two (50%) chance her sons will inherit the disease allele and a one in two (50%) chance her daughters will be carriers.
- Affected males transmit the disease allele to all of their daughters, who are then carriers, but to none of their sons.
- Women are affected when they have two copies of the disease allele. All of their sons will be affected, and all of their daughters will be unaffected carriers.
ORGAN-BASED X-LINKED RECESSIVE DISEASES
- Alport syndrome (X-linked dominant disease)
- Bruton agammaglobulinemia
- Chronic granulomatous disease of childhood
- Duchenne muscular dystrophy
- Ehlers-Danlos syndrome (certain forms)
- Glucose-6-phosphatase deficiency
- Fragile X syndrome
- Hemophilia A
- Hemophilia B
- Hyper IgM syndrome
- Ocular albinism
- Wiskott-Aldrich syndrome