Question
Identify the type of inheritance shown in the Pedigree analysis.
A. Autosomal Dominant
B. Autosomal Recessive
C. X linked Dominant
D. X linked Recessive
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Correct Answer » D Explanation |
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Ans:D. X linked recessive.
Modes of Inheritance
- Inheritance patterns for single gene disorders are classified based on whether they are autosomal or X-linked and whether they have a dominant or recessive pattern of inheritance. These disorders are called Mendelian disorders.
Autosomal Dominant Inheritance
- Only one copy of a disease allele is necessary for an individual to be susceptible to expressing the phenotype.
- With each pregnancy, there is a one in two (50%) chance the offspring will inherit the disease allele.
- Unless a new mutation has occurred, all affected individuals will have at least one parent who carries the disease allele.
- Autosomal dominant inheritance is often called vertical inheritance because of the transmission from parent to offspring.
- Across a population, the proportion of affected males should be equal to the proportion of affected females.
- Male-to-male transmission can be observed.
- Examples of diseases with autosomal dominant inheritance include myotonic muscular dystrophy and Huntington disease.
Autosomal Recessive Inheritance
- In autosomal recessive inheritance, two copies of a disease allele are required for an individual to be susceptible to expressing the phenotype.
- Typically, the parents of an affected individual are not affected but are gene carriers.
- With each pregnancy of carrier parents:
- There is a one in four (25%) chance the offspring will inherit two copies of the disease allele and will therefore have the phenotype.
- There is a one in two (50%) chance the offspring will inherit one copy of the disease allele and will be a carrier.
- There is a one in four (25%) chance the offspring will inherit no copies of the disease allele and will not express the phenotype or be a carrier. This individual would not be at risk for passing the disorder on to his/her offspring.
- As with autosomal dominant inheritance, the proportion of affected males should be equal to the proportion of affected females in a given population.
- Examples of diseases with autosomal recessive inheritance include sickle cell anemia and cystic fibrosis.
X-Linked Dominant Inheritance
- As in autosomal dominant inheritance, only one copy of a disease allele on the X chromosome is required for an individual to be susceptible to an X-linked dominant disease.
- Both males and females can be affected, although males may be more severely affected because they only carry one copy of genes found on the X chromosome.
- When a female is affected, each pregnancy will have a one in two (50%) chance for the offspring to inherit the disease allele. When a male is affected, all his daughters will be affected, but none of his sons will be affected.
- Examples of diseases with X-linked dominant inheritance are hypophosphatemic ricketsm, oral-facial-digital syndrome type I, and Fragile X syndrome.
X-Linked 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.
- Examples of diseases with X-linked recessive inheritance include Duchenne muscular dystrophy, hemophilia A and hypohidrotic or anhidrotic ectodermal dysplasia.
