Question

Question #a5cdf

Answer 1

The easiest way to solve a pedigree question is to first look for the trait being dominant/recessive and then check for autosomal or X-linked inheritance

Explanation:

**

• Step 1:
  
  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.
  

**

• Step 2:
  **
• *Determine if the pedigree chart shows an autosomal or Xlinked
  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.
  

* * Rules of Inheritance: * *

Autosomal Recessive:

• Appears in both sexes with equal frequency.
• Trait tend to skip generations.
• Affected offspring are usually born to unaffected parents.
• When both parents are heterozygous, approx. 1/4 of the progeny will be affected.
• Appears more frequently among the children of consanguinous marriages.

Autosomal Dominant:

• Appears in both sexes with equal frequency.
• Both sexes transmit the trait to their offspring.
• Does not skip generations.
• Affected offspring must have an affected parent unless they posses a new mutation.

X-Linked Dominant:

• Both males and females are affected; often more females than males are affected.
• Does not skip generations.
• Affected sons must have an affected mother; affected daughters must have either an affected mother or an affected father.
• Affected fathers will pass the trait on to all their daughters.
• Affected mothers if heterozygous will pass the trait on to 1/2 of their sons and 1/2 of their daughters.

X-Linked Recessive:

• More males than females are affected.
• Affected sons are usually born to unaffected mothers, thus the trait skips generations.
• Approximately 1/2 of carrier mothers’ sons are affected.
• It is never passed from father to son.
• All daughters of affected fathers are carriers.

Besides this one should check for Y-linked dominant and mitochondrial disease as well.