Question #a5cdf

1 Answer
Oct 2, 2015

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.