How to solve Pedigree Analysis related questions

July 30, 2016

Pedigree analysis related questions are pretty simple to solve if you understand the concept. These questions just involve plain logic.

Some Important Concepts 

That you must know

  • Dominant traits: - The phenotype appears every generation
  • Recessive traits: - Traits which are not dominant.
  • X-linked dominant/ recessive: Trait is never passed from father to son.

Some Important Rules

Rule #1: Every gamete carries exactly one allele for every gene.
Exception: Sex linked genes in males.
There is an exception to this rule because some genes are carried on the X or Y chromosomes, and sperm get one or the other, but not both

Rule #2: Any individual with the recessive phenotype (appearance) must be homozygous recessive.

Rule #3: Any individual with a homozygous recessive offspring must have at least one recessive allele.

Rule #4: Any individual with a homozygous recessive parent must have at least one recessive allele.

Rule #5: Don't guess. If the information isn't available, don't guess.

Also you need to remember these points:
1. Since the pedigrees presented are simple pedigrees, the following assumptions can be made:
a. No new mutations occur in the families. This means that a dominant trait must occur in a parent if it appears in a child. This also means that if a recessive trait appears in a child but does not occur in either parent; both parents must be heterozygous carriers for the trait.
b. All traits are 100% penetrant. This means that if the genotype for the trait occurs in an individual, the trail will appear.

2. If neither parent is affected,
a. The trait cannot be dominant.
b. The trait could be recessive and either parent or both could be heterozygous carriers.

3. If one parent is affected,
a. the trait could be dominant and the affected parent could be heterozygous while the unaffected parent is not a carrier
b. the trait could be recessive and the affected parent is homozygous while the unaffected parent could be a heterozygous carrier

4. If both parents are affected
a. the trait could be dominant and both parents could be heterozygous carriers which means that some of the children could be unaffected
b. the trait could be recessive meaning that both parents would have to be homozygous and all the children would have to be affected

5. Consideration of X-linked or Y-linked traits:
a. X-linked recessive
      i. affected male children must have a heterozygous mother
      ii. an affected female will transmit the trait to all male children
b. X-linked dominant
      i. affected males transmit the trait to all daughters
      ii. affected sons must have an affected mother
c. Y-linkage
      i. male to male transmission only
      ii. no affected females

Enjoy solving questions!

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