Simple Pedigree Analysis

By A.K. Matai

Pedigrees are diagrams that chronicle the phenotype/genotype of organisms over multiple generations. They can be used to determine the mode of inheritance of a particular disease as well as determine the probability of a future offspring inheriting it. Real pedigrees can be complicated due to incomplete penetrance (age of onset, etc) and variable expressivity but they will not be covered in this post which will just focus on the basics of pedigrees.

Pedigree Symbol Conventions

Simple Pedigree Analysis 1

Modes of Inheritance

A) Autosomal Dominant

Pedigree Autosomal Dominant

In an autosomal dominant disease, having one copy of the dominant mutant allele is enough for an individual to be affected.

B) X-Linked Dominant

In an x-linked dominant disease, the gene responsible for causing the disease is on the x chromosome and having one copy of the mutant allele is enough for a male or female to be affected.

Pedigree X-linked Dominant

 

C) Autosomal Recessive

Pedigree Autosomal Recessive

In an autosomal recessive disease, an individual has to have two copies of the mutant allele to be affected.

D) X-Linked Recessive

Pedigree X-linked Recessive

In an x-linked recessive disease, the gene responsible for causing the disease is on the x chromosome. A female requires two copies of the mutant allele to be affected whereas males only require one copy of the mutant allele to be affected since they only have one x chromosome.

E) Mitochondrial

Pedigree Mitochondrial

Mutations in mitochondrial DNA (mtDNA) are inherited through the maternal line. Therefore, all offspring of an affected mother are affected.

 

Sample Problem

Here is a pedigree of the Potter family where some members have a rare disease that has complete penetrance.

Pedigree 2.pnga) What type of inheritance could this be? Classify the following types of inheritance as having a high probability, low probability, or impossible to be the actual type.

Autosomal Recessive (AR)

Low probability as it would require individuals II 1 and II 8 to be carriers for this rare disease.

Autosomal Dominant (AD)

Impossible as it would require all individuals in generation II to be affected and they are not. 

X-linked Dominant (XD)

Impossible as it would require all individuals in generation II to be affected and they are not. 

X-linked Recessive (XR)

High probability as generation III males that have mothers related to individual I 1 are affected. 

Mitochondrial

Impossible as mitochondrial DNA is maternally inherited and no mothers are affected in this pedigree. 

b) If this trait was autosomal recessive, what could be concluded about the genotypes of the following members of the Potter family?

Pedigree 1

Individual A

A/_

Individual B

A/a

Individual C

A/_

Individual D

A/a

Questions, Comments or Concerns?

Feel free to post a your questions, comments or concerns down below!

References

Locke, John; Deyholos, Michael; Harrington, Michael; Canham, Lindsay; Kang, Min, “Open Genetics Lectures (OGL) Fall 2016”

Sinara Rossatto. Dog Animal Pet Pedigree. 2017. https://pixabay.com/p-2070148/?no_redirect. Web. 19 Mar. 2017.

 

 

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