Identifying heredity models

Basic concepts

The genotype is the genetic information we deduce from the phenotype.

It is usually represented by two letters.

Each gene responsible of a character can exist in different versions -we call it alleles- , which are associated with a certain phenotype.

As each of us have two alleles for each gene, we represent our genotype by two letters.

As we can show only one phenotype, the resulting phenotype will result from the combination of our two alleles, depending on the relationships of dominance or recessivity between the alleles, and the heredity pattern.

For a single gene, we can have four main inheritance patterns:

• Autosomal dominant
• Autosomal recessive
• Sex-Linked recessive
• Sex-Linked dominant

Procedure

To determine if a character is following a determined pattern, we should assume this pattern and follow the following steps.

1. Identify which allele would correspond to the observed characteristic (marked in black).
2. For each individual, identify its alleles from the phenotype. In some cases, we will only be able to determine one of the alleles, or none.
3. Beggining from the sons, try to determine where are the alleles of each individual came from: the father? the mother?
4. Try to complete the genotypes. In some individuals, you won't be able to complete it.

Two possibilities:

1. some individuals are impossible to solve: the pattern is not consistent, you can discard this model of inheritance for this character.
2. all the individuals have a possible explanation according to the assumed heredity model: the pattern is consistent.

Even if you have found a consistent model, you have to test the rest of the models to discard that two models are consistent at the same time: it woul mean that, even if only one model is driving the inheritance of this character, you are not able to determine which it it. Don't worry: to discard some models is also information!

Look at the videos to learn how to discard heredity models from a pedigree.

Image extracted from: Michael Gregory Website

More training exercises at: Michael Gregory's Pedigree Exercises.