Geneticists analyze pedigrees to follow the inheritance of genetically controlled conditions. Three things must be determined in a pedigree analysis:

The mode of inheritance of the condition. In this tutorial, this means deciding if a condition is caused by an autosomal dominant or autosomal recessive allele.

The genotypes of individuals in the pedigree (as far as can be known) based on their phenotype and the phenotypes of their parents or children.

The probability that certain individuals will have the condition. This requires assigning probability values to some individuals whose genotypes cannot be determined with certainty. It also requires an understanding of how and when to apply the product and sum rules.

Part A - Determining the mode of inheritance

The pedigrees below show the inheritance of three separate, rare autosomal conditions in different families. For each pedigree, decide if the condition is betterexplained as recessive or dominant.

Drag the correct label to the appropriate location. Labels can be used once, more than once, or not at all.

It has been given that the traits are autosomal i.e. they are not controlled by X and Y genes. We have to further find out if they are inherited in recessive pattern (autosomal recessive) or in dominant pattern (autosomal dominant).

Pedigree 1: Out of the three generations, the trait is only observed in one. This usually happens in recessive inheritance as two recessive alleles must be together to express the condition and this event has less chances of occurring. Also, it is visible how normal parents in first generation had a daughter with the recessive trait. This is possible if the parents were heterozygous for the trait or "carriers". They contributed one recessive allele each in the next generation due to which the daughter showed the recessive phenotype. Thus, this is most probably a recessive condition.

Pedigree 2: Out of the four generations, the trait is expressed in three. This usually happens in dominant inheritance as only one allele is needed to express the trait so both dominant homozygous and heterozygous individuals will show it. Thus, this is most probably a dominant condition.

Pedigree 3: This trait also skipped generations which is a feature of recessive traits. As with pedigree 1, the trait disappeared in pedigree and reappeared when two recessive alleles came together again. Thus, this is most probably a recessive condition.