Question

Assume that a particular unusual phenotype is caused by an autosomal recessive allele. John has a brother who appears normal. Jane has a sister with the unusual phenotype. No one else in either family has the unusual phenotype. John marries Jane, and they have a daughter with the unusual phenotype. Draw a 3-generation pedigree that depicts this family and assign the most likely genotypes to each individual. (You can use any letter you wish to represent the gene.)

Answer 1

John and Jane's daughter must be homologous recessive to show the unusual trait

Step-by-step explanation:

Given that John and his brother both appear normal there is a possibility that at least one of his parents are heterozygous for the trait. On the other hand, since Jane's sister shows the unusual trait, this means that both the parents are heterozygous for the trait since once must be homozygous recessive to show the unusual phenotype. This also means that Jane is a carrier of the trait or she is heterozygous, like her parents.

John and Jane's daughter shows the unusual phenotype which means she is also homozygous for the recessive trait inheriting one copy from her father and one copy form her mother.

Finally, this confirms that at least one of John's parents MUST have the recessive gene since his daughter did inherit a copy form him.