A couple and some of their relatives are screened for Gaucher disease in a community-based screening program. The woman is homozygous for the dominant allele, represented by G. The woman's father, sister, and paternal grandmother are heterozygous carriers of the mutant allele, represented by g. Her paternal grandfather, her mother, and both of her mother's parents are homozygous for the dominant allele. The man is heterozygous and he has a brother with Gaucher disease. The man's parents and grandparents have not been tested, but it is known that none of them has Gaucher disease.

Explain why you are able to assign genotypes to the man's parents despite their not being tested.

Key Concepts

Dominant and Recessive Alleles

In genetics, alleles can be classified as dominant or recessive. A dominant allele, represented by a capital letter (e.g., G), will express its trait even if only one copy is present. In contrast, a recessive allele (e.g., g) requires two copies to express the trait. Understanding this distinction is crucial for determining genotypes based on phenotypic expressions in family members.

Homozygosity and Heterozygosity

Homozygosity refers to having two identical alleles for a particular gene (e.g., GG or gg), while heterozygosity means having two different alleles (e.g., Gg). In the context of the question, knowing that the woman is homozygous dominant (GG) and the man is heterozygous (Gg) helps infer the genotypes of their relatives based on their phenotypes and inheritance patterns.

Inheritance Patterns

Inheritance patterns describe how traits and genetic conditions are passed from parents to offspring. In this case, Gaucher disease is an autosomal recessive condition, meaning that an individual must inherit two copies of the recessive allele (gg) to express the disease. By analyzing the genotypes of the couple and their relatives, one can deduce the genotypes of the man's parents based on the presence of the disease in his brother.