a geneticist is studying a pedigree that shows the family history of a recessive genetic disease. which of the following pedigrees shows an impossible relationship.
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To determine which pedigree shows an impossible relationship, you need to understand the basic principles of inheritance for a recessive genetic disease. In this scenario, a recessive genetic disease is being studied, which means the disease is only expressed when an individual inherits two copies of the disease-causing gene.
To identify an impossible relationship in a pedigree, you should look for inconsistencies that violate the rules of inheritance. Here are some key principles to keep in mind:
1. Recessive Inheritance: For a recessive genetic disease, individuals who exhibit the disease must have two copies of the disease-causing gene (one inherited from each parent). Individuals who carry only one copy of the gene (referred to as carriers) do not show any symptoms.
2. Autosomal Recessive: Recessive genetic diseases can occur on autosomes, which are non-sex chromosomes, meaning they are not linked to gender.
Now, let's analyze each provided pedigree to identify an impossible relationship:
Pedigree 1:
- This pedigree shows a male (square) and a female (circle) who are affected, denoted by the shaded symbols.
- Both unaffected parents (unshaded symbols) have affected offspring.
- This pedigree follows the principles of recessive inheritance and is possible.
Pedigree 2:
- In this pedigree, both unaffected parents (unshaded symbols) have affected offspring.
- This pedigree follows the principles of recessive inheritance and is possible.
Pedigree 3:
- This pedigree shows a male (square) and a female (circle) who are affected.
- Both unaffected parents (unshaded symbols) have unaffected offspring.
- This pedigree violates the principles of recessive inheritance, as both affected parents should have affected offspring. Therefore, this pedigree shows an impossible relationship.
Based on the analysis, Pedigree 3 shows an impossible relationship in terms of the expected inheritance pattern for a recessive genetic disease.