III-3: homozygous recessive
II-1: heterozygous
I-1: homozygous dominant
II-4: heterozygous
1. This trait is recessive. Affected individuals (III-3 and III-4) have unaffected parents (II-3 and II-4), which means both parents must carry a recessive allele to pass it on. If the trait were dominant, at least one parent would have to be affected.
2. We know II-3 and II-4 are heterozygous because they are unaffected but have affected children (III-3 and III-4). For a child to express a recessive trait, both parents must contribute a recessive allele; therefore, each parent must be a carrier (heterozygous).
3.
Pedigree:
Generation I (grandparents):
- Maternal grandfather: brown eyes → genotype Bb (certain, since his daughter has blue-eyed father)
- Maternal grandmother: brown eyes → genotype Bb (certain, since her daughter has blue-eyed father)
- Paternal grandfather: brown eyes → genotype BB or Bb (uncertain — could be either)
- Paternal grandmother: brown eyes → genotype BB or Bb (uncertain — could be either)
Generation II (parents):
- Mother: brown eyes → genotype Bb (certain — she inherited b from her blue-eyed father)
- Father: brown eyes → genotype Bb (certain — he must carry b to have a blue-eyed son)
Generation III (son):
- Son: blue eyes → genotype bb (certain)
Individuals with certain genotypes: maternal grandparents (Bb), mother (Bb), father (Bb), son (bb).
Individuals with uncertain genotypes: paternal grandparents (each could be BB or Bb).
Parent Tip: Review the logic above to help your child master the concept of pedigree worksheet biology.
