Final Answer:
III-3: homozygous recessive
II-1: heterozygous
I-1: heterozygous
II-4: heterozygous
1. This trait is recessive. Explanation: Affected individuals (filled symbols) appear only in generations where both parents are unaffected — which is only possible if the trait is recessive. For example, II-3 and II-4 are unaffected but have affected children (III-3 and III-4), so they must both carry a hidden recessive allele.
2. We know II-3 and II-4 are heterozygous because they are unaffected (so they can’t be homozygous recessive) but they have affected children (who must be homozygous recessive). To pass on two recessive alleles (one to each affected child), both parents must carry one recessive allele — meaning they are heterozygous.
Parent Tip: Review the logic above to help your child master the concept of genetics pedigree worksheet.
