Final answer:
The two phenotypes recognized in the family of Labrador retrievers with an autosomal recessive allele for dwarfism are 4) Dwarf and normal. Dwarf dogs weigh less than 13 kg, while normal dogs weigh more than 23 kg. The normal phenotype can arise from either homozygous dominant or heterozygous genotypes. Option 4 is correct..
Step-by-step explanation:
In the context of genetics within a family of Labrador retrievers, we are examining a scenario where there is an autosomal recessive allele that causes dwarfism. Two distinct phenotypic expressions are present: dwarf, which is characterized by a weight of less than 13 kg, and normal, which is characterized by a weight of greater than 23 kg. Considering the provided options, the correct answer is 4) Dwarf and normal, as these terms accurately describe the phenotypes in the Labrador retriever family that correspond to the recessive allele and the dominant condition, respectively.
Recessive alleles, like the one that leads to dwarfism in Labradors, require two copies (homozygosity) for the associated trait to be phenotypically expressed. When a dominant allele is present, it will mask the expression of the recessive allele. Therefore, 'normal' sized Labradors can be either homozygous dominant or heterozygous, having one dominant and one recessive allele without displaying dwarfism. The presence of dwarfism in the puppies indicates both parents must carry the recessive allele.