The two DNA and polypeptide sequences shown are for alleles at a hypothetical locus that produce different polypeptides, both five amino acids long. In each case, the lower DNA strand is the template strand:

Based on DNA and polypeptide sequences alone, is there any way to determine which allele is dominant and which is recessive? Why or why not?

Verified step by step guidance

1. Step 1: Understand the concept of dominance and recessiveness in genetics. Dominance refers to an allele that expresses its trait even in the presence of another allele, while recessiveness refers to an allele whose trait is masked by a dominant allele. Dominance is determined by phenotypic expression, not by DNA or polypeptide sequences alone.
2. Step 2: Analyze the DNA sequences provided for allele A₁ and allele A₂. Note the difference in the sequences: allele A₁ has 'ATGCATGTAAGTGCATGA' while allele A₂ has 'ATGCAAGTAAGTGCATGA'. The difference lies in the third codon, where A₁ has 'ATG' (coding for His) and A₂ has 'AAG' (coding for Gln).
3. Step 3: Examine the polypeptide sequences. Allele A₁ produces a polypeptide sequence N–Met–His–Val–Ser–Ala–C, while allele A₂ produces N–Met–Gln–Val–Ser–Ala–C. The difference in the polypeptides is due to the single nucleotide change in the DNA sequence, resulting in a substitution of His with Gln.
4. Step 4: Consider the relationship between DNA sequence, polypeptide sequence, and dominance. Dominance is determined by the observable phenotype, which may involve the functional impact of the polypeptides produced by the alleles. Without information about the phenotypic effects of these polypeptides, it is impossible to determine dominance or recessiveness based solely on the DNA and polypeptide sequences.
5. Step 5: Conclude that additional information about the phenotypic expression of these alleles is required to determine dominance or recessiveness. For example, if one allele produces a functional protein while the other does not, or if one allele's protein has a greater impact on the phenotype, this could indicate dominance. However, such information is not provided in the problem.

Key Concepts

Alleles and Dominance

Alleles are different versions of a gene that can produce variations in traits. Dominance refers to the relationship between alleles, where a dominant allele can mask the expression of a recessive allele in a heterozygous individual. To determine dominance, one typically needs to observe the phenotypic effects of the alleles in a biological context, rather than just their sequences.

DNA and Polypeptide Translation

DNA sequences are transcribed into messenger RNA (mRNA), which is then translated into polypeptides (proteins). The specific sequence of nucleotides in DNA determines the sequence of amino acids in a polypeptide. While the polypeptide sequences can indicate functional differences, they do not inherently reveal dominance without additional context about their effects on phenotype.

Phenotypic Expression

Phenotypic expression refers to the observable traits or characteristics of an organism, which result from the interaction of its genotype with the environment. To determine which allele is dominant, one must assess how each allele affects the phenotype when expressed, as the mere presence of different polypeptide sequences does not provide sufficient information about their dominance without experimental evidence.