A number of comparisons of nucleotide sequences among...

A number of comparisons of nucleotide sequences among hominids and rodents indicate that inbreeding may have occurred more often in hominid than in rodent ancestry. Bakewell et al. (2007. Proc. Nat. Acad. Sci. IUSAJ 104: 7489-7494) suggest that an ancient population bottleneck that left approximately 10,000 humans might have caused early humans to have a greater chance of genetic disease. Why would a population bottleneck influence the frequency of genetic disease? Shown below are two homologous lengths of the alpha and beta chains of human hemoglobin. Consult a genetic code dictionary (Figure 12.7 ), and determine how many amino acid substitutions may have occurred as a result of a single nucleotide substitution. For any that cannot occur as a result of a single change, determine the minimal mutational distance.

Key Concepts

Population Bottleneck

A population bottleneck is a sharp reduction in the size of a population due to environmental events or other pressures, which leads to a loss of genetic diversity. It results in a small number of individuals contributing genetically to future generations, often amplifying effects of inbreeding and genetic drift.

Inbreeding

Inbreeding refers to the mating of individuals who are genetically related, increasing the probability that deleterious recessive alleles come together. In small populations, such as those resulting from a bottleneck, inbreeding can lead to a higher frequency of genetic diseases due to the accumulation of harmful mutations.

Genetic Disease in Small Populations

When a population undergoes a bottleneck, the limited genetic pool often results in an increased incidence of genetic diseases. This is because harmful mutations that might have been rare in a larger population can become more frequent when there is less genetic variability and more inbreeding.

Nucleotide Substitution

Nucleotide substitution is a type of mutation where one nucleotide in the DNA sequence is replaced by another. This can lead to changes in the codon, potentially altering the amino acid sequence of proteins if the change is non-synonymous, impacting the protein's structure and function.

Genetic Code and Codon Mutations

The genetic code translates nucleotide sequences into amino acids via codons, which are sets of three nucleotides. Mutations in these codons, such as single nucleotide substitutions, can either result in the same amino acid (a synonymous change) or a different one (a non-synonymous change). Evaluating these changes involves using a genetic code dictionary to determine the nature and consequence of the mutation.

Mutational Distance

Mutational distance refers to the minimum number of nucleotide changes required to alter one codon into another. This concept is important when assessing how likely a specific amino acid substitution is to occur; a single substitution might change a codon to one encoding a different amino acid, but some substitutions may require multiple nucleotide changes.

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