Final answer:
Normal alleles become abnormal through mutations, which are changes in the DNA sequence that can alter protein structure and function. Some mutations can be beneficial, while others, like the dominant lethal alleles that cause Huntington's disease, are detrimental but may not be immediately selected against.
Step-by-step explanation:
Normal alleles become abnormal alleles through the process of mutation. A mutation is a heritable change in the DNA sequence that can lead to a change in the amino acid sequence of proteins, potentially resulting in abnormal proteins and phenotypes. Some mutations occur spontaneously due to errors during DNA replication or as a result of environmental factors such as radiation or exposure to toxic substances. While some mutations can be beneficial and increase an allele's frequency in the population through natural selection, others can be detrimental or even lethal, as in the case with dominant lethal alleles such as those responsible for Huntington's disease. This disease is an example of how an abnormal allele can become part of the human population when the effects of the lethal allele appear after the individual has reproduced.