Question

which statement describes the mechanism by which new alleles are formed?
mutations accumulate in a genetic sequence over time, which brings about new alleles.
natural selection, occuring over many generations, leads to the formation of new alleles.
random segregation of homologous chromosomes during meiosis leads to new allele format
new alleles are formed in the offspring when two heterozygous parents mate.
all of the possible alleles already exist, and new alleles cannot be formed.

Explanation:

Mutations are the source of new alleles. They can occur spontaneously or due to environmental factors. Natural selection acts on existing alleles, random segregation during meiosis and mating of heterozygous parents shuffle existing alleles, and it's incorrect to say all alleles already exist as mutations can create new ones.

Answer:

Mutations accumulate in a genetic sequence over time, which brings about new alleles.