1. DNA Sequence:
* The fundamental basis: Alleles are variations in the sequence of DNA that makes up a gene. These sequence differences can be small (a single nucleotide change) or larger (insertions, deletions, or even whole gene duplications).
2. Mutations:
* The source of variation: Mutations are the ultimate source of new alleles. They occur randomly during DNA replication and can alter the gene's sequence. Some mutations have no effect, while others can change the protein produced by the gene, or even prevent its production altogether.
3. Recombination:
* Shuffling existing alleles: During sexual reproduction, chromosomes from the mother and father exchange genetic material through a process called recombination. This shuffling of existing alleles creates new combinations, further increasing genetic diversity.
4. Environmental Factors (Less Direct):
* While not directly determining alleles, environmental factors can influence which alleles are more likely to be passed on to future generations. For example, if a particular allele provides an advantage in a specific environment, individuals carrying that allele may be more likely to survive and reproduce, increasing its frequency in the population.
In summary: Alleles are determined by the sequence of DNA within a gene, which can be altered by mutations. Recombination shuffles existing alleles to create new combinations. While environmental factors can influence allele frequency, they do not directly determine the alleles themselves.
