Answer:
Step-by-step explanation:
Everyone has their own unique set of chromosomes which make up a gene. A different variation of gene is called an allele. These 'mutations' are caused by reproduction. A person with a certain trait who reproduces with another certain trait could create a child with both certain traits. The more people reproduce with other people the more varied traits there are.
E.g.
For example in the image (a Punnett square) there is one person with brown eyes, a dominant Brown allele and a recessive Blue allele hence Bb. The other has blue eyes with two recessive Blue alleles hence bb.
The offspring they produce will have a 50-50 chance of Blue or Brown eyes. Only two recessive alleles will allow a recessive phenotype to show whereas only one dominant allele is needed to show a dominant phenotype.