Final answer:
Alleles can be dominant, recessive, or exhibit codominance or incomplete dominance. Dominant alleles mask recessive ones in heterozygotes, while codominance allows both alleles to be expressed equally. Multiple alleles can exist in a population, contributing to genetic diversity.
Step-by-step explanation:
Each individual has two alleles for every gene, one from each parent, which can be the same (homozygous) or different (heterozygous). When different, they can show various patterns of inheritance. Dominant alleles mask the presence of recessive alleles in heterozygous states, causing the dominant phenotype to be expressed. However, dominance can sometimes be incomplete, where the resulting phenotype is an intermediate blend of both alleles. Furthermore, there are cases of codominance, where both alleles are fully and equally expressed in the phenotype.
Apart from the classic dominance or recessiveness, multiple alleles can exist at the population level, providing a greater diversity of genetic traits. The most common expression in a population is referred to as the wild type, and variations from this norm are considered mutants, which can be dominant or recessive. In certain instances, the allelic relationship does not fit typical Mendelian patterns, exemplified by incomplete dominance and codominance. For example, in human blood types, the M and N alleles exhibit codominance.
The complete question is:
Assertion: Everyone has two copies of the gene(2 alleles)
Reason: Dominant alleles are always expressed.