This is the result of nondisjunction of chromosomes during meiosis. this mutation results in three copies , instead of normal two. it can happen with sex or autosomal chromosomes.

In a population that is in hardy-weinberg equilibrium, there are two possible alleles for a certain gene, a and a. if the frequency of allele a is 0.4, what fraction of the population is heterozygous? a. 0.40 b. 0.60 c. 0.16 d. 0.48