It is a long-standing observation that most mutations are recessive. That is, they do not lead to visible phenotypic effects when in heterozygous combination with the wild-type allele. The reason for this has long been debated. probably it is easier to get one chromosomes in a pair mutated than both the chromosomes in a pair.
We have 46 chromosomes or 23 pairs of chromosomes. Out of which 22 pairs or 44 chromosomes are identical called as autosomes and one pair i.e sex chromosome is non homologous containing one X and one Y chromosome.