Which of the following describes human sex chromosomes?

An allele that exerts its effects whenever it is present is:

If the two alleles for a particular gene are identical the gene pair is:

If an individual is homozygous for a particular trait:

If an allele must be present on both the maternal and paternal chromosomes to affect the phenotype, the allele is said to be 

(a) Dominant

(b) Recessive

(c) Complementary

(d) Heterozygous