5. A single-gene disorder is one that is determined primarily by the alleles at a single locus. Individuals with a pair of identical alleles are said to be homozygous (a homozygote) Individuals with different alleles are said to be heterozygous (a heterozygote or carrier). These terms (homozygous, and heterozygous) can be applied either to a person or to a genotype.

6. In the special case in which a male has an abnormal allele for a gene located on the X chromosome and there is no other copy of the gene, he is neither homozygous nor heterozygous and is referred to as hemizygous.

9. Mendel’s Law of Dominance