For a single gene, the following are possible inheritance patterns. Autosomal dominance or autosomal recessive are two possible patterns on the first 22 pairs of chromosomes known as autosomes. In autosomal dominance, only one gene is necessary for the trait to appear in the phenotype. In autosomal recessive inheritance, two genes(from each parent) must be present in order for the trait to appear in the phenotype. If a person is heterozygous for the trait, they will have one dominant and one recessive trait and are considered to be a carrier. Another pattern of inheritance is sex-linked. That is when a gene is linked to the sex chromosomes. If the gene is X linked, it is carried on the X chromosome. Males have one X and females have two X chromosomes. If a trait is X-linked, if males inherit the gene on their X chromosome, it will be expressed, as males are hemizygous for traits on the X chromosome. All males get their X chromosome from their mothers. However, in females, if one X chromosome has the normal gene and the other is carrying the X linked gene like colorblindness, the female will be a carrier, since this gene is X linked recessive. However, if a gene is X- linked dominant, it would be expressed. Another pattern of inheritance is Y-linked inheritance. This only passes from fathers to sons in a pedigree. Only males have the Y chromosome in their genotype. Mitochondrial inheritance is yet another inheritance pattern--mothers pass these genes to all their children. This is because the mitochondrial DNA is present in the egg cell, but not in the sperm cell at fertilization.