Human characteristics are inherited through DNA that the child receives from his mother and father. Traits may be determined by a single gene on the DNA or multiple genes. Each person has two sets of each gene, one from each parent.
One simple example is whether a person has attached ear lobes or hanging earlobes. The gene for hanging earlobes is dominant. If a person has two copies of the recessive earlobe gene, he has attached earlobes. If one or both of the genes are dominant, he has free-hanging earlobes. Many other characteristics, including skin and eye color, are determined by multiple genes.
Gender is determined by a single set of chromosomes. Females have two X chromosomes, while males have one X and one Y chromosome. Genes for some characteristics are located on these sex chromosomes. For example, red-green color blindness is determined by a gene on the X chromosome. If a male receives a normal copy of the gene, he sees color normally, but if he receives the gene for red-green colorblindness, he has the condition. For a woman to be red-green colorblind, she would have to receive two copies of the gene for this colorblindness, one from each of her parents.