Dominant and recessive illnesses occur with equal frequency in males and females. This is because the genes involved are located on autosomes, which are the same in both genders. Many physical traits, however, obviously do differ between the two genders. In addition, gender dramatically affects the inheritance of certain traits and illnesses that have no obvious connection to sexual

 

    These sex-linked traits are controlled by genes located on the sex chromosomes. Humans have 46 chromosomes, including 44 autosomes (nonsex chromosomes) and the two sex chromosomes, which can be either X or Y. The autosomes come in 22 homologous pairs, present in both males and females. Females also possess a homologous pair of X chromosomes, while males have one X chromosome and one Y chromosome (the master gene for “maleness” is located on the Y chromosome). All eggs have an X chromosome, so the sex of a child is determined at the time of fertilization by the type of sperm. If the fertilizing sperm carries an X chromosome, the child will be female; if it carries a Y chromosome, the child will be male. The X chromosome is much larger than the tiny Y chromosome, and most of the genes on the X chromosome do not have a homologous counterpart on the Y.

 

    Genes on autosomes will always be present in two copies: one inherited from the maternal parent, the other from the paternal parent. The traits controlled by such autosomal genes will be generally unaffected by gender and will follow Mendelian patterns of inheritance (with the exceptions noted in previous sections). In contrast, genes on the X chromosome (X-linked genes) are present in two copies in females but only one copy in males. Female offspring will inherit one copy of an X-linked gene from each parent, but male offspring must inherit the Y chromosome from their father and therefore always inherit only the maternal allele of any X-linked gene. For example, color blindness and hemophilia are sex-linked disorders. The mutated gene that causes these disorders is recessive and exists on the X chromosome. In order for a female, who is XX, to have a phenotype that is color blind or hemophiliac, both of her parents have to have the recessive gene. But since males have only one X chromosome inherited from their mother, if their mother expresses the recessive mutation, that trait will automatically be expressed in the male child’s phenotype, since the male has no other gene to assert dominance over the recessive mutation.

 

    The pedigree shown below diagrams three generations of a hypothetical family affected by hemophilia A.

    This pedigree demonstrates many of the  features of X-linked recessive inheritance. Heterozygous females are carriers who do not express the disease. In contrast, all males with the mutated allele will express the disease; there are no male carriers. Affected males will transmit the mutated allele to none of their sons but to all of their daughters, who will then all be carriers. Heterozygous females will transmit the disease to one-half of their sons, and one-half of their daughters will be carriers. Affected males generally have an unaffected father and a mother who is a carrier; 50 percent of their maternal uncles will have the disease.