Sex linked recessive disorders

Every cell in the human body has 23 pairs of chromosomes. One of these pairs is made up of 2 sex chromosomes. These are called X and Y chromosomes. One of these is inherited from the mother and the other from the father. Females have 2 X chromosomes, while males have 1 X and 1 Y chromosome. Chromosomes are made of genes.

Some genetic conditions are caused by mutations in a single gene. These conditions are usually inherited in one of several patterns, depending on the gene involved:. Many health conditions are caused by the combined effects of multiple genes described as polygenic or by interactions between genes and the environment. Such disorders usually do not follow the patterns of inheritance listed above.

This page has been archived and is no longer updated. Knowledge about recessive X-linked gene-disease associations has led to the development and widespread use of prenatal diagnostic tests that can provide parents with information about whether their embryo or fetus might be carrying a disease mutation. Based on that information, the parents can either terminate the pregnancy or prepare for their future child, depending on their beliefs. You might expect that this kind of widespread testing has had the ultimate effect of decreasing the frequency of disease alleles in the population.

When completing this pedigree with X-linked recessive inheritance, use the symbols X and Y in the genotype to represent the sex chromosomes passed on from the previous generation. The X chromosome will contain the alleles for the trait and the Y chromosome will have no alleles for this trait. When completing this pedigree with X-linked recessive inheritance, shaded females who are expressing the recessive phenotype and can only have the genotype of XrXr, the shaded males who are expressing the recessive phenotype and can only have the genotype of XrY, and the non-shaded males who are expressing the dominant phenotype and can only have the genotype XRY. Use this knowledge and additional knowledge about how genes are passed from generation to generation to complete the remainder of the pedigree.