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.
Patterns for X-linked Recessive Inheritance
After filling in the genotypes for individuals in several family trees that exhibit this mode of inheritance, some patterns that can be noticed are:
- The trait is more common in males than in females.
- If a mother has the trait, all of her sons should also have it.
- There is no male to male transmission.
- It has the same inheritance patterns as autosomal recessive for human females.
- The son of a female carrier has a 50 percent chance of having the trait.
- Mothers of males who have the trait are either heterozygous carriers or homozygous and express the trait.
- The father passes his X sex chromosome (and all its genes) to his daughters and his Y sex chromosome (with its genes) to his sons.
- Genes act in pairs, one from each parent for the females. For this mode of inheritance, males get their gene for the trait from their mother.
- Gene pairs separate during meiosis and the formation of the sex cells along with the chromosomes.
- When the sperm fertilizes the egg, the father’s genes (and chromosomes) join the mother’s, or both contribute to the genetic makeup of the offspring.
- One form of a gene may be dominant over another form which is recessive and the dominant form would be expressed.
X-inactivation, also known as lyonization after Mary Lyon who discovered the phenomenon, is a process by which one of the copies of the X chromosome present in female mammals is inactivated very early in embryonic development. In XX females, the majority of the genes on the X chromosome are silenced. This stops females from having twice as much gene product compared to males. The choice of which X chromosome is inactivated is random and should occur at approximately a 50/50 ratio. If this inactivation does not occur at a 50/50 ratio it is called skewed X-inactivation, or skewed lyonization. Skewed X-inactivation causes more gene product from one X to be expressed, which can disrupt the typical “X-linked recessive” model. If more of the gene product from the variant (mutated) gene is expressed in a female, the “X-linked recessive” condition may be expressed. A female can have symptoms ranging from severe (like a male) to a milder signs and symptoms. A woman with skewed X-inactivation will still pass down the gene variant to half of her children, with males being affected and females typically being unaffected ‘carriers’. It is important to remember that some conditions, such as hemophilia, may not always follow the typical “X-linked recessive” model and may have heterozygous females that show signs and symptoms.
Other things to keep in mind:
- A female with Turner syndrome will only have one X and will therefore express “X-linked recessive” conditions just as a male would
- A female with an ‘XXX’ genotype may have a different X-inactivation pattern
- A male with an ‘XXY’ genotype may express milder phenotype due to his X-inactivation pattern