Assigning genotypes for Sex Limited traits can be difficult because the genes can be found in both sexes and probably on the autosomes but they can only be expressed in the sex that is anatomically or physiologically correct. For example, only males can have prostate cancer and only females can have ovarian cancer, although both males and females can carry the genes for these conditions. These traits would usually involve primary or secondary sex traits.
When completing this pedigree with sex limited traits, shaded females would be rr, assuming this sex limited trait acts like a recessive trait on an autosome. Use this knowledge and additional knowledge about how genes are passed from generation to generation to complete the remainder of the pedigree.
Patterns for Sex Limited 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:
- These are genes that occur in both sexes (probably on the autosomes) but are normally expressed only in the gender having the appropriate hormonal determiner (activator).
- Throughout the pedigree the trait appears in only one sex, but it need NOT occur in all member of that sex.
- The genes for the trait can be carried and transmitted by the opposite sex although it is NOT displayed in that sex because of anatomical or physiological differences.
Real example: X-linked Hereditary Prostate Cancer.
- Genes act in pairs, one from each parent.
- 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.