Assigning genotypes for a sex influenced female dominant trait can be challenging. The trait is dominant in females while at the same time it is recessive in males. It is difficult to use “R” to represent the dominant allele and “r” to represent the recessive allele because they behave differently as they pass from females to males. It is customary to use R’ to represent the allele for the unusual condition and R to represent the normal condition. If the shaded individuals in the tree were expressing the trait we call Herberden’s nodes (bony excrescences on fingers) which is sex influenced female dominant, then:
- RR would be expressed as no Herberden’s nodes in both males and females (not shaded)
- RR’ would be expressed as Herberden’s nodes in females (shaded) and no Herberden’s nodes in males (not shaded). This is the difficult one.
- R’R’ would be expressed as Herberden’s nodes in both males and females (shaded)
When completing this pedigree, begin with females with no Herberden’s nodes, these females would have to be RR (if they had an R’ allele they would have Herberden’s nodes because it is dominant in females) and males with Herberden’s nodes would have to be R’R’ (it takes two alleles for it to express in males because it is recessive in males).
Real Examples: Heberden nodes or a variety of osteoarthritis.
Patterns for Sex Influenced, Female Dominant 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:
- If the father possesses the trait, all of his daughters will have it.
- Two parents having the trait may have sons without it.
- Parents without the trait may have a daughter with it.
- Generally, more females than males show the trait.
- 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.