Assigning genotypes for a sex influenced male dominant trait can be challenging. The trait is dominant in men while at the same time it is recessive in women. 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 males to females. 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 pedigree were expressing the trait we call Male Patterned Baldness then:
- RR would be expressed as no baldness in both males and females (not shaded)
- RR’ would be expressed as patterned baldness in males (shaded) and no baldness in females (not shaded). This is the difficult one.
- R’R’ would be expressed as patterned baldness in both males and females (shaded)
When completing this pedigree, begin with males with no baldness, these males would have to be RR (if they had an R’ allele they would be bald because it is dominant in males) and females with patterned baldness would have to be R’R’ (it takes two alleles for it to express in females because it is recessive in females).
Patterns for Sex Influenced, Male 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 mother has the trait, all of her sons will have it.
- Two parents who have the trait may have daughters without it.
- Parents without the trait may have sons with it.
- Generally, more males than females have 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.