Sex Influenced male dominant Inheritance

Pedigree chart showing Sex-influenced male dominant inheritance example

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 tree 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)
Once again, they should first assign genotypes to the individuals that have only one possibility, males with no baldness 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). They can then assign genotypes to the rest of the individuals as they study their relationships and think about how genes/chromosomes are passed from generation to generation. If they can't determine the second allele in any pair they can again use "-" to signify that the allele could be R or R', and that there isn't enough information to determine which it is.

Sex Influenced Male Dominant Inheritance Blank Pedigree (PDF)
Sex Influenced Male Dominant Inheritance Pedigree Answers (PDF)

Real examples: Index finger shorter than ring finger and Male-patterned baldness.

Patterns for Sex Influenced, Male Dominant Inheritance

(taken from "Dichotomous Key for Human Pedigree Analysis," William Kimmich and Thomas Mertens, Ball State University)

After filling in the genotypes for individuals in several family trees that exhibit this mode of inheritance, your students will notice that:

  • 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.

Student Difficulties

Students who have difficulty assigning genotypes are usually not using their understanding of how meiosis and fertilization help move the chromosomes with their genes from generation to generation. This mode of inheritance is especially difficult because it appears dominant in males and recessive in females. They need to remember:

  • 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.
  • Like anything else, this takes practice. We find the mistakes in our thinking and correct them, which leads to a better understanding of the benchmark.