Sex Limited Inheritance

Pedigree chart showing Sex-limited inheritance example

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.

As students assign genotypes they should once again begin with the individuals for which there is only one choice. In this case the shaded females would be rr, assuming this sex limited trait acts like a recessive trait on an autosome. 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. When assigning genotypes for sex limited traits there are often alleles that can't be determined so the students should once again use "-" to represent the unknown allele. It can be frustrating when there are so many that can't be determined through pedigree analysis. This is why we often need another tool to determine what alleles are present.

Sex Limited Inheritance Blank Pedigree (PDF)
Sex Limited Inheritance Pedigree Answers (PDF)

Real example: X-linked Hereditary Prostate Cancer.

Patterns for Sex Limited 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:

  • 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 NOToccur 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.

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 both sexes can carry the genes for it but only one sex can express the phenotype. 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.