Sex(X)-linked Recessive Inheritance

Pedigree chart showing Sex(X)-linked recesive inheritance example

Ask students to assign a genotype for each individual in the pedigree by writing it on the blank line below the circle or square. With this type of inheritance we use the symbols X and Y in the genotype to represent the sex chromosomes passed on from the previous generation. The X chromosome will contain the alleles for the trait and the Y chromosome will have no alleles for this trait. When working with genotypes and pedigrees they should always fill in the ones that have only one possibility first. For this mode of inheritance it would be the shaded females who are expressing the recessive phenotype and can only have the genotype of XrXr, the shaded males who are expressing the recessive phenotype and can only have the genotype of XrY, and the non-shaded males who are expressing the dominant phenotype and can only have the genotype XRY. Using their knowledge of how genes are passed from generation to generation (Benchmark Heredity: HS#1), they should then determine whether or not the females expressing the dominant phenotype are XRXr or XRX- (XRXR or XRXr, not enough information to determine). To do this they need to study the relationships between individuals and how chromosomes with their genes are passed from generation to generation. For example, who did the individuals with Xr get them from and who did they give them to? Who did the males get their X and Y from and who did they give them to?

X-linked Recessive Inheritance Blank Pedigree (PDF)
X-linked Recessive Inheritance Pedigree Answers (PDF)

Real Examples: Hemophilia, Muscular Dystrophy and Fragile X.

Patterns for X-linked Recessive Inheritance

(taken from "Puzzling Pedigrees," BSCS, 1997 and "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:

  • The trait is more common in males than in females.
  • If a mother has the trait, all of her sons should also have it.
  • There is no male to male transmission.
  • It has the same inheritance patterns as autosomal recessive for human females.
  • The son of a female carrier has a 50 percent chance of having the trait.
  • Mothers of males who have the trait are either heterozygous carriers or homozygous and express 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. They need to remember:

  • The father passes his X sex chromosome (and all its genes) to his daughters and his Y sex chromosome (with its genes) to his sons.
  • Genes act in pairs, one from each parent for the females. For this mode of inheritance, males get their gene for the trait from their mother.
  • 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.

Incorrect Ideas Students May Have, Watch Out for Them

  • They often forget that males (usually) have one X sex chromosome and will often give them two, like females, when assigning genotypes.
  • They believe that offspring receive their genes from the parent that shows the same phenotype as they do.
  • They believe that daughters only get their traits from their mothers and sons only get their traits from their fathers.
  • They believe that parents expressing the dominant phenotype can ONLY have children with that same phenotype.