Ask students to assign a genotype for each individual in the pedigree by writing it on the blank line below the circle or square. When working with genotypes and pedigrees they should always fill in the ones that have only one possibility first. In this case however, all are known. The trait is carried on the Y chromosome and is transmitted from father to son only. When writing genotypes we still use X and Y as symbols for the chromosomes passed on from the previous generation, but only the Y chromosome will have an allele for the gene. For example, all females will have the genotype XX (no alleles). Males with the trait will have the genotype XYR and males without the trait will have the genotype XYr (with the allele on the Y). The alleles are not dominant over one another so the R and r were just randomly assigned as a symbol to represent each allele.

Copy of blank pedigree to print

Answers!

Real Example: Hairy ears or Retinitis Pigmentosa

Patterns for Y-linked 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:

• Trait expression and transmission is only in males, the individuals with the Y chromosome.
• If a male has a trait, so should his father and paternal grandfather as well as his sons and their sons. It follows the inheritance of the Y chromosome.

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:

• When writing genotypes, not to forget that the gene is located on the Y chromosome. They will often write it on the X as in X-linked inheritance.
• 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.
• Males get their gene for the trait only from their father.
• The X and Y chromosomes separate during meiosis and the formation of the sex cells.
• 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.
• 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.

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