When completing this pedigree with Y linked inheritance, 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.
Patterns for Y-linked 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:
- 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.
- 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.