When completing this pedigree with autosomal recessive inheritance, individuals that are shaded are expressing the recessive phenotype and have a genotype of “rr”. Use this knowledge and additional knowledge about how genes are passed from generation to generation to complete the remainder of the pedigree.
Patterns for Autosomal Recessive 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:
- Males and females have the same chance of expressing the trait
- You can only express the trait if you are homozygous
- If both parents express the trait, then all their offspring should also express the trait
- If the offspring express the trait but their parents don’t, then both parents are heterozygous
- If one parent expresses the trait, then their offspring who don’t express it are heterozygous
- The trait can skip generations
Real examples: Sudden Cardiac Death of the Young/Long QT Syndrome (has a recessive and dominant form; recessive is associated with profound deafness), Cystic Fibrosis, PKU, Sickle Cell Anemia, Tay-Sachs Disease and Alzheimer’s Disease.
- 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.