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. For this mode of inheritance it would be the shaded individuals who are expressing the recessive phenotype and can only have the genotype of "rr." Using their knowledge of how genes are passed from generation to generation (Benchmark Heredity: HS#1), they should then determine whether or not the individuals expressing the dominant phenotype are "Rr" or "R-"(RR or Rr, 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 who are "rr" get their "r's" from and who can they give them to.

Copy of blank pedigree to print

Answers!

Real examples: Hemochromatosis (rusty organ disease) or Sudden Cardiac Death of the Young (Long QT syndrome) (has two forms recessive and dominant; recessive form is associated with profound deafness), Cystic Fibrosis, PKU, Sickle Cell Anemia, Tay-Sachs Disease, and Alzeimer Disease.

Patterns for Autosomal Recessive Inheritance

(taken from "Puzzling Pedigrees," BSCS, 1997)

After filling in the genotypes for individuals in several family trees that exhibit this mode of inheritance, your students will notice that

• 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 offpsring 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

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:

• 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.

Incorrect Ideas Students May Have, Watch Out for Them

• Offspring receive their genes from the parent that shows the same phenotype as they do
• Daughters only get their traits from their mothers and sons only get their traits from their fathers
• Parents expressing the dominant phenotype can ONLY have children with that same phenotype

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