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. For this mode of inheritance it would be the non-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 pedigree to print

Answers!

Real examples: Cardiovascular disease or Huntington Disease or Sudden Cardiac Death of the Young (Long QT Syndrome) (has two forms dominant and recessive; recessive form is associated with profound deafness), Tuberous Sclerosis Complex, Marfan Syndrome, Neurofibromatosis, and Polycystic Kidney Disease.

Patterns for Autosomal Dominant 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:

• Males and females are equally likely to have the trait.
• There is male to male transmission.
• Traits do not skip generations (generally). If the trait is displayed in offspring, at least one parent must show the trait.
• If parents don't have the trait, their children should not have the trait. (except for situations of gene amplification)
• The trait is present whenever the corresponding gene is present (generally). If both parents possess the trait, but it is absent in any of their offspring, then the parents are both heterozygous ("carriers") of the recessive allele.
• Homozygotes for the dominant condition have a more severe form of the condition.

Student Difficulties

It can be difficult for students to switch to a dominant trait after learning how to assign genotypes for a recessive trait because they assume the shaded ones are showing the recessive phenotype rather than the shaded individuals are the ones with the condition following the dominant mode of inheritance. 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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