Copy of blank pedigree to print

Answers!

Real example: Heberden nodes or a variety of osteoarthritis

Assigning genotypes for a sex influenced female dominant trait can be challenging. The trait is dominant in females while at the same time it is recessive in males. It is difficult to use "R" to represent the dominant allele and "r" to represent the recessive allele because they behave differently as they pass from females to males. It is customary to use R' to represent the allele for the unusual condition and R to represent the normal condition. If the shaded individuals in the tree were expressing the trait we call Herberden's nodes (bony excrescences on fingers) which is sex infuenced female dominant, then:

• RR would be expressed as no Herberden's nodes in both males and females (not shaded)
• RR' would be expressed as Herberden's nodes in females (shaded) and no Herberden's nodes in males (not shaded) This is the difficult one.
• R'R' would be expressed as Herberden's nodes in both males and females (shaded)

Once again, they should first assign genotypes to the individuals that have only one possibility, females with no Herberden's nodes would have to be RR (if they had an R' allele they would have Herberden's nodes because it it dominant in females) and males with Herberden's nodes would have to be R'R' (it takes two alleles for it to express in males because it is recessive in males). They can then assign genotypes to the rest of the individuals as they study their relationships and think about how genes/chromosomes are passed from generation to generation. If they can't determine the second allele in any pair they can again use "-" to signify that the allele could be R or R', and that there isn't enough information to determine which it is.

Patterns for Sex Influenced, Female Dominant 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:

• If the father possesses the trait, all his daughter will have it.
• Two parents having the trait may have sons without it.
• Parents without the trait may have daughter with it.
• Generally more females than males show the trait.

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. This mode of inheritance is especially difficult because it appears dominant in females and recessive in males. 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.

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