Activity: Developing a Pedigree for the Inheritance of the BRCA 1 Gene

Activity: Developing a Pedigree for the Inheritance of the BRCA 1 Gene

Introduction: In the last year, the location of a gene responsible for one type of breast cancer has been identified to be the lower right leg of chromosome 15, thanks to a three-year study led by Dr. Mary Claire King at the University of California, Berkeley.

Breast cancer afflicts 2% of women in the US, yet it is a leading cause of death. Why? Metastasis! Before growing big enough to be obvious, the cancer spreads into the lymph nodes, and from there, anywhere in the body. By the time it’s detected, the cancer has spread to numerous organs.

Some forms of breast cancer runs in families. The odds soar from 2% risk nationally to 50% if it is in the family. To identify one’s risk, a marker can be pinpointed in DNA analysis, but a family pedigree can determine the need for such a test.

Your Job…Be Mary Claire King for a day!

Here’s a dialogue with a patient named Elizabeth, who had a preventive mastectomy at age 28 because of the frequency of breast cancer in her family. (Remember, Dr. King had no inkling as to the hereditary pattern of the BR gene. Here’s a hint that Dr. King did not know when she began her study: males can carry the gene and not be affected).

• “It’s my two year old daughter I’m worried about. Will she get this?”
• “I married Steven, who has no family history of breast cancer”
• “I have three sisters, and two already have breast cancer.”
• “Our dad is Frank, who had six sisters (my aunts) one of them never showed any signs of breast cancer, though one got ovarian cancer, and three got breast cancer. The other one died in a car accident at age 20.”
• “Frank’s parents (my grandparents) are free of breast cancer, but that’s as far back as I can go with information.”
• “Frank’s sister, Rhonda (one of my aunts who had it), married a guy with no family history of it. They had three kids (my cousins), two girls and a boy. One of the girls was recently diagnosed with breast cancer.”

Tracking the gene…

First generate a pedigree as much as possible form the above discussion with Elizabeth. From the pedigree, can you determine if the gene is sex linked? Dominant or recessive?

Then, have a discussion with the members in your group. Write a brief paper which explains the disease’s mode of transmission. Include the likelihood of Elizabeth’s daughter developing breast cancer. Then consider moral issues such as testing for the DNA marker and insurance regulations regarding knowledge of the carriers of such markers.