s CF respectively? Punnett Squares “So, what’s next?” asked Mike. “First, we’ll collect DNA samples from both of you. We’ll then analyze your CF genes for the most common mutations to see if you are carriers for this recessive genetic disease.” A few weeks later, the genetic counselor welcomed back Sarah and Mike. “We’ve received the results of your genetic tests for common CF mutations. Michael, you’re a carrier of the most common disease allele, delta-F508, and Sarah, you tested negative for the most common CF mutations.” “Thank goodness,” Sarah replied with
The Meeting
Sarah stared blankly at the blue paisley wallpaper. Her husband Mike sat by her side, bending and unbending a small paper clip.
“Sarah and Michael, it’s good to meet you,” welcomed the genetic counselor, as she entered the room. “I apologize for being late, but I was just meeting with another couple. Let’s see, you’d like to have a child, but you’re concerned because of your family history of cystic fibrosis.”
“Yes,” Sarah replied softly. “Mike and I met at a CF support group meeting a few years ago. He had a younger brother who died of cystic fibrosis, and I had a younger sister. We saw the painful lives they had—difficulty breathing, the constant respiratory infections. Although the treatments for CF are better now, we just don’t know if we can…” she trailed off.
“I can certainly understand your concern,” the genetic counselor responded sympathetically. “That’s where I hope to help, by providing as much information and advice as I can. I’m glad that you came to see me before you became pregnant so I can fully advise you of all options beforehand.”
“To start, let’s go over what we know about your case so far,” continued the genetic counselor. She pulled out a pad of paper, which she placed on the table in front of Sarah and Michael, and began to draw a series of circles and squares connected by lines.
“As I recall, both sets of parents did not display any of the symptoms of cystic fibrosis, right?”
“Yes,” said Sarah and Michael in unison.
“Ok, well that means….”
1. Draw a pedigree showing the family history for CF in Sarah’s and Michael’s families. Be sure to distinguish between individuals with the disease, those that are carriers for the disease, and individuals who do not possess a copy of the disease allele. Based on the pedigree, what is the chance of Sarah or Michael being carriers of the mutant allele that causes CF respectively?
Punnett Squares
“So, what’s next?” asked Mike.
“First, we’ll collect DNA samples from both of you. We’ll then analyze your CF genes for the most common mutations to see if you are carriers for this recessive genetic disease.”
A few weeks later, the genetic counselor welcomed back Sarah and Mike. “We’ve received the results of your genetic tests for common CF mutations. Michael, you’re a carrier of the most common disease allele, delta-F508, and Sarah, you tested negative for the most common CF mutations.”
“Thank goodness,” Sarah replied with relief. “That means it’s safe for us to have a child, right?”
“Not exactly,” cautioned the genetic counselor.
“Since Sarah tested negative for CF, it seems we don’t have much to worry about, right?” said Michael, pointing to the pedigree in which Sarah is not a carrier. “So, what did you mean when you said not exactly?”
The genetic counselor grabbed her laptop computer and positioned it in front of the three of them. Her fingers quickly typed out a web address and the words “OMIM - Online
Construct a Punnett square to demonstrate why Sarah concluded that she and Michael could not have an affected child (assuming that she does not carry a CF mutation).
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