[Photo by audi_insperation.]
[In The Birthday Surprise, Alex discovered her family was expecting a new member…]
What will the baby look like, Alex wondered. “Dad, is there any way to tell whether the baby will have blue eyes like I do, or brown like the rest of the family?”
Dr. Jones shuffled the papers on his desk and found a blank page. “Over 100 years ago, the Austrian monk Gregor Mendel studied genetics, or how various traits are passed down from one generation to another.” He began to draw a diagram as he talked.
Brown and Blue Eyes
“Mendel analyzed pea plants, but the principles he discovered apply to many living things — including people,” Dr. Jones explained. “This is a Punnett square, which shows all the possible combinations of a pair of genes.”
“Let me guess,” Alex said. “Mendel planted a huge garden and used the Law of Large Numbers?”
Her father laughed. “Well, he may have done it the other way around. When he published his data, it matched his theory so well that at least one scientist has suggested that he decided the percentages he wanted and tweaked the numbers until they fit . . . Here we have the brown-eyed (B) or blue-eyed (b) genes that you kids could inherit from your mother and me.”
“How do you know what your genes are?” Alex asked.
“Maria and I both have brown eyes, but we have a blue-eyed child. Since brown eyes are dominant, BB or Bb genes will make brown eyes.” He pointed to the chart. “Only a bb combination would give you your blue eyes. And only two Bb parents can have brown eyes and still make a baby with bb genes.”
And a Puzzle
“Now, Alex, count out the chances,” Dr. Jones said. “What is the probability of a blue-eyed baby?”
Alex pouted. “None, since I’ve already used the blue-eyed genes. I guess this baby will have to have brown eyes.”
(Hint: No, but why not?)
To Be Continued…
Read all the posts from the July/August 1999 issue of my Mathematical Adventures of Alexandria Jones newsletter.
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