The Blue People
Over the years, there have been a number of families in which some of the individuals have blue skin. WhatÕs going on here?
There is a human gene that codes for an enzyme called ŅNADH diaphorase,Ó which we will give the abbreviation ND. This enzyme repairs hemoglobin that has been damaged by oxidization. Humans in whom this gene is defective cannot make the ND enzyme, and accumulate damaged hemoglobin, which is blue. As a result, their skin appears blue.
In the blue families, some of the individuals are blue, some are not. How can we explain this?
LetÕs model one of these families.
Martin and his wife homestead a plot of land in a fertile valley. They have kids. Over the years, a few more families settle in this valley, and the children of the several families intermarry. Many generations later, there are quite a number of people who think they might be descendents of Martin. They are quite proud of this, because legend says that he was blue.
To model what happens in this valley, letÕs follow the genetic inheritance of MartinÕs descendents. Martin was blue, so he must not have had a functional copy of the ND gene. We can model this by representing Martin with two blue pieces of yarn—one of each of MartinÕs ND genes. MartinÕs wife was unrelated to him, and had two functional ND genes, so we can represent her with two white pieces of yarn. Let's put two blue pieces of yarn on a desk at the back of the room to represent Martin, and two pieces of white yarn on the desk next to it to represent his wife.
The way genetics works, we each have two sets of genes, one set from Mom, one set from Dad. MartinÕs children, therefore, received one blue gene from Martin, and one white gene from his wife. We can represent MartinÕs children with one blue piece of yarn, and one white piece of yarn. LetÕs consider only two of MartinÕs children, William and Hanna, to keep things simple. To represent William, let's put a blue piece of yarn (from Martin) and a white piece of yarn (from Martin's wife) on a desk in front of the desks representing Martin and his wife. To represent Hanna, let's put a blue and a white piece of yarn on another desk in the same row.
Are William and Hanna blue? Think about it and discuss it. They have one non-functional gene from Martin (blue yarn), but they have a perfectly good gene from MartinÕs wife (white yarn). If the perfectly good, functional gene can make the ND enzyme, can they repair damaged hemoglobin?
Now, William and Hanna grow up and marry newcomers to the valley. We can model WilliamÕs wife with two white pieces of yarn and HannaÕs husband with two white pieces of yarn. Let's put pieces of yarn on two more desks to represent the people whom William and Hanna marry.
From these marriages, William has several children and Hanna has several children. How can we represent these children? WilliamÕs children each inherit a ŅwhiteÓ gene from WilliamÕs wife, which we can represent with a piece of white yarn. Each of the children must also inherit a gene from William—but which one? To make the choice, flip a coin. If it comes up heads, give the child a white piece of yarn. If it comes out tails, give the child a blue piece of yarn. Represent these children with the appropriate pieces of yarn on desks in a new row.
We can do the same for HannaÕs children. Each has one white piece of yarn, and a second piece of yarn that is determined by the flip of the coin.
Why do we use a coin flip? Think about it and discuss it. The answer, quite simply, is that neither Hanna nor William can decide which gene to put into a particular egg cell or sperm cell. That choice is not up to them, and occurs entirely at random.
If you like, you can follow this family through the generations, each time having the children inherit one ND gene from each parent. As long as each of MartinÕs descendents marries someone from a different family, the blue ND gene will always be paired with a white one.
But, what if one of MartinÕs descendents marries another of his descendents? LetÕs model this by allowing marriages between HannaÕs children and WilliamÕs children. These would be marriages between cousins, which has been known to occur, especially in small villages in isolated areas.
The most interesting thing occurs if one of HannaÕs children, represented by blue and white yarn, marries one of WilliamÕs children, also represented by blue and white yarn. It this occurs, what will their children be like? For each child, we need to determine whether the child inherits the white or the blue gene from Mom, and whether the child inherits the white or the blue gene from Dad. Model this family—give them 10 children. Do any of them happen to inherit blue genes from both Mom and Dad? If they do, then they are blue, just like Martin was.
Now consider what happens over many generations. MartinÕs blue gene, represented by the blue yarn, is passed on from generation to generation. It may not get passed on in every lineage, if the luck-of-the-coin-flip works out just right, but most likely it moves through the generations. Usually, it is paired with a white gene. But sometimes there may be a great-great-great-great grandchild of MartinÕs who marries another great-great-great-great grandchild of his, and the blue trait may show up again.
In actual fact, this is what has happened in the Blue Families. As you will see when you read The Blue People of Troublesome Creek, there is a simple cure for blueness, so this trait is no longer troublesome. You will also see that the individuals we have represented with blue and white yarn are not entirely free of blueness—they are bluer than normal at birth, but Ņpinken upÓ quickly, and their lips and fingernails can be somewhat blue. The message from Ben Stacy tells us this. What is the value, to you, of the blue people and the bits of yarn? They should help you see how inheritance works, and how our traits are determined by genes and chemistry. It sometimes helps to know that real people, in real families, have helped us learn how it works.