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Other Traits

Hey, why do some people get a lot taller than their parents? And why am I a natural athlete and my parents are totally different? My hair is kinda the same as theirs but my eyes are way different. Please explain.

-A middle school student from Tennessee

March 9, 2005

We often hear that a child is the "spitting image" of one of their parents. An identical carbon copy, a couple decades younger.

How is it then that some of us end up being so different from our parents? The way we look, our interests, and our abilities just don't seem to reflect them at all.

There is no single explanation for the differences we see between generations. Several factors play a part.

Genes have a starring role in determining our physical attributes. For instance, a single gene decides if we can roll our tongue into a tube. Two genes determine our base eye color as brown, blue, or green (/ask/ask2).

Unfortunately, the genetics governing most of our traits is not nearly as simple. Nor as well understood. This is because most traits utilize not just one or two genes, but many genes, to make us the way we are.

When lots of genes are at work, as with height and skin color, we see an entire spectrum of the trait across the population. For example, there aren't just tall people and short people. There are people of all heights falling anywhere between tall and short.

Figuring out which genes are responsible for these "many-gene" traits is challenging. Scientists have identified a few genes that are thought to be involved in our height. Although the exact number is not known, they estimate that as few as 7 to more than 20 genes may be working together on our stature. With so many players in the "height game", it is no wonder we vary so much from one another.

This explains why we may not stand eye to eye with our friends. But what about the differences we see in our own families?

As you mentioned, some children are considerably taller than their parents. Basketball legend, Wilt Chamberlain, was a towering 7'1". Strangely, neither of his parents stood above 5'9". Of course, children can be much shorter than their parents, too.

So how do a bunch of genes work together to determine our height?

Let's say, for the sake of simplicity, only three genes control our height. We will call them A, B, and C. An uppercase letter, like A, means "tall". A lowercase letter, like a, means "short".

Remember that we have two copies of every gene, one from dad and one from mom. Keeping that in mind, we would have 6 copies of these 3 genes. Our final height is then an average of the number of "tall" and "short" copies we inherit.

For an example, let's take a father whose gene makeup is AaBbCc and a mother with aaBBCc. With 3 "tall" and 3 "short" copies each, both are of average height. How tall will their kids grow?

Which copy of each gene we inherit from our parents is random. In this family, the children will most likely inherit 3 "tall" and 3 "short" copies. Like their parents, they will be average height. But this will not always be the case.

There is a smaller chance that their child could inherit five "tall" copies, ABC from dad and aBC from mom. This child will be quite tall. On the other hand, dad could pass down abc and mom could pass down aBc. With five "short" copies, this child would be considerably shorter.

Imagine how many possibilities there would be if twenty genes controlled our height!

All the burden of our stature cannot be placed solely on our genes. Many factors contribute to our final adult height. These are called Environmental Effects. They act on our "genetic height" to make us taller or shorter.

Some of these factors are at work very early, before we are even born! A mother's health while she is pregnant is thought to be a key player. Poor diet and toxic substances, like alcohol and nicotine, can be harmful to the growth of her child.

After we are born, healthy eating habits are vital in reaching our maximum height. Avoiding drugs, environmental pollutants, and childhood illnesses is important. As is getting enough exercise and sleep.

Many of our traits follow a similar pattern. They are a blending of lots of different genes as well as our environment.

Is this also true regarding how athletic we are? Sure. As we just talked about, our genes are important in designing our bodies. Height is important for success in sports like basketball and gymnastics. Endurance and lean muscles are critical to a long distance runner.

But even at seven feet tall, you may not excel at basketball. Many things can affect how successful we are at different activities.

Do we enjoy the activity? Are there local teams or clubs that we can join to help us practice? As the old adage says, practice makes perfect!

Do we have strong interests in other areas, like music or art, which limit our time? Can we afford to do the activity? Downhill skiing, photography, and painting are expensive hobbies.

There are many reasons that we are the way we are. Our genes make a significant contribution. But our lifestyle is important, too!

By Anne Tecklenburg Strehlow, Stanford University


People come in all shapes and sizes