# Genetics

## Building Pedigrees

Let's Build a Family Chart: a Pedigree of My Family

Aunt Molly has brown hair. She married Uncle Sven, who's blonde, and all five of their kids have brown hair like Aunt Molly. But her sister, Aunt Jess, also has brown hair, and Uncle Tage has blonde hair, but their son is blonde! Why doesn't Aunt Jess's son have brown hair, like Aunt Molly's children do?

Let's see if we can figure this out. Hmm....what do we know? Let's start by drawing a couple of simple diagrams to keep track of Molly and Sven's family, and to keep track of Jess and Tage's family. We'll use circles for females, and squares for males, and we'll color these symbols brown or light tan to match the hair color. This would give us:

Of course, Molly and Jess are sisters, and they have a number of brothers and sisters (including Dad!). Let's add them, and Grandma and Grandpa, to the diagram.

It still isn't clear why Jess and Tage have a blonde child, but Molly and Sven do not. Maybe we can find out about Grandma's and Grandpa's parents, and their parents, and so on. ... After some searching, we've learned a bit about Grandpa, but we haven't learned very much about Grandma. Grandpa's mother was blonde! But his father had brown hair, and his father's parents had brown hair. Let's diagram them this way:

This helps somewhat! Grandpa's mother was blonde, so maybe Jess inherited something from her that made it possible for her to have a blonde son (my cousin John). But what could this be? Maybe we'll have to wait until I get married, and we can see what my children are like.

..... many years pass .....

Now I have 3 children, and one of them is blonde! John has two sons, one of whom is blonde. Here's what our family pedigree is like now:

I can understand how John would have a blonde child, since he's blonde himself, and his dad is blonde. But how did I get a blonde daughter? I have brown hair, my husband has brown hair, my parents have brown hair, and so did my grandparents. My great grandmother had blonde hair, but how could her hair color reappear so many generations later?

Section 2

The Difficulty in Discovering the Mechanisms of Inheritance by Studying Human Populations

As we see in the pedigree chart above, Jess and Molly both have brown hair, and married men with blonde hair. Yet, their children show different hair colors--Jess's son is blonde, while Molly's children are all brown-haired. Shouldn't Jess have had some brown-haired children, too? It's hard to know...she had just the one son. Could Molly have had a blonde child if she'd had more children? Maybe, but neither family has a large enough number of children to be able to discover what all of the possibilities are. We know, for example, that in humans, children tend to be either boys or girls, with equal probability. Jess's one child could have been male or female, but happens to be male. Hed she had another child, that one might have been male or female...but she didn't have another child, so we can't tell for sure. Molly had more children, 2 girls and 3 boys, which is closer to the 50:50 ratio that we think is reasonable. Still, it's not perfect. Rather than being a ratio of 1:1, it's a ratio of 2:3. Their parents' generation might be better to study, since they had more children. For them, the ratio of females to males is 5:4. It looks like the trait of maleness or femaleness might be inherited in a simple manner: each child has a 50% chance of being male and a 50% chance of being female. But it's hard to test this idea with single human families, because we just don't have enough children for the numbers to get closer to the 1:1 ratio that this idea suggests we should find.

Hair color--and probably most other traits as well--is trickier. At each generation, members of other families come in, bringing their own versions of the traits into the family. For example, here is a more complete pedigree chart of the family above, with arrows pointing to the individuals who married into the extended family, but for whom we have no information.

The fact that many people join the family, bringing with them their own genetic history, makes it difficult to work out the Rules of Inheritance from human pedigrees, unless we have a huge amount of information concerning a very large number of families. Perhaps, however, we can begin to identify some of the rules if we study hair color in some other mammals. Some mammals, such as dogs, have larger litter sizes than the typical human (we usually have one child at a time). Also, with domesticated animals, there are many varieties, or "breeds," in which particular hair colors "breed true" -- that is, in which every individual of that breed has the same coat color. Perhaps, if we study some model systems, we can identify the rules that govern inheritance in those species. Then, we can come back to our study of humans, and see if the same rules apply to us.