5 Steps to a 5 AP Biology, 2014-2015 Edition

Heredity ❮ 107

from male to female represents mates that have produced offspring. The offspring are listed

below their parents from oldest to youngest. A fully shaded individual possesses the trait

being studied. If the condition being studied is a monogenic recessive condition (rr), then

those shaded gray have the genotype rr. If the condition being studied is a dominant con-

dition (Rr or RR), then those that are unshaded have the genotype rr. A line through a

symbol indicates that the person is deceased. A sample pedigree is shown in Figure 10.6.

Pedigrees can be used in many ways. One use is to determine the risk of parents passing

certain conditions to their offspring. Imagine that two people want to have a child, and they

both have a family history of a certain autosomal recessive condition (dd). Neither has the

particular condition, but the man has a brother who died of the disease and the woman’s

mother died of the disease at an older age. They want to know the probability of having a

child with the condition. You must first determine the probability that each parent is a car-

rier, and then determine the probability of the parents having a child with the disease, given

that they are carriers. See the pedigree in Figure 10.7.

First, we can determine the father’s (second generation) probability of being a carrier. We

know that both of his parents must be carriers with a genotype of Dd. Why is this the

case? Although neither parent has the condition, they must both be carriers for his brother

Figure 10.6 Schematic of a pedigree.

Figure 10.7 Three-generation pedigree indicating probability of inheriting a par-

ticular disease.

1 st generation

2 nd generation

3 rd generation

A B

1 st generation

2 nd generation

3 rd generation?

KEY IDEA

CT teacher:
“Test almost
always has two to
three questions
about this topic.”

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