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It Runs in the Family
Investigating Sex-Linked Traits and Developing a Pedigree
Background
You are a genetic counselor. As a friend of the Garcia family you have volunteered to develop
a pedigree for them. The gene for red-green color blindness runs in this family. Using the
pedigree you develop, you will determine or predict the genotypes for several of the family
members.
Most of the traits we have studied so far are autosomal. These genes are located on
homologous chromosomes that are not sex chromosomes (not X or Y). Today we will be
dealing with an X-linked trait. X-linked genes are located on the X-chromosome. The most
common type of red-green color blindness is due to a mutation on the X-chromosome (i.e. a
red-green color blind allele). X-linked red-color blindness is a recessive trait. We represent the
presence of the allele as Xb. The “normal” vision allele is represented as XB. Females
heterozygous for this trait (XB Xb) have normal vision, but are considered “carriers” of the allele
because they can still pass the trait on to their children. The color perception defect manifests
itself in females only when it is inherited from both parents, thus making them homozygous
for the trait (Xb Xb). By contrast, males inherit their single X-chromosome from their mothers
and become red green color blind if this X-chromosome has the color perception defect (XbY).
Data
You will develop a pedigree using the following information. Remember to use all the
notations discussed at the beginning of class. You may want to refer to a textbook to refresh
your memory.
Garcia Family:
Jerome Garcia married Barbara (Garcia). Barbara is a carrier for colorblindness. Jerome is
colorblind. They have three daughters; Amy, Nicole, and Laura, none of which are colorblind.
Nicole does not marry, but Laura marries Michael Kang who has normal vision.
Anastasio Family:
Giuseppe Anastasio II and Susan Anastasio have four children; Heather, Melissa, Trey, and
Steve. Giuseppe is not colorblind, but his wife Susan is a carrier for colorblindness. Steve (the
son) is colorblind, while Trey, Melissa, and Heather are not. Melissa was tested and discovered
that she was not a carrier. Heather is never tested and marries Bobby Weir. They (Bobby and
Heather) have two sons; Mike and Joshua. Mike has “normal” vision while Joshua is
colorblind.
The Anastasio/Garcia Connection:
Steve Anastasio and Amy Garcia were married and had two children; Patricia and James. Patricia
and James are both colorblind. Patricia married Ziggy Morley and the two are expecting a son.
Your Task
You will develop a pedigree for this family based on the information provided above. It’s a
good idea to lay out the pedigree without the trait first. Then write the genotypes of each
individual in the pedigree and color in the symbol for individuals who carry or express the
trait. You will notice that some of the genotypes are missing. You should be able to determine
these genotypes by doing Punnett’s squares. Answer the following:
1. Find Heather Anastasio-Weir’s genotype. Show your work below. (Hint: you must
look at her sons and work backwards)
2. If Nicole and Laura Garcia are not colorblind, what are their genotypes? Show your
work!!
3. If Laura and Michael Kang have a child, what is the probability of it being colorblind?
Show your work!!!
4. Ziggy Morley and Patricia Anastasio-Morley are expecting a son. What is the
probability that their son will be colorblind? Show your work!!!!