Download number of colorblind males

Name _______________________________________________
Date __________
Activity: How is Colorblindness Transmitted?
Goal: To model how a sex-linked gene is transmitted.
Background Information: Is there a special pattern of inheritance for genes located
on the X chromosome or the Y chromosome? The answer is yes. Because these
chromosomes determine sex, genes located on them are said to be sex-linked genes.
Many sex-linked genes are found on the X chromosome. More than 100 sex-linked
genetic disorders have now been mapped to the X chromosome. These include
colorblindness and hemophilia. The Y chromosome is much smaller than the X
chromosome and appears to contain only a few genes.
Three human genes associated with color vision are located on the X
chromosome. In males, a defective version of any one of these genes produces
colorblindness, an inability to distinguish certain colors. The most common form of this
disorder, red-green colorblindness, is found in about 1 in 10 males in the United States.
Among females, however, colorblindness is rare – only about 1 in 100 has
colorblindness. Why the difference?
Males have just one X chromosome. Thus, all X-linked alleles are expressed in
males, even if they are recessive. In order for a recessive allele, such as the one for
colorblindness, to be expressed in females, there must be two copies of the allele, one
on each of the two X chromosomes. This means that the recessive phenotype for a
sex-linked genetic disorder tends to be much more common among males than among
females.
Materials: 2 paper bags, 3 white beans, black marker, and one red bean
Procedure:
1. What I Know: Write a sentence or two about what you already know
about sex-linked traits.
2. Label one bag “female” and the other bag “male.”
3. The white beans represent X chromosomes. Use the marker to make a dot on 1
white bean to represent the X-linked allele for colorblindness. Place this
bean, plus 1 unmarked white bean, into the bag labeled “female.”
4. Place a white bean and a red bean into the bag labeled “male.” The red bean
represents a Y chromosome.
5. Close your eyes and pick one bean from each bag to represent how each
parent contributes a sex chromosome to a fertilized egg.
6. Fill in the attached data table. Record the color of each bean and the sex of an
individual who would carry this pair of sex chromosomes. Also record how many Xlinked alleles for colorblindness this individual has. Put the beans back in the bags
they came from.
7. Determine whether the individual would have colorblindness.
8. Continue choosing beads from the bags for a total of 10 trials.
9. What I Did: Write a short summary of the procedure you used to complete
this activity.
10. What I Observed: This section should include the handout provided.
A. Expected Results: Create a Punnett square showing your expected results
for your cross. The cross represented in the activity is XXC x XY. XC is the
allele for colorblindness.
Punnett square:
X
XC
X
Y
% of colorblind males
_______
% of colorblind females _______
B. Observed Results: Complete the following table to obtain your observed results.
Trials
Example
1
2
3
4
5
6
7
8
9
10
Alleles
White/Red
X cY
Sex of
Individual
Male
Number of Xlinked Alleles for
Colorblindness
1
Colorblind?
Yes/No
yes
C. Summary of Observed Results:
Number of Colorblind Females
Total Number of Females
Percentage of Colorblind Females
number of colorblind females
total number of females
Number of Colorblind Males
Total Number of Males
Percentage of Colorblind Males
number of colorblind males
total number of males
11. What I Learned: Based on the data you collected, write a complete
explanation of what you learned from this activity.
12. What I Wonder: Pose a question that arises from this activity or topic.
13. Questions: Answer the following questions in complete statements.
a. Why did you select one bean from each bag?
b. From your observed results, what percentage of females were colorblind?
c. From your observed results, what percentage of males were colorblind?
d. How do your expected and observed results compare?
e. Is it possible for a son to inherit red-green color blindness from his father?
Explain your answer.
f. How would the results be different if both parents carried a recessive allele for
colorblindness?
Take the colorblind test!
Go to: http://www.copresco.com/links/colblind.htm