Name ______Period ______
Sex-Linked Inheritance Activity
Objectives:
- Apply concepts of statistics and probability to explain the variation and distribution of expressed traits in a population.
Materials and Resources:
- 1 black bean (represents the Y chromosome)
- 3 white beans (each white bean represents an X chromosome)
- 2 plastic cups
- Marker
Procedure:
Cross 1
1. Obtain 2 cups, 3 white beans, and one black bean.
2. Label one cup ‘mother’ and the other cup ‘father.’
3. Each white bean represents the X chromosomes. Put a dot on one white bean to represent the X-linked allele for colorblindness (remember that the dot represents the recessive allele “b” for colorblindness). Place this bean, plus one unmarked white bean in the ‘mother’ cup. The unmarked bean would have the dominant allele or “B” allele that is not colorblind. The mother is a carrier for this sex-linked trait.
4. Place one unmarked white bean, plus 1 black bean, into the cup labeled ‘father.’
5. Close your eyes and pick one bean from each parental cup. This represents each parent contributing a sex chromosome to a fertilized egg.
6. Examine the two beans in your hand. In your data table, record the colors of the two beans.
7. Determine the sex of this individual carrying this pair of chromosomes. Remember that white beans are X chromosomes and black beans are Y chromosomes.
8. Record the genotype of the individual. Remember that the dot represents the lower case allele on the X chromosome and no dot on the white bean represents the upper case allele on the X chromosome.
9. Put the beans back in the cups they came from.
10. REPEAT STEPS 1 THROUGH 5 for nine more trials.
Cross 2
1. Obtain 2 cups, 3 white beans, and one black bean.
2. Label one cup ‘mother’ and the other cup ‘father.’
3. Each white bean represents the X chromosomes. Put a dot on one white bean to represent the X-linked allele for colorblindness. Place this bean, plus one unmarked white bean in the ‘mother’ cup. The unmarked bean would have the dominant allele or “B” allele that is not colorblind. The mother is a carrier for this sex-linked trait.
4. Mark a black dot on a different white bean. Place this bean, plus 1 black bean, into the cup labeled ‘father.’ (remember that the dot represents the recessive allele “b” for colorblindness)
5. Close your eyes and pick one bean from each parental cup. This represents each parent contributing a sex chromosome to a fertilized egg.
6. Examine the two beans in your hand. In your data table, record the colors of the two beans.
7. Determine the sex of this individual carrying this pair of chromosomes. Remember that white beans are X chromosomes and black beans are Y chromosomes.
8. Record the genotype of the individual. Remember that the dot represents the lower case allele on the X chromosome and no dot on the white bean represents the upper case allele on the X chromosome.
9. Put the beans back in the cups they came from.
10. REPEAT STEPS 1 THROUGH 5 for nine more trials.
Cross 1 Group Data: Cross 1 Class Data:
Trial / Colors of the two beans / Sex of the individual(male or female) / Alleles present
(genotype)
1
2
3
4
5
6
7
8
9
10
Total # of normal males:
Total # of colorblind males:
Total # of normal females:
Total # of carrier females:
Total # of colorblind females:
Group / Normal Males / Colorblind Males / Normal Females / Carrier Females / Colorblind Females
1
2
3
4
5
6
7
8
9
10
TOTALS:
Cross 2 Group Data: Cross 2 Class Data:
Trial / Colors of the two beans / Sex of the individual(male or female) / Alleles present
(genotype)
1
2
3
4
5
6
7
8
9
10
Total # of normal males:
Total # of colorblind males:
Total # of normal females:
Total # of carrier females:
Total # of colorblind females:
Group / Normal Males / Colorblind Males / Normal Females / Carrier Females / Colorblind Females
1
2
3
4
5
6
7
8
9
10
TOTALS:
Questions:
1. In general, how do the sex chromosomes keep the number of males and females roughly the same?
2. In cross 1, why were none of the female offspring colorblind?
3. In cross 1, why were roughly half of the male offspring colorblind?
4. In cross 2, why is the mother considered to be a “carrier”? How is the heterozygous genotype related to being a “carrier”?
5. In cross 2, the father is color blind. Why is the father color blind when he ONLY has one recessive allele?
6. In cross 2, why were there no normal (non-carrier) females?
7. Use the data to explain the special pattern of inheritance for sex-linked genes. Why does this pattern exist?