olVirtual Genetics Lab: The Inheritance of Color Blindness Worksheet

Learning Goal: To explore the inheritance pattern of color blindness in humans.
Prerequisite Knowledge: Before beginning this lab, you should be familiar with these concepts: * the definition of sex-linked genes, and examples of sex-linked genes in humans and other organisms * how the inheritance pattern of sex-linked genes differs from that of autosomal genes * the use of Punnett squares to understand patterns of inheritance
Introduction: The gene that codes for red-green color blindness, the most common form of color blindness in humans, is found on the X chromosome. One in 12 males is color blind. In females, however, color blindness affects only about 1 in 200. Why is there such a difference?

In this lab activity, you will act as a genetic counselor and help parents determine their likelihood of having color-blind children. You will also use this information to understand why males are more likely than females to be color blind.
Part A
Couple 1 comes into your office. The husband has normal vision; the wife is heterozygous for the color-blindness allele.

Enter the Virtual Genetics Lab Room by clicking the button. Then, follow the Lab Procedure.

Lab Procedure 1. Make sure that Human is chosen from the Species Selector and that Color Blindness is selected from the menu of traits. 2. In the Set Alleles box, set the parents’ alleles according to the information about Couple 1. Then, set the number of offspring to the maximum of 6. 3. Click the Cross button repeatedly until these parents have produced about 100 F1 offspring—a large enough sample size to determine the pattern of inheritance of the trait. Make sure Include Male/Female is checked in the lower left corner of the results box. |
What percentage of Couple 1’s male offspring will be color blind?
What percentage of their female offspring will be color blind? According to the results, it came up with 0 percent of the female generation being color blind. The male in both graphs will be 25 to 28 percent of the males will be color-blind.

ANSWER: |

Part B
Couple 2 comes into your office. The husband is color blind; the wife is homozygous for the normal vision allele. 4. Click the Reset button at the bottom. Then, choose Color Blindness again and set the parents’ alleles according to the information about Couple 2. 5. Again, set the number of offspring to the maximum of 6. Then, click the Cross button repeatedly until these parents have produced about 100 F1 offspring. |
What percentage of Couple 2’s male and female offspring will be color blind? The percentage of males and females being colorblind is 0 percent when the male is carrying the color blind gene.

ANSWER: |

Part C
Couple 3 comes into your office. The husband is color blind; the wife is heterozygous for the color-blindness allele. 6. Click the Reset button at the bottom. Then, choose Color Blindness again and set the parents’ alleles according to the information about Couple 3. 7. Again, set the number of offspring to the maximum of 6. Then, click the Cross button repeatedly until these parents have produced about 100 F1 offspring. |
What percentage of Couple 3’s male and female offspring will be color blind? In this case when they both carry the gene females were 27 percent color blind and 25 percent of males are color blind.

ANSWER: |

Part D
Review the results you obtained for the female offspring of the three couples.
Based on your results for the female offspring, predict whether color blindness is a dominant or recessive trait. Explain your reasoning. I believe if both parents carry the trait then the 1 and 4 chance that the child will have the trait in females and the same in males. If the just the gene is carried by the female it is more likely that the males will carry the gene and no females will have the gene. I believe that it is a dominant trait in females but not males or at least carrying them. However it is a dominant trait in males when the female carries the gene.

ANSWER: |

Part E
Complete the Punnett squares below to determine the possible genotypes of each couple’s male and female offspring. (X represents the normal color vision allele. X represents the color-blindness allele. Y represents the Y chromosome, which does not carry the color-vision gene.)

X X | XX | XY | XY |

Couple 1

Wife

| X | X | X | xx | xx | Y | xy | xy |

Husband

Couple 2

Wife

| X | X | X | xx | xx | Y | xy | xy |

Husband

Couple 3

Wife

| X | X | X | xx | xx | Y | xy | xy |

Husband

Part F
Review the results you obtained from the lab for the female offspring of all three couples. Then, look at the female offspring (the offspring with two X chromosomes) in your three Punnett squares.
What genotype must a female child have in order to be color blind? Xx gene which is guaranteed to carry the trait.

ANSWER: |

Part G
Review the results you obtained from the lab for the male offspring of all three couples. Then, look at the male offspring (the offspring with a Y chromosome) in your three Punnett squares.
What genotype must a color-blind male have? Explain. Males carry the x and a y chromosome in order to have the trait.

ANSWER: |

Part H
Think it over
Explain why color blindness occurs more commonly in males than females.
Because the males carry a x and y chromosome where the x is commonly carries the trait. Because woman carry two x chromosomes it is more likely that the gene carries a defective trait and that is why it is more likely that the man gets the trait then the woman. ANSWER: |

Part I
Think it over
Based on your Punnett squares, determine whether a son can inherit color blindness from his father. Explain your reasoning. It is possible that the son can inherit the gene but only in one way and that is if the father and mother both have the genes. If the father doesn’t have the gene the son will not get the genes but the females are more likely to pass the gene to the sons as the fathers are more likely to pass it to the daughters if they are carriers.

ANSWER: |