Question

At Dinner That Night Awilda: We should try to find a way to make sure we only have sons, no daughters. I don't want to have any daughters who might be color blind and have so many problems like I do. Color blindness wouldn't matter so much for a boy. Frank: Remember, the doctor said that, since I'm not color blind, none of our daughters would be color bliyd, only our sons. Awilda: Ickagree. I think our daughters will be color blind like me and our sons will be normal like you. Frank: No, the doctor said that the gene for color blindness is on the X chromosome, so only our sons will inherit your colorblindness. Awilda: That doesn't make any sense. Girls have more X chromosomes than boys, so girls should be more likely to be color blind. Help Frank to explain to Awilda why their sons will be color blind and their daughters will not be color blind by answering the following questions. 1. What are the genotypes of Awilda and Frank? (Since the allele for color blindness is located on the X chromosome, use the symbol X.. for an X chromosome with the recessive allele for color blindness and Xcs for an X chromosome with the dominant normal allele.) Awilda: Frank: 2. Draw the Punnett square for this couple and their children. In this Punnett Square, circle each daughter and use arrows to indicate any colorblind offspring. 3. Write an explanation to help Awilda understand why their sons will be colorblind even though their father has normal color vision. 4. Explain why their daughters will not be colorblind like their mother. Explain why having two X chromosomes decreases a person's risk of color blindness, instead of increasing her risk.

Answer 1

1. The genotype of Awilda should be Xcb Xcb. ( Since she is colorblind, she should be homozygous for recessive colorblind allele )

The genotype of Frank should be XCBY. ( Since he is not colorblind, X chromosome should have normal dominant allele)

2. Punnett Square

3. The gene responsible for colorblindness is located on the X chromosome, not on the Y chromosome. Since males have only one X chromosome, they have high chances of expressing the trait ( colorblindness in this case ).

In this condition, Frank is normal, which means the X chromosome transferred by him onto the next generation daughters will have the normal dominant allele.

Alwilda, on the other hand, is colorblind, which means her both X chromosomes have abnormal colorblind allele which will be transferred onto the next generation daughters and sons. Since sons will receive an abnormal colorblind X chromosome from Awilda, they will be colorblind.

4. Alwilda is colorblind, which means her both X chromosomes have abnormal colorblind alleles which will be transferred onto the next generation daughters and sons.

Daughters will be receiving on the normal X chromosome from Frank also, so, they will become heterozygous for the condition, i.e., daughters will become the carriers of colorblindness and will not manifest the condition.

Colorblindness will be manifested only when daughters are homozygous for recessive allele, which is not the case.

5. Having two X chromosomes decreases the risk of colorblindness because a defect in one is typically compensated by the other, making the person just a carrier not the sufferer.