Chapter 4. Animation Activity: X-Linked Inheritance

2

Correct. Crosses between true breeding white-eyed females and red-eyed males would result in 50% red-eyed females and 50% white-eyed males because the white eye trait is linked to the X-chromosome and males only inherit their X chromosome from their mother. If the white eye trait were autosomal recessive, this cross would have yielded all red-eyed F1 progeny. All of the other crosses described would yield identical results whether or not the white-eyed trait was X-linked.

You have one more chance to answer the question correctly. Try again.

Incorrect. Crosses between true breeding white-eyed females and red-eyed males would result in 50% red-eyed females and 50% white-eyed males because the white eye trait is linked to the X-chromosome and males only inherit their X chromosome from their mother. If the white eye trait were autosomal recessive, this cross would have yielded all red-eyed F1 progeny. All of the other crosses described would yield identical results whether or not the white-eyed trait was X-linked.

2

Correct. This is, in fact, the intercross performed by T. H. Morgan after first crossing a red-eyed female with a white-eyed male. When these F1 were intercrossed, they yielded all red-eyed females with 50% of the males having white and 50% of the males having red eyes.

You have one more chance to answer the question correctly. Try again.

Incorrect. This is, in fact, the intercross performed by T. H. Morgan after first crossing a red-eyed female with a white-eyed male. When these F1 were intercrossed, they yielded all red-eyed females with 50% of the males having white and 50% of the males having red eyes.

2

Correct. In this case, the male’s X chromosome, carrying the recessive white-eye trait, would have no bearing on the phenotype of the progeny. Whether the progeny are male or female, their eye color depends entirely on which X chromosome they inherit from their mother. Since their mother is heterozygous, half of her offspring will inherit the dominant red-eye trait and half will inherit the recessive white-eye trait.

You have one more chance to answer the question correctly. Try again.

Incorrect. In this case, the male’s X chromosome, carrying the recessive white-eye trait, would have no bearing on the phenotype of the progeny. Whether the progeny are male or female, their eye color depends entirely on which X chromosome they inherit from their mother. Since their mother is heterozygous, half of her offspring will inherit the dominant red-eye trait and half will inherit the recessive white-eye trait.

2

Correct. In this case, the male’s X chromosome, carrying the recessive red-eye trait, would have no bearing on the phenotype of the progeny. Whether the progeny are male or female, their eye color depends entirely on which X chromosome they inherit from their mother. Since their mother is heterozygous, half of her offspring will inherit the dominant white-eye trait and half will inherit the recessive red-eye trait.

You have one more chance to answer the question correctly. Try again.

Incorrect. In this case, the male’s X chromosome, carrying the recessive red-eye trait, would have no bearing on the phenotype of the progeny. Whether the progeny are male or female, their eye color depends entirely on which X chromosome they inherit from their mother. Since their mother is heterozygous, half of her offspring will inherit the dominant white-eye trait and half will inherit the recessive red-eye trait.

2

Correct. If the white-eye trait were X-linked dominant, all of the female progeny would have white eyes because they would inherit the white-eye trait from their father’s X chromosome. In contrast, the males would inherit their single X chromosome from their mother, with half receiving the white-eye trait and half receiving the red-eye trait.

You have one more chance to answer the question correctly. Try again.

Incorrect. If the white-eye trait were X-linked dominant, all of the female progeny would have white eyes because they would inherit the white-eye trait from their father’s X chromosome. In contrast, the males would inherit their single X chromosome from their mother, with half receiving the white-eye trait and half receiving the red-eye trait.

2

Correct. If the white-eye trait were Y-linked, only flies inheriting a Y chromosome would be expected to possess white eyes. Under normal circumstances, male Drosophila are the only flies having a Y chromosome; if the trait is dominant, all flies inheriting the trait will have white eyes.

You have one more chance to answer the question correctly. Try again.

Incorrect. If the white-eye trait were Y-linked, only flies inheriting a Y chromosome would be expected to possess white eyes. Under normal circumstances, male Drosophila are the only flies having a Y chromosome; if the trait is dominant, all flies inheriting the trait will have white eyes.

2

Correct. Each nondisjunction of X chromosomes in the red-eyed female will generate gametes with two X chromosomes and with no X chromosomes with equal frequency. The white-eyed male will generate gametes with an X chromosome, bearing the white trait, or a Y chromosome, again with equal frequency. The union of these gametes generates four potential types of offspring. If an egg with two X chromosomes is fertilized by a sperm with one X chromosome, the result would be an X⁺X⁺X^w metafemale fly that is nonviable. If an egg bearing two X chromosomes is fertilized by a sperm bearing a Y chromosome, the result would be an X⁺X⁺Y female that will be phenotypically indistinguishable from the red-eyed females resulting from gametes without any nondisjunction. If an egg with no X chromosomes is fertilized with an X chromosome bearing sperm, the result will be an X X^wmale with white eyes. If an egg with no X chromosomes is fertilized with a Y-bearing sperm, the zygote will die because survival requires at least one X chromosome. So if there are three white-eyed males and 1234 red-eyed offspring, we can assume that there were (3×4) 12 zygotes resulting from nondisjunction gametes. Of these, we would have expected only six to survive. Therefore the rate of nondisjunction is 12/(1234 + 9) = 0.0097. The total number of offspring was 1243 to account for the six zygotes that would not have survived and the three white offspring which led to the discovery of the nondisjunction.

You have one more chance to answer the question correctly. Try again.

Incorrect. Each nondisjunction of X chromosomes in the red-eyed female will generate gametes with two X chromosomes and with no X chromosomes with equal frequency. The white-eyed male will generate gametes with an X chromosome, bearing the white trait, or a Y chromosome, again with equal frequency. The union of these gametes generates four potential types of offspring. If an egg with two X chromosomes is fertilized by a sperm with one X chromosome, the result would be an X⁺X⁺X^w metafemale fly that is nonviable. If an egg bearing two X chromosomes is fertilized by a sperm bearing a Y chromosome, the result would be an X⁺X⁺Y female that will be phenotypically indistinguishable from the red-eyed females resulting from gametes without any nondisjunction. If an egg with no X chromosomes is fertilized with an X chromosome bearing sperm, the result will be an X X^wmale with white eyes. If an egg with no X chromosomes is fertilized with a Y-bearing sperm, the zygote will die because survival requires at least one X chromosome. So if there are three white-eyed males and 1234 red-eyed offspring, we can assume that there were (3×4) 12 zygotes resulting from nondisjunction gametes. Of these, we would have expected only six to survive. Therefore the rate of nondisjunction is 12/(1234 + 9) = 0.0097. The total number of offspring was 1243 to account for the six zygotes that would not have survived and the three white offspring which led to the discovery of the nondisjunction.

2

Correct. Each nondisjunction of X chromosomes in the red-eyed female will generate gametes with two X chromosomes and with no X chromosomes with equal frequency. The white-eyed male will generate gametes with an X chromosome, bearing the white trait, or a Y chromosome, again with equal frequency. The union of these gametes generates four potential types of offspring. If an egg with two X chromosomes is fertilized by a sperm with one X chromosome, the result would be an X⁺X⁺X^w metafemale fly that is nonviable. If an egg bearing two X chromosomes is fertilized by a sperm bearing a Y chromosome, the result would be an X⁺X⁺Y female that will be phenotypically indistinguishable from the red-eyed females resulting from gametes without any nondisjunction. If an egg with no X chromosomes is fertilized with an X chromosome bearing sperm, the result will be an X^w male with white eyes. If an egg with no X chromosomes is fertilized with a Y-bearing sperm, the zygote will die because survival requires at least one X chromosome. Therefore, if the nondisjunction rate is 2%, this means that 20 of the 1000 offspring were the result of nondisjunction-bearing gametes, of which 1/4 would be white-eyed males.

You have one more chance to answer the question correctly. Try again.

Incorrect. Each nondisjunction of X chromosomes in the red-eyed female will generate gametes with two X chromosomes and with no X chromosomes with equal frequency. The white-eyed male will generate gametes with an X chromosome, bearing the white trait, or a Y chromosome, again with equal frequency. The union of these gametes generates four potential types of offspring. If an egg with two X chromosomes is fertilized by a sperm with one X chromosome, the result would be an X⁺X⁺X^w metafemale fly that is nonviable. If an egg bearing two X chromosomes is fertilized by a sperm bearing a Y chromosome, the result would be an X⁺X⁺Y female that will be phenotypically indistinguishable from the red-eyed females resulting from gametes without any nondisjunction. If an egg with no X chromosomes is fertilized with an X chromosome bearing sperm, the result will be an X^w male with white eyes. If an egg with no X chromosomes is fertilized with a Y-bearing sperm, the zygote will die because survival requires at least one X chromosome. Therefore, if the nondisjunction rate is 2%, this means that 20 of the 1000 offspring were the result of nondisjunction-bearing gametes, of which 1/4 would be white-eyed males.

2

Correct. In the mating described, a nondisjunction of X chromosomes in meiosis II would be expected to generate two gametes with one X chromosome each, one gamete with two X chromosomes, and one gamete with no X chromosomes. If an egg bearing no X chromosomes was fertilized by a sperm bearing an X chromosome, an X^w white-eyed male will result.

You have one more chance to answer the question correctly. Try again.

Incorrect. In the mating described, a nondisjunction of X chromosomes in meiosis II would be expected to generate two gametes with one X chromosome each, one gamete with two X chromosomes, and one gamete with no X chromosomes. If an egg bearing no X chromosomes was fertilized by a sperm bearing an X chromosome, an X^w white-eyed male will result.

2

Correct. XXY females are particularly prone to nondisjunction of the X chromosomes during meiosis. These nondisjunction events can generate both X⁺Y red-eyed males (resulting from a Y-bearing egg being fertilized by an X- bearing sperm) and X^wX^wY white-eyed females (resulting from an egg bearing two X chromosomes being fertilized by a Y-bearing sperm).

You have one more chance to answer the question correctly. Try again.

Incorrect. XXY females are particularly prone to nondisjunction of the X chromosomes during meiosis. These nondisjunction events can generate both X⁺Y red-eyed males (resulting from a Y-bearing egg being fertilized by an X- bearing sperm) and X^wX^wY white-eyed females (resulting from an egg bearing two X chromosomes being fertilized by a Y-bearing sperm).