Drosophila melanogaster (D. melanogaster) is a species of fruit fly frequently used by researchers in genetic studies. Members of this species have two of each of four different chromosomes: the sex chromosome (fies have X and Y) and three autosomes (chromosomes 2, 3, and 4). Researchers studying D. melanogaster conducted genetic crosses to investigate a particular X-linked recessive trait encoded by a single gene (Table 1). Affected flies have the trait. TABLE 1. CROSSES PERFORMED AND THE PHENOTYPES OF THE RESULTING OFFSPRING Phenotype of Females Phenotype of Males Percent of Total Population Percent of Male Offspring Percent of Female Offspring Cross Number Used in the Cross Used in the Cross That Are Affected That Are Affected That Are Affected Unaffected Affected 2. Affected Unaffected 50 100 3. Unaffected Unaffected 25 50 4. Affected Affected 100 100 100

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A. Identify the genotypes of the male and female used in cross 2. 

B. Identify the cross in which the female parent was most likely heterozygous.

C. Explain how the results exclude the possibility that the trait is encoded by a mitochondrial gene.

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**Study of Drosophila melanogaster Genetic Crosses**

Drosophila melanogaster, commonly known as the fruit fly, is a species frequently utilized in genetic research. This species possesses two of each of four distinct chromosomes, including sex chromosomes (X and Y) and autosomes (chromosomes 2, 3, and 4). Researchers exploring D. melanogaster conducted genetic crosses to examine a specific X-linked recessive trait encoded by a single gene.

**Table 1. Crosses Performed and the Phenotypes of the Resulting Offspring**

| Cross Number | Phenotype of Females Used in the Cross | Phenotype of Males Used in the Cross | Percent of Total Population That Are Affected | Percent of Male Offspring That Are Affected | Percent of Female Offspring That Are Affected |
|--------------|----------------------------------------|--------------------------------------|---------------------------------------------|--------------------------------------------|-----------------------------------------------|
| 1            | Unaffected                              | Affected                             | 0                                           | 0                                          | 0                                             |
| 2            | Affected                                | Unaffected                           | 50                                          | 100                                        | 0                                             |
| 3            | Unaffected                              | Unaffected                           | 25                                          | 50                                         | 0                                             |
| 4            | Affected                                | Affected                             | 100                                         | 100                                        | 100                                           |

**Explanation of Table:**

- **Cross Number**: This column lists the identification number for each genetic cross performed.
  
- **Phenotype of Females/Males Used in the Cross**: These columns indicate whether the female and male fruit flies used in each cross exhibited the trait (affected) or did not (unaffected).
  
- **Percent of Total Population That Are Affected**: This column shows the percentage of the offspring showing the X-linked recessive trait across the entire population resulting from each cross.
  
- **Percent of Male/Female Offspring That Are Affected**: These columns provide detailed percentages of affected traits among male and female offspring separately for each cross.

This table helps to illustrate the inheritance patterns of an X-linked recessive trait in Drosophila melanogaster.

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Transcribed Image Text:--- **Study of Drosophila melanogaster Genetic Crosses** Drosophila melanogaster, commonly known as the fruit fly, is a species frequently utilized in genetic research. This species possesses two of each of four distinct chromosomes, including sex chromosomes (X and Y) and autosomes (chromosomes 2, 3, and 4). Researchers exploring D. melanogaster conducted genetic crosses to examine a specific X-linked recessive trait encoded by a single gene. **Table 1. Crosses Performed and the Phenotypes of the Resulting Offspring** | Cross Number | Phenotype of Females Used in the Cross | Phenotype of Males Used in the Cross | Percent of Total Population That Are Affected | Percent of Male Offspring That Are Affected | Percent of Female Offspring That Are Affected | |--------------|----------------------------------------|--------------------------------------|---------------------------------------------|--------------------------------------------|-----------------------------------------------| | 1 | Unaffected | Affected | 0 | 0 | 0 | | 2 | Affected | Unaffected | 50 | 100 | 0 | | 3 | Unaffected | Unaffected | 25 | 50 | 0 | | 4 | Affected | Affected | 100 | 100 | 100 | **Explanation of Table:** - **Cross Number**: This column lists the identification number for each genetic cross performed. - **Phenotype of Females/Males Used in the Cross**: These columns indicate whether the female and male fruit flies used in each cross exhibited the trait (affected) or did not (unaffected). - **Percent of Total Population That Are Affected**: This column shows the percentage of the offspring showing the X-linked recessive trait across the entire population resulting from each cross. - **Percent of Male/Female Offspring That Are Affected**: These columns provide detailed percentages of affected traits among male and female offspring separately for each cross. This table helps to illustrate the inheritance patterns of an X-linked recessive trait in Drosophila melanogaster. ---
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