When the cloned cat Carbon Copy (CC) was born, she had black patches and white patches, but completely lacked any orange patches. The knowledgeable students of genetics were not surprised at this outcome. Starting with the somatic ovarian cell used as the source of the nucleus in the cloning process, explain how this outcome occurred.
Table of contents
- 1. Introduction to Genetics51m
- 2. Mendel's Laws of Inheritance3h 37m
- 3. Extensions to Mendelian Inheritance2h 41m
- 4. Genetic Mapping and Linkage2h 28m
- 5. Genetics of Bacteria and Viruses1h 21m
- 6. Chromosomal Variation1h 48m
- 7. DNA and Chromosome Structure56m
- 8. DNA Replication1h 10m
- 9. Mitosis and Meiosis1h 34m
- 10. Transcription1h 0m
- 11. Translation58m
- 12. Gene Regulation in Prokaryotes1h 19m
- 13. Gene Regulation in Eukaryotes44m
- 14. Genetic Control of Development44m
- 15. Genomes and Genomics1h 50m
- 16. Transposable Elements47m
- 17. Mutation, Repair, and Recombination1h 6m
- 18. Molecular Genetic Tools19m
- 19. Cancer Genetics29m
- 20. Quantitative Genetics1h 26m
- 21. Population Genetics50m
- 22. Evolutionary Genetics29m
2. Mendel's Laws of Inheritance
Sex-Linked Genes
Problem 28c
Textbook Question
While vermilion is X-linked in Drosophila and causes the eye color to be bright red, brown is an autosomal recessive mutation that causes the eye to be brown. Flies carrying both mutations lose all pigmentation and are white-eyed. Predict the F₁ and F₂ results of the following crosses:
white females x wild-type males

1
Step 1: Identify the genotypes of the parents. White-eyed females have both the vermilion mutation (X-linked recessive, denoted as Xᵛ) and the brown mutation (autosomal recessive, denoted as bb). Their genotype is XᵛXᵛ bb. Wild-type males have no mutations, so their genotype is X⁺Y B⁺B⁺, where X⁺ represents the wild-type X chromosome and B⁺ represents the wild-type autosomal allele.
Step 2: Determine the gametes produced by each parent. The white-eyed female can produce gametes with Xᵛ b and Xᵛ b (since she is homozygous for both mutations). The wild-type male can produce gametes with X⁺ B⁺ and Y B⁺.
Step 3: Perform a Punnett square to predict the F₁ offspring. Combine the gametes from the female and male parents. For the X chromosome, combine Xᵛ from the female with X⁺ or Y from the male. For the autosomal alleles, combine b from the female with B⁺ from the male.
Step 4: Analyze the F₁ phenotypes. Since all F₁ offspring will inherit one Xᵛ and one X⁺ (or Y for males), they will not be homozygous for the vermilion mutation. Similarly, all F₁ offspring will inherit one b and one B⁺, so they will not be homozygous for the brown mutation. Therefore, all F₁ flies will have wild-type pigmentation.
Step 5: Predict the F₂ results by crossing the F₁ flies. Set up a Punnett square for both the X-linked vermilion gene and the autosomal brown gene. Include all possible combinations of alleles from the F₁ gametes. Analyze the resulting genotypes and phenotypes to determine the proportions of wild-type, vermilion, brown, and white-eyed flies in the F₂ generation.

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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
X-linked Inheritance
X-linked inheritance refers to genes located on the X chromosome. In Drosophila, traits such as vermilion eye color are determined by alleles on this chromosome. Males, having one X and one Y chromosome, express X-linked traits directly, while females, with two X chromosomes, can be carriers or express the trait depending on their genotype.
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X-Inactivation
Autosomal Recessive Inheritance
Autosomal recessive inheritance involves genes located on non-sex chromosomes (autosomes) where two copies of a recessive allele are necessary for the trait to be expressed. In this case, the brown eye color in Drosophila is an example, where flies must inherit two brown alleles to exhibit the brown phenotype, while carriers with one brown allele remain unaffected.
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Autosomal Pedigrees
Punnett Square and Genetic Crosses
A Punnett square is a tool used to predict the genotypic and phenotypic outcomes of genetic crosses. By organizing the alleles from each parent, it allows for the visualization of potential offspring combinations. In this scenario, analyzing the crosses between white females (homozygous for both mutations) and wild-type males will help predict the F₁ and F₂ generations' eye color outcomes.
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Punnet Square
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