Predict the F₁ and F₂ results of crossing a male fowl that is cock-feathered with a true-breeding hen-feathered female fowl. Recall that these traits are sex limited.
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 35
Textbook Question
Red–green color blindness is a relatively common condition found in about 8% of males in the general population. From this, population, biologists estimate that 8% is the frequency of X chromosomes carrying a mutation of the gene encoding red and green color vision. Based on this frequency, determine the approximate frequency with which you would expect females to have red–green color blindness. Explain your reasoning.

1
Understand that red-green color blindness is an X-linked recessive trait. This means that males (XY) need only one mutated X chromosome to express the condition, while females (XX) need two mutated X chromosomes to express the condition.
The frequency of X chromosomes carrying the mutation is given as 8% (0.08). For females to have red-green color blindness, they must inherit two mutated X chromosomes, one from each parent.
To calculate the frequency of females with red-green color blindness, use the Hardy-Weinberg principle. The probability of a female inheriting two mutated X chromosomes is the square of the frequency of the mutated X chromosome: \( q^2 \), where \( q \) is the frequency of the mutated X chromosome.
Substitute the given frequency of the mutated X chromosome (\( q = 0.08 \)) into the formula \( q^2 \). This represents the proportion of females who are homozygous for the mutation and thus have red-green color blindness.
Conclude that the calculated value of \( q^2 \) represents the expected frequency of females with red-green color blindness in the population. This value is much lower than the 8% observed in males because females require two copies of the mutation to express the condition.

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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
X-Linked Inheritance
Red-green color blindness is an X-linked recessive trait, meaning the gene responsible for this condition is located on the X chromosome. Males, having one X and one Y chromosome, express the trait if their single X carries the mutation. In contrast, females have two X chromosomes, so they must inherit two copies of the mutated gene to express the condition.
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X-Inactivation
Hardy-Weinberg Principle
The Hardy-Weinberg principle provides a mathematical framework for understanding allele frequencies in a population at equilibrium. It allows biologists to estimate the frequency of genotypes based on known allele frequencies. In this case, knowing that 8% of males are affected helps calculate the expected frequency of affected females using the principle.
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Hardy Weinberg
Genotype Frequency Calculation
To determine the frequency of red-green color blindness in females, we can use the allele frequency of the mutated X chromosome. If 8% of males are affected, this indicates that the frequency of the mutated allele (X^c) is 0.08. For females, the frequency of homozygous recessive individuals (X^cX^c) can be calculated as (0.08)^2, leading to an expected frequency of approximately 0.0064 or 0.64%.
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