QQuestionBiology
QuestionBiology
The 9:3:3:1 ratio is obtained:
A. All of the above
B. In a dihybrid cross in which the genes are linked on the same chromosome
C. In a dihybrid cross in which the genes are on different chromosomes
D. When crossing over occurs
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Answer
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Step 1:Let's solve this problem step by step:
Step 2:: Understanding Dihybrid Cross Ratios
In a typical dihybrid cross with genes on different chromosomes (independent assortment), the expected phenotypic ratio is 9:3:3:1. This ratio occurs when two different genes are inherited independently.
Step 3:: Analyzing the Conditions for 9:3:3:1 Ratio
The 9:3:3:1 ratio specifically occurs when: - The genes are located on different chromosomes - The genes are not linked - The alleles are not sex-linked - No crossing over interferes with the gene inheritance
Step 4:: Examining the Genetic Mechanism
In a dihybrid cross with independent assortment: - 9 parts represent individuals with dominant alleles for both traits - 3 parts represent individuals with dominant allele for first trait, recessive for second - 3 parts represent individuals with recessive allele for first trait, dominant for second - 1 part represents individuals with recessive alleles for both traits
Step 5:: Comparing the Given Options
A. All of the above - Incorrect B. Genes linked on the same chromosome - Incorrect (would NOT produce 9:3:3:1 ratio) C. Genes on different chromosomes - CORRECT D. When crossing over occurs - Incorrect (crossing over would disrupt the 9:3:3:1 ratio)
Final Answer
In a dihybrid cross in which the genes are on different chromosomes Key Insight: The 9:3:3:1 ratio is a classic result of independent assortment, which occurs when genes are located on different chromosomes and segregate independently during meiosis.
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