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If a trait is controlled by a single gene with two alleles, where A is dominant

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Question: If a trait is controlled by a single gene with two alleles, where A is dominant over a, what is the expected phenotypic ratio in the offspring of a cross between Aa and aa?

Options:

  1. 1:1
  2. 3:1
  3. 2:1
  4. 1:2

Correct Answer: 1:1

Solution: 

The cross Aa x aa will produce offspring in a 1:1 phenotypic ratio (A:a).

If a trait is controlled by a single gene with two alleles, where A is dominant

Practice Questions

Q1
If a trait is controlled by a single gene with two alleles, where A is dominant over a, what is the expected phenotypic ratio in the offspring of a cross between Aa and aa?
  1. 1:1
  2. 3:1
  3. 2:1
  4. 1:2

Questions & Step-by-Step Solutions

If a trait is controlled by a single gene with two alleles, where A is dominant over a, what is the expected phenotypic ratio in the offspring of a cross between Aa and aa?
  • Step 1: Identify the genotypes of the parents. One parent is Aa (heterozygous) and the other is aa (homozygous recessive).
  • Step 2: List the possible gametes (sperm or egg cells) each parent can produce. The Aa parent can produce gametes A and a, while the aa parent can only produce gametes a.
  • Step 3: Set up a Punnett square to visualize the cross. Place the gametes from one parent on the top and the gametes from the other parent on the side.
  • Step 4: Fill in the Punnett square by combining the gametes. The combinations will be Aa (from A and a) and aa (from a and a).
  • Step 5: Determine the phenotypes from the genotypes. Aa will show the dominant trait (A), and aa will show the recessive trait (a).
  • Step 6: Count the phenotypes. You will have 1 Aa (dominant) and 1 aa (recessive), giving a total of 2 offspring.
  • Step 7: Calculate the phenotypic ratio. There is 1 dominant phenotype (A) and 1 recessive phenotype (a), resulting in a 1:1 ratio.
  • Mendelian Genetics – Understanding the inheritance patterns of traits controlled by single genes and the concept of dominant and recessive alleles.
  • Punnett Square – Using a Punnett square to predict the genotypic and phenotypic ratios of offspring from genetic crosses.
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