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Difficulty: MediumGenetics and Heredity

An organism displaying a dominant phenotype is crossed with a homozygous recessive individual (aaaa). If 50% of the resulting offspring exhibit the recessive phenotype, what is the genotype of the dominant parent?

  1. A
    Homozygous dominant (AAAA)
  2. Heterozygous (AaAa)Answer
  3. C
    Homozygous recessive (aaaa)
  4. D
    Codominant (ABAB)

Answer

Heterozygous (AaAa)
The correct answer is the heterozygous genotype (AaAa). Because offspring with the recessive phenotype must inherit a recessive allele (aa) from each parent, the parent with the dominant phenotype must carry at least one recessive allele (aa). Since this parent also displays the dominant trait, it must carry a dominant allele (AA), confirming its genotype as AaAa.

Step-by-Step Solution

1
Determine the required genotype of the recessive offspring.
Offspring showing the recessive phenotype must be homozygous recessive (aaaa).
Recessive traits are only physically expressed when an organism carries two recessive alleles.
2
Identify the parental origin of the alleles in the recessive offspring.
Each aaaa offspring receives one recessive allele (aa) from the homozygous recessive parent and one recessive allele (aa) from the dominant parent.
Offspring inherit exactly one allele for each gene from each biological parent.
3
Deduce the dominant parent's complete genotype.
The dominant parent must possess one dominant allele (AA) to express the dominant phenotype and one recessive allele (aa) to produce aaaa offspring, resulting in the genotype AaAa.
A test cross between a heterozygous individual (AaAa) and a homozygous recessive individual (aaaa) yields a 1:1 phenotypic ratio (50% dominant, 50% recessive).

Key Concept

Test Cross and Mendelian Allele Segregation
Estimated Time:1m 0s
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Topics

Genetics and Heredity

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