Question

Assuming classical Mendelian inheritance, how can one differentiate between a homozygous dominant individual and one who is heterozygous for the dominant trait? A. By crossing each individual with a known homozygous dominant and examining the offspring B. By crossing each individual with a known homozygous recessive and examining the offspring C. By crossing each individual with a known heterozygote and examining the offspring D. Both (B) and (C)

Short answer

D. Both (B) and (C)

Step-by-step solution

- Understanding the problem

To differentiate between a homozygous dominant individual (AA) and a heterozygous individual (Aa) with respect to a dominant trait, one must analyze their genotypic differences. This can be done by examining the offspring produced from specific crosses.

- Analyze cross with homozygous recessive (Option B)

Crossing a homozygous dominant (AA) with a homozygous recessive (aa) will result in all offspring showing the dominant trait (Aa). Crossing a heterozygous (Aa) with a homozygous recessive (aa) will result in a 1:1 ratio of offspring showing the dominant trait (Aa) and the recessive trait (aa). This difference in offspring ratios helps to distinguish between AA and Aa individuals.

- Analyze cross with heterozygote (Option C)

Crossing a homozygous dominant (AA) with a heterozygote (Aa) will result in all offspring exhibiting the dominant trait (AA or Aa). Crossing a heterozygous individual (Aa) with another heterozygote (Aa) will produce offspring in a 3:1 ratio of dominant to recessive traits (AA, Aa, aa). This ratio difference can also help identify if the original individual is AA or Aa.

- Conclusion

Both methods (Option B and C) can be used to differentiate between a homozygous dominant individual and a heterozygous individual. Therefore, the answer is D.

Key concepts

Genotypic Analysis

Genotypic analysis is a fundamental method in genetics to determine the genetic makeup of an organism. It involves examining the genes and alleles present in an individual. In the context of Mendelian inheritance, this analysis helps identify whether the individual is homozygous (possessing two identical alleles) or heterozygous (possessing two different alleles) for a particular trait. By analyzing an organism's genotype, one can predict the possible inheritance patterns and traits that may appear in subsequent generations. In genetics exercises such as the one presented, genotypic analysis is essential to understand the distinctions between different genetic compositions.

Homozygous Dominant

An individual is described as homozygous dominant when they have two copies of the dominant allele for a trait. Represented as AA, this genotype ensures that the dominant trait is always expressed in the phenotype. Since both alleles are the same and dominant, there is no variation in the trait manifestation.

In genetic crosses, when a homozygous dominant individual (AA) is crossed with a homozygous recessive (aa), all resulting offspring will be heterozygous (Aa) but will display the dominant trait. This makes it challenging to differentiate AA from Aa without further analysis.

Heterozygous Dominant

An individual is heterozygous dominant if they have one dominant and one recessive allele for a particular trait. Represented as Aa, this genotype leads to the dominant trait being expressed due to the presence of the dominant allele.

In genetic crosses, a heterozygous dominant individual (Aa) can reveal more information about their genotype based on the offspring ratios. For instance, crossing Aa with aa results in a 1:1 ratio of dominant to recessive phenotypes. This differs from the cross of AA with aa (where all offspring display the dominant trait), thus helping to differentiate between homozygous and heterozygous dominants.

Offspring Ratios

Offspring ratios provide crucial insights into the genotypes of the parent organisms in Mendelian genetics. The proportion of dominant to recessive traits in the offspring can indicate the genotype of the parents. Here's a breakdown:

• If a homozygous dominant (AA) is crossed with a homozygous recessive (aa), all offspring will display the dominant trait (Aa).
• If a heterozygous dominant (Aa) is crossed with a homozygous recessive (aa), the resulting offspring will show a 1:1 ratio of dominant (Aa) to recessive (aa) traits.
• If two heterozygous individuals (Aa) are crossed, the expected ratio of offspring will be 3:1, with three showing the dominant trait (AA or Aa) and one showing the recessive trait (aa).

These different ratios are a pivotal tool in genotypic analysis, shedding light on the genetic structure of parents involved in each cross.