Question

If a test cross on a species of plants reveals the appearance of a recessive phenotype in the offspring, what must be true of the phenotypically dominant parent? A. It must be genotypically heterozygous. B. It must be genotypically homozygous. C. It could be either genotypically heterozygous or homozygous. D. It must have the same genotype as the testcross control parent.

Short answer

A. It must be genotypically heterozygous.

Step-by-step solution

- Understand the Test Cross

In a test cross, an organism with a dominant phenotype is crossed with an organism that is homozygous recessive for the trait in question.

- Identify Possible Genotypes

The dominant parent can be either homozygous dominant (AA) or heterozygous (Aa). The recessive parent is always homozygous recessive (aa).

- Determine the Outcome of Each Cross

If the dominant parent is homozygous (AA), the offspring will all have the genotype Aa and display the dominant phenotype. If the dominant parent is heterozygous (Aa), the offspring will be a mix of Aa (dominant phenotype) and aa (recessive phenotype).

- Analyze the Given Outcome

The appearance of a recessive phenotype in the offspring signifies that some offspring have the genotype aa. This means the dominant parent must have contributed a recessive allele, so it must be heterozygous (Aa).

- Choose the Correct Option

Given that the phenotypically dominant parent must be heterozygous to produce offspring with the recessive phenotype, the correct answer is option A.

Key concepts

genotype

A genotype is the genetic makeup of an organism, specifically the set of alleles it possesses. Alleles are different forms of a gene, and they come in pairs. One allele is inherited from each parent. For example, in pea plants, the allele for flower color can be either purple (P) or white (p). Therefore, the genotype for this trait could be PP, Pp, or pp.

The combination of these alleles determines the specific characteristics (phenotype) of the organism. In the case of Mendelian genetics, the genotype plays a crucial role in predicting the outcomes of genetic crosses, such as those observed in test crosses.

phenotype

The phenotype is the observable traits or characteristics of an organism, such as its physical appearance, behavior, and biochemical properties. Phenotypes result from the interaction of the organism's genotype with the environment.

For example, if a plant has a genotype of PP or Pp for flower color, it will have a purple phenotype because the purple allele (P) is dominant. However, if it has a genotype of pp, it will have a white phenotype because both alleles are recessive. Phenotypes are what you can see or measure, but knowing the observable traits alone doesn't always reveal the exact underlying genotype.

dominant and recessive traits

Genes can have dominant and recessive alleles, which affects how traits are expressed in the phenotype. Dominant traits are represented by uppercase letters (e.g., P for purple flowers), and recessive traits are represented by lowercase letters (e.g., p for white flowers).

If an organism inherits two dominant alleles (PP) or one dominant and one recessive allele (Pp), the dominant trait will be expressed. On the other hand, the recessive trait (white flowers) will only be expressed if the organism inherits two recessive alleles (pp).

Understanding dominant and recessive traits is essential for predicting the outcomes of genetic crosses and interpreting the results of test crosses.

Mendelian inheritance

Mendelian inheritance refers to the set of rules proposed by Gregor Mendel that explain how traits are passed from parents to offspring. These rules include the law of segregation and the law of independent assortment.

The law of segregation states that each individual has two alleles for each gene, but only one allele is passed on to each offspring. The alleles segregate (separate) during the formation of gametes (sperms and eggs).

The law of independent assortment states that alleles of different genes sort independently of one another during gamete formation. This means the inheritance of one trait generally does not affect the inheritance of another.

These principles help explain the genetic variation observed in populations and are fundamental to understanding genetic crosses, such as test crosses.