Question

The genotype of an individual with the dominant phenotype can be determined best by reproduction with a. the recessive genotype or phenotype. b. a heterozygote. c. the dominant phenotype. d. the homozygous dominant. e. Both a and b are correct.

Short answer

The most accurate and efficient method to determine the genotype of an individual with the dominant phenotype is by breeding them with the recessive genotype or phenotype (aa). This allows us to observe the offspring's phenotypes and determine whether the individual is homozygous dominant (AA) or heterozygous (Aa). The correct answer is option (a).

Step-by-step solution

Understand Dominant and Recessive

Dominant traits are those that are more likely to be expressed in an organism, while recessive traits are less likely to be expressed. An individual with a dominant phenotype can have a homozygous dominant genotype (AA) or a heterozygous genotype (Aa). Our goal is to figure out the genotype of the individual with the dominant phenotype. Step 2: Analyzing the available options

Evaluate the Options

We will compare the outcomes of the offspring when the individual with the dominant phenotype is bred with organisms representing each of the available options: a. Recessive genotype or phenotype (aa): Breeding an individual with a dominant phenotype (AA or Aa) with a recessive genotype (aa) will help us determine the individual's genotype. If all offspring display the dominant phenotype, the individual is homozygous dominant (AA). If a percentage of offspring display the recessive phenotype, the individual is heterozygous (Aa). b. Heterozygote (Aa): In this case, offspring could display either dominant or recessive phenotypes regardless of whether the individual is homozygous dominant (AA) or heterozygous (Aa). It would be difficult to accurately determine the individual's genotype. c. Dominant phenotype (AA or Aa): Breeding with another individual with a dominant phenotype won't help determine the genotype, as all offspring will likely have a dominant phenotype, whether the parent is homozygous dominant or heterozygous. d. Homozygous dominant (AA): Similarly, breeding with a homozygous dominant organism will produce offspring with dominant phenotypes, and we won't be able to determine the genotype of the initial individual. Step 3: Determine the most accurate and efficient method

Identifying the Best Approach

Based on the analysis of the available options, it's clear that breeding the individual with the recessive genotype or phenotype (aa) will provide the most accurate and efficient method of determining the individual's genotype. Therefore, the correct answer is option (a).

Key concepts

Dominant and Recessive Traits

Understanding the concepts of dominant and recessive traits is foundational in genetics.
Dominant traits are those that are expressed in an organism even if there's only one copy of the gene present. Think of them as the 'louder' traits; they can mask the presence of other traits. For example, if the gene for brown eyes is dominant over the gene for blue eyes, a person with one brown-eye gene and one blue-eye gene will have brown eyes.

Recessive traits, on the other hand, typically require two copies of the gene to be expressed—that is, they appear only when an individual is homozygous for the recessive gene. So, if our person has two blue-eye genes, they will have blue eyes, but if they have a brown-eye gene and a blue-eye gene, the dominant brown will be expressed.

• The presence of a dominant trait can be due to two different genotypes: homozygous dominant where two dominant alleles are present (AA) or heterozygous where one dominant and one recessive allele are present (Aa).
• For a recessive trait to be expressed, both alleles must be recessive (aa).

This distinction is crucial when trying to determine an organism's genotype based on phenotypic traits, as demonstrated in the exercise presented.

Homozygous Dominant

When we speak of a 'homozygous dominant' genotype, we are referring to an individual who has inherited two dominant alleles of a gene, one from each parent. In notation, we represent this as 'AA'.

With a homozygous dominant genotype, the individual will definitely express the dominant trait. However, it's important to note that a dominant phenotype does not automatically mean the genotype is homozygous dominant; a heterozygous (Aa) individual will also display the dominant trait.

• Homozygous dominant is signified by a pair of uppercase letters (like TT for tallness in pea plants).
• The dominant trait will always be expressed, regardless of what the other allele is.

In the context of the exercise, if an individual with a dominant phenotype produces offspring with someone of a recessive phenotype (aa) and all offspring show the dominant trait, we can infer that the parent is homozygous dominant (AA).

Heterozygous Genotype

A heterozygous genotype occurs when an individual possesses two different alleles of a single gene—one dominant and one recessive; this is represented as 'Aa'.

In such a case, the dominant allele masks the presence of the recessive allele, yielding a dominant phenotype. However, the recessive allele is still present and can be passed on to the individual's offspring.

• Someone with a heterozygous genotype has a 50% chance of passing on the dominant allele and a 50% chance of passing on the recessive allele to their offspring.
• When the heterozygous individual reproduces with a homozygous recessive individual, the dominant trait will be expressed in approximately half of their offspring, if we assume simple Mendelian inheritance without any other complexity like incomplete dominance or epistasis.

In the exercise, if breeding with a recessive genotype (aa) yields offspring that exhibit both dominant and recessive phenotypes, we can conclude that the dominant phenotype parent is heterozygous (Aa).