Question

During a_____ , an individual with the dominant phenotype and an unknown genotype is crossed with a_____ individual to determine the unknown genotype. a. single-factor cross, homozygous recessive b. two-factor cross, heterozygous c. testcross, homozygous dominant d. single-factor cross, homozygous dominant e. testcross, homozygous recessive

Short answer

The correct answer is e. testcross, homozygous recessive.

Step-by-step solution

Understand the Definitions

A dominant phenotype is manifested when at least one dominant allele exists in the genotype. Homozygous means having two identical alleles (could be recessive or dominant) for a particular gene. Heterozygous means having two different alleles for a particular gene. A testcross is a way to explore the genotype (genetic composition) of an individual with a dominant phenotype.

Use the Definitions to Evaluate the Options

In a testcross, you're testing an unknown, dominant phenotype against a homozygous recessive to determine the genotype. This is because any offspring displaying the recessive phenotype must have inherited the recessive gene from the dominant parent. So it eliminates b,c, and d which mention different methods or different than homozygous recessive for the second individual.

Select the Correct Answer

By using the process of elimination from our understanding of a testcross, the correct answer is e. testcross, homozygous recessive. The original individual with an unknown genotype displaying dominant phenotype is crossed with a homozygous recessive individual to discern the genotype. It ensures any expression of recessive trait in offspring will translate to the original individual being heterozygous dominant.

Key concepts

Dominant Phenotype

Understanding the concept of a dominant phenotype is essential in genetics. A phenotype refers to the observable traits of an organism, such as eye color or height. These traits come from the interaction of the organism's genetic makeup and its environment. When we say an individual expresses a dominant phenotype, this means the displayed traits can be attributed to at least one dominant allele.

A dominant allele is a variation of a gene that can express its trait in the presence of another different allele—known as the recessive allele. In simpler terms, if the dominant allele is like a loud voice in a conversation, it can be heard even if there is a whisper (recessive allele) alongside it. This is why an individual with a dominant phenotype for a particular trait might have either one or two dominant alleles—referred to as being heterozygous dominant or homozygous dominant, respectively.

For instance, if we're looking at pea plants, a dominant phenotype might be purple flowers. A plant with at least one dominant allele for purple flowers would show this phenotype even if it carries another allele for white flowers, provided that the purple flower allele is dominant.

Genotype

The term genotype refers to the actual genetic makeup of an organism—specifically, the set of alleles it carries. Each trait, such as flower color or hair type, is typically controlled by one or more genes, with each gene having different variations or alleles.

Your genotype is like the complete genetic 'blueprint' that dictates your potential traits. However, remember that not all the traits may be expressed visibly, since some may be masked by dominant alleles when it comes to observable characteristics, or phenotypes. Geneticists use letters to symbolize alleles; for example, a capital letter (e.g., 'A') represents a dominant allele, and a lowercase letter (e.g., 'a') represents a recessive allele.

Here's a quick guide to allele combinations and genotype terms:

• Homozygous dominant: both alleles are dominant (AA).
• Heterozygous dominant: one dominant, one recessive allele (Aa).
• Homozygous recessive: both alleles are recessive (aa).

Determining genotype is a fundamental task in genetics, as it allows scientists and breeders to predict and understand the inheritance patterns of various traits.

Homozygous Recessive

A homozygous recessive genotype occurs when an individual has two copies of the recessive allele for a specific gene. Remember, for a trait or characteristic governed by a recessive allele to be displayed, an organism must have both alleles as recessive since the presence of a dominant allele would mask it.

Therefore, a homozygous recessive individual exhibits the recessive phenotype because there is no dominant allele to override it. A common example in human genetics is blue eyes—a person with blue eyes typically has two recessive alleles for eye color.

Understanding homozygous recessive genotypes is also key to conducting what's called a testcross. In this method, a breeder or geneticist can determine the genotype of an organism that shows the dominant phenotype but has an unknown genotype. By crossing it with a homozygous recessive individual, the resulting offspring's phenotypes can reveal whether the subject is homozygous dominant or heterozygous dominant. If any offspring display the recessive trait, it confirms that the subject is heterozygous dominant since it must have contributed a recessive allele. This is a powerful tool to reveal the inheritable traits within a population or gene pool.