Question

In guinea pigs, an allele for rough fur \((R)\) is dominant over an allele for smooth fur \((r) ;\) an allele for black coat \((B)\) is dominant over that for white ( \(b\) ). You have an animal with rough, black fur. What cross would you use to determine whether the animal is homozygous for these traits? What phenotype would you expect in the offspring if the animal is homozygous?

Short answer

Perform a test cross with an animal that is homozygous recessive (rrbb) for both traits. If the test animal is homozygous (RRBB), then all offspring will have rough, black fur.

Step-by-step solution

Understanding the Genetics

First, consider the genetic background provided. Rough fur (R) is dominant over smooth fur (r) and black coat (B) is dominant over white coat (b). An animal displaying the dominant phenotype could be either homozygous dominant (RR or BB) or heterozygous (Rr or Bb).

Choosing the Cross

To determine whether the animal is homozygous dominant for both traits (RRBB), perform a test cross with an animal that is homozygous recessive for both traits (rrbb). The homozygous recessive genotype means the animal has smooth, white fur.

Predicting the Phenotypes

If the test animal is indeed homozygous dominant (RRBB), all offspring will display the dominant phenotypes (rough, black fur), because they will inevitably inherit one dominant allele for each trait from the test parent.

Interpreting Results of the Cross

The genotype of the offspring can confirm the test animal's genotype. If any offspring display the recessive phenotype (smooth fur, white coat), the test animal must be heterozygous for those traits. If all offspring display the dominant phenotype, it supports that the test animal is homozygous dominant.

Key concepts

Dominant and Recessive Alleles

To comprehend the underlying principles of genetics, it's essential to grasp the concepts of dominant and recessive alleles. These are two different versions of a gene that can determine specific traits in an organism. Dominant alleles, represented here by letters such as \(R\) for rough fur and \(B\) for black coat in guinea pigs, mask the expression of recessive alleles paired with them. In contrast, recessive alleles like \(r\) for smooth fur and \(b\) for white coat, only manifest their characteristics when they are paired together without the dominant allele.

For a visual aid, if a guinea pig inherits the allele \(R\) from one parent and \(r\) from another, the \(R\) allele's dominance means the guinea pig will exhibit rough fur, a concept pivotal to the exercise. Thus, a dominant allele's presence confirms the associated phenotype, while the recessive allele only shows when it exists in a double dose, as in \(rr\).

Homozygous and Heterozygous

The terms homozygous and heterozygous are critical to interpreting the genetic makeup, or genotype, of an individual. An organism is homozygous for a trait when it possesses two identical alleles for a gene, such as \(RR\) (homozygous dominant) or \(rr\) (homozygous recessive). Conversely, an organism is heterozygous when it has two different alleles, like \(Rr\), for a single trait.

Understanding the distinction between these genetic compositions is crucial for predicting offspring phenotypes. For example, a homozygous dominant guinea pig for rough fur would have a genotype of \(RR\), always passing the \(R\) allele to its offspring, ensuring they also have rough fur, as illustrated in the textbook exercise solution. This contrasts with a heterozygous guinea pig \(Rr\), which could pass on either the \(R\) or \(r\) allele, introducing variability into the progeny's phenotypic outcomes.

Mendelian Inheritance Patterns

Introduced by Gregor Mendel through his work on pea plants, Mendelian inheritance patterns provide a foundational framework for genetics. These patterns describe how traits are transmitted from parents to offspring. Mendelian genetics contemplates dominant and recessive alleles, as well as homozygous and heterozygous genotypes, to determine the phenotype of an organism.

In the context of the exercise solution, these patterns allow us to predict that a cross between a homozygous dominant individual \(RRBB\) and a homozygous recessive individual \(rrbb\) will produce offspring that all exhibit the dominant phenotypes. This prediction is formed on the law of segregation, stating that each parent contributes one allele for each trait, allowing simple predictions of genetic outcomes. If none of the offspring inherit the recessive phenotype, it confirms the homozygous dominant nature of the parent guinea pig.

Phenotype and Genotype

Phenotype refers to the observable traits of an organism, such as rough or smooth fur and black or white coat in guinea pigs. On the other hand, genotype is the genetic makeup at a particular genetic locus and includes the alleles inherited from the parents. The connection between genotype and phenotype is demonstrated through the expression of traits governed by genetic information.

In the exercise held here, we discuss determining an unknown genotype based on observable phenotypes. When conducting a test cross, if the resulting offspring only display dominant phenotypes (rough, black fur), it suggests the parent's genotype to be homozygous dominant. This illustrates how phenotypes, the physical expressions, can reveal clues about genotypes, the genetic codes, an invaluable insight for students learning about genetics.