Problem 11 - Biology Concepts and Applications

Question

A dominant allele \(W\) confers black fur on guinea pigs. A guinea pig that is homozygous recessive \((ww)\) has white fur. Fred would like to know whether his pet black-furred guinea pig is homozygous dominant \((WW)\) or heterozygous \((W w) .\) How might he determine his pet's genotype?

Step-by-Step Solution

Answer

Conduct a test cross with a homozygous recessive guinea pig; observe offspring's fur color to infer genotype.

Step 1: Understand Dominance in Alleles

In genetics, a dominant allele masks the effect of a recessive allele. Here, the allele \(W\) (black fur) is dominant over \(w\) (white fur). Therefore, both \(WW\) and \(Ww\) genotypes result in black fur, while \(ww\) results in white fur.

Step 2: Set Up a Test Cross

To determine if Fred’s guinea pig is homozygous dominant \( (WW) \) or heterozygous \( (Ww) \), a test cross can be conducted. This involves mating the black guinea pig with another guinea pig that is known to have the homozygous recessive genotype \( (ww) \).

Step 3: Analyze Possible Offspring Outcomes

If the black guinea pig is \(WW\), all offspring from this cross will have black fur, i.e., \(Ww\). If the black guinea pig is \(Ww\), some offspring will have black fur \((Ww)\) and some will have white fur \((ww)\).

Step 4: Conduct the Test and Observe Offspring

Allow the guinea pig to mate and observe the fur color of their offspring. If any offspring have white fur, then Fred’s guinea pig is heterozygous \((Ww)\). If all offspring have black fur, it is likely homozygous dominant \((WW)\).

Key Concepts

Dominance in Genetics

In genetics, some alleles have the ability to mask the presence of other alleles, which is known as dominance. A dominant allele is symbolized by an uppercase letter (e.g., \(W\)), and it overshadows the effects of a recessive allele, represented by a lowercase letter (e.g., \(w\)). This means that in the presence of a dominant allele, the phenotype, or observable trait, is determined by the dominant allele.
For instance, in guinea pigs, the dominant allele \(W\) results in black fur. This allele is strong enough to hide the effects of the recessive allele \(w\), which results in white fur only when present in two copies (\(ww\)). Consequently, a guinea pig with either \(WW\) or \(Ww\) genotypes will display black fur, demonstrating the concept of dominance. By understanding dominance, we can predict the appearance of organisms based on their genetic makeup.

Homozygous and Heterozygous Genotypes

Genotypes describe the genetic constitution of an organism in terms of its alleles. When we talk about homozygous and heterozygous genotypes, we're referring to whether the alleles an organism inherits from its parents are the same or different. An organism is **homozygous** if it has two identical alleles for a trait. This could be either **homozygous dominant** (\(WW\)), where both alleles express the dominant trait of black fur, or **homozygous recessive** (\(ww\)), where both alleles express the recessive trait of white fur.
On the other hand, when an organism is **heterozygous**, it means that the alleles are different (\(Ww\)). In this case, the dominant allele \(W\) manages to mask the effect of the recessive allele \(w\), resulting in the dominant phenotype, black fur.

• **Homozygous Dominant (\(WW\))**: Both alleles lead to black fur.
• **Homozygous Recessive (\(ww\))**: Both alleles lead to white fur.
• **Heterozygous (\(Ww\))**: One dominant and one recessive allele lead to black fur.

This distinction is key in predicting the traits of the offspring.

Test Cross

A test cross is a practical way in genetics to determine the unknown genotype of an organism showing a dominant trait. In cases like Fred's guinea pig, whose phenotype (black fur) is dominant, we want to know if its genotype is homozygous dominant (\(WW\)) or heterozygous (\(Ww\)). To perform a test cross, the organism in question is bred with an organism that is homozygous recessive (\(ww\)), which will exhibit the recessive phenotype (white fur).
By analyzing the offspring from this cross, we can make inferences about the unknown genotype:

• If all offspring have black fur, the unknown genotype is likely homozygous dominant (\(WW\)).
• If some offspring are black and some are white, the unknown genotype is heterozygous (\(Ww\)), as the presence of at least one white offspring indicates that the recessive allele \(w\) was passed down.

This method is a straightforward approach to uncover the genetic makeup of organisms, helping us to understand inheritance patterns and predict future generation traits.