Problem 10 - Campbell Biology

Question

In \(1981,\) a stray black cat with unusual rounded, curled-back ears was adopted by a family in California. Hundreds of descendants of the cat have since been born, and cat fanciers hope to develop the curl cat into a show breed. Suppose you owned the first curl cat and wanted to develop a true breeding variety. How would you determine whether the curl allele is dominant or recessive? How would you obtain true-breeding curl cats? How could you be sure they are true-breeding?

Step-by-Step Solution

Answer

Mate the first curl cat with a normal-eared cat and observe the offspring. If all have curled ears, the allele is likely dominant. Ensure true breeding by consistent phenotypes over generations.

Step 1 - Observe the Phenotype

Examine whether the characteristic of curled-back ears is present in the first curl cat. This is your initial phenotype of interest.

Step 2 - Perform a Test Cross

Mate the first curl cat with a cat that has normal ears (homozygous recessive). Observe the offspring's ear shape.

Step 3 - Analyze Offspring Phenotypes

If all offspring have curled-back ears, the curl allele is likely dominant. If some offspring have normal ears, the curl allele is likely recessive.

Step 4 - Establish True Breeding Lines

If the curl allele is dominant, mate two curl cats and examine if all their offspring have curled-back ears. Continue breeding for several generations to confirm.

Step 5 - Confirm True Breeding

Ensure that all offspring for several generations exhibit the curled-back ears. If there is no variation, the cats are true breeding.

Step 6 - Validate Results Through Genetic Testing

Optionally, perform genetic tests to confirm the genetic makeup of the true breeding cats, ensuring no hidden recessive alleles.

Key Concepts

dominant and recessive alleles

Alleles are different forms of the same gene. Each gene can have multiple alleles, which can be classified as either dominant or recessive. A dominant allele only needs one copy to express a trait, while a recessive allele needs two copies. In the case of the curl cat, we need to determine if the curled-back ear trait is dominant or recessive. If the trait is dominant, it will appear in the offspring even if only one parent carries the allele. If it's recessive, both parents must carry a copy for the offspring to exhibit the trait.

test cross

A test cross helps determine the genetic makeup of an organism with a dominant phenotype by crossing it with an organism that has a known recessive phenotype. In our case, to find out if the curl allele is dominant or recessive, breed the first curl cat (which might have either genotype) with a cat that has normal ears (homozygous recessive). Observe the offspring:

• If all offspring have curled-back ears, the curl allele is dominant.
• If some offspring have normal ears, the curl allele is recessive.

This step is vital for establishing the genetic basis of the curl trait.

phenotype

Phenotype refers to the observable physical traits of an organism, such as the curled-back ears in the curl cat. These traits are a result of the interaction between the genotype (the genetic makeup) and the environment. When developing a true breeding variety, the phenotype is our visible indicator of whether the desired trait is being consistently passed down. By observing the ear shapes of the offspring in each generation, we can infer the underlying genetic pattern and ensure we are breeding for the right trait.

true breeding

True breeding means producing offspring that always display the same phenotype for a particular trait across many generations. To develop true breeding curl cats, follow these steps:

• Determine if the curl allele is dominant or recessive using a test cross.
• If the curl allele is dominant, mate two curl cats and check if all offspring have curled-back ears.
• Continue breeding for several generations, ensuring each generation displays the curled-back ear trait without variation.

True breeding is confirmed when all offspring consistently show the desired phenotype, indicating a stable genetic line.

genetic testing

Genetic testing can be used to validate the genetic makeup of your true breeding cats. Even if your cats display the desired phenotype, it's essential to confirm that they don't carry hidden recessive alleles that can appear in future generations. By analyzing the DNA, you can ensure that the curl cats are genetically pure and will consistently produce the desired trait in their offspring. This step adds an extra layer of certainty to your breeding program, making sure that the curl trait is here to stay.