Problem 8 - AP Biology Premium Prep 2021

Question

A breeder of black Labrador puppies notices that in his line a gene predisposing his dogs for white spots has arisen. He believes the white spot allele is autosomal recessive, and he wants to prevent it from continuing in his dogs. He sees that one of his female dogs, Speckle, has white spots, and he therefore assumes she is homozygous for the gene. But he must determine which males in his pack are heterozygous to avoid breeding them in the future. What is a reasonable plan? (A) Stop breeding Speckle. (B) Pair Speckle with a male with white spots. (C) Perform a testcross of Speckle with a black Labrador male. If any pups are born with white spots, stop breeding that male because he is heterozygous. (D) Breed Speckle with a chocolate Labrador.

Step-by-Step Solution

Answer

The best plan is to perform a testcross of Speckle with a black Labrador male. If any pups are born with white spots, stop breeding that male because he is heterozygous. (C)

Step 1: Understanding the given options

It's important to first understand the given options and their implications: (A) Stop breeding Speckle: This option suggests that by not breeding Speckle, the issue of spreading the white spot allele can be prevented. However, this would not help in identifying heterozygous males. (B) Pair Speckle with a male with white spots: This option will mate Speckle with another dog showing the white spot trait. This may perpetuate the white spot allele but does not address the concern of identifying heterozygous males. (C) Perform a testcross of Speckle with a black Labrador male: If any pups are born with white spots, stop breeding that male because he is heterozygous. This option suggests using a testcross to identify heterozygous males. (D) Breed Speckle with a chocolate Labrador: This option suggests breeding with a different color Labrador. However, it doesn't specifically help in identifying heterozygous males or preventing the spread of the white spot allele.

Step 2: Choosing the best option

Option C is the best choice because it proposes a focused and logical way to identify heterozygous males, which was the main concern of the breeder. A testcross is performed with a black Labrador male, as they cannot have white spots (meaning they have at least one dominant allele). If they produce offspring with white spots, it means that the male carries the recessive white spot allele (heterozygous), and the breeder should avoid breeding that male. Therefore, the best plan is: (C) Perform a testcross of Speckle with a black Labrador male. If any pups are born with white spots, stop breeding that male because he is heterozygous.

Key Concepts

Autosomal Recessive Traits

In genetics, many traits are influenced by alleles that follow an autosomal recessive pattern. These alleles are located on non-sex chromosomes known as autosomes. For a trait to be expressed in this manner, an individual must possess two copies of the recessive allele. This means they inherit one recessive allele from each parent to display that particular trait.

In the case of the Labrador puppies with white spots, the white spot allele is assumed to be recessive. This means that a dog will only display white spots if they are homozygous recessive—having two identical alleles that create the characteristic.

Some important points about autosomal recessive traits are:

• They can be carried by individuals who do not show the trait themselves (carriers).
• If two carriers breed, there is a 25% chance their offspring will be affected, 50% chance they will be carriers, and 25% chance they will inherit no recessive alleles.
• Both males and females are equally affected.

Testcross

A testcross is a genetic technique used to determine the genotype of an individual showing a dominant trait. The process involves crossing the individual in question with another individual known to be homozygous recessive for the trait.

In the scenario with the Labradors, performing a testcross helps identify which male dogs carry the recessive allele for white spots. By mating the female dog, Speckle—already known to express white spots—with a male suspected of having a dominant black coat, we can observe the offspring to deduce the male's genotype.

Here’s a step-by-step look at how a testcross works:

• Mate the suspect dog (assumed to be heterozygous or homozygous dominant) with a homozygous recessive dog (Speckle).
• Examine the offspring: if any display the recessive trait (white spots), the suspect dog is heterozygous.
• If no offspring display white spots, the suspect male is likely homozygous dominant.

Homozygous Organisms

An organism is described as homozygous when it has two identical alleles for a particular gene. This uniformity can be either two dominant alleles (homozygous dominant) or two recessive alleles (homozygous recessive).

In the context of the exercise, the female dog Speckle has white spots, indicating she is homozygous recessive for that trait, carrying two recessive alleles. Knowing Speckle's genotype as homozygous is crucial for breeding decisions because she will only pass on the recessive allele to her offspring.

Some key notes about homozygous organisms include:

• They consistently pass on one type of allele to their offspring.
• Homozygous recessive individuals will always show the recessive trait.
• Knowing homozygous individuals is helpful in predicting genetic outcomes.

Heterozygous Individuals

Heterozygous refers to possessing two different alleles for a single trait—one being dominant and the other recessive. Consequently, the dominant trait is typically expressed phenotypically, while the recessive allele is hidden but can be passed to the next generation.

Identifying heterozygous males in the breeder's pack is key to controlling the spread of the white spot trait. A heterozygous male has a 50% chance of passing the recessive allele to his offspring, which might yield white spots if paired with a homozygous recessive female like Speckle.

Looking deeper into heterozygotes:

• They are often referred to as carriers if they have one dominant and one recessive allele.
• Breeding two heterozygous individuals can produce a mix of homozygous dominant, heterozygous, and homozygous recessive offspring.
• Their genetic contribution is unpredictable without a testcross, hence the need for genetic tests to verify their allele combinations.