Question

In Dexter and Kerry cattle, animals may be polled (hornless) or horned. The Dexter animals have short legs, whereas the Kerry animals have long legs. When many offspring were obtained from matings between polled Kerrys and horned Dexters, half were found to be polled Dexters and half polled Kerrys. When these two types of \(\mathrm{F}_{1}\) cattle were mated to one another, the following \(\mathrm{P}_{2}\) data were obtained: \(3 / 8\) polled Dexters \(1 / 8\) horned Dexters \(3 / 8\) polled Kerrys \(1 / 8\) horned Kerrys A geneticist was puzzled by these data and interviewed farmers who had bred these cattle for decades. She learned that Kerrys were true-breeding. Dexters, on the other hand, were not true- breeding and never produced as many offspring as Kerrys. Provide a genetic explanation for these observations.

Short answer

Answer: The proposed genetic explanation for the observed ratios in the F2 generation is that polled Kerrys have a dominant allele for the polled (hornless) trait and a dominant allele for the long legs trait, while Dexters have a recessive allele for the horned trait and a dominant allele for the short legs trait. The true-breeding nature of Kerrys results from them being homozygous dominant for both polled and long legs traits, and the non-true-breeding nature of Dexters, along with a genetic factor causing reduced offspring viability, can explain the difference in offspring numbers between Dexters and Kerrys.

Step-by-step solution

Understand the Initial Cross

First, we need to understand that polled Kerrys are crossed with horned Dexters. As a result, half of the offspring were found to be polled Dexters, and half polled Kerrys.

Determine the \(\mathrm{F}_{1}\) Cattle Types

From the initial cross, two types of \(\mathrm{F}_{1}\) cattle are obtained: polled Dexters and polled Kerrys.

Analyze the \(\mathrm{F}_{2}\) Generation

When these two types of \(\mathrm{F}_{1}\) cattle are mated with one another, the following data are obtained for the \(\mathrm{F}_{2}\) generation: - \(3 / 8\) polled Dexters - \(1 / 8\) horned Dexters - \(3 / 8\) polled Kerrys - \(1 / 8\) horned Kerrys This information indicates that there are specific genetic factors affecting the ratios of the cattle types in the \(\mathrm{F}_{2}\) generation.

Consider True-Breeding Traits

Recall that Kerrys are true-breeding, which means they consistently pass on specific traits to their offspring. Dexters, however, are not true-breeding and have less offspring than Kerrys. This information is crucial to understanding the genetic explanation for these observations.

Propose a Genetic Explanation

We can propose that Kerrys have a dominant allele for the polled (hornless) trait and a dominant allele for the long legs trait, while Dexters have a recessive allele for the horned trait and a dominant allele for the short legs trait. The true-breeding nature of Kerrys would result from them being homozygous dominant (having two copies of the dominant allele) for both polled and long legs traits. The smaller number of offspring from Dexters could be due to a genetic factor that causes some of the potential offspring to be non-viable during development, leading to fewer overall offspring.

Conclusion

Considering the provided data and true-breeding information of Kerrys, we propose that polled Kerrys and polled Dexters have specific genetic factors (dominant/recessive alleles) that affect the inheritance of certain traits such as horn status and leg length. These genetic factors and the true-breeding nature of Kerrys can lead to the observed ratios in the \(\mathrm{F}_{2}\) generation, while the non-true-breeding nature of Dexters, accompanied by a genetic factor causing reduced offspring viability, can explain the difference in offspring numbers between Dexters and Kerrys.

Key concepts

True-Breeding

In genetics, the term "true-breeding" refers to organisms that consistently pass down certain phenotypic traits to their offspring. This means that, over generations, true-breeding animals produce offspring that exhibit the same characteristics as the parent. This is because the organism is homozygous for a particular trait, possessing two identical alleles that contribute equally to the trait's expression.
In the context of the Kerry cattle, true-breeding implies that these cattle pass on the traits for being polled (hornless) and having long legs consistently through generations. This is because Kerrys have homozygous dominant alleles for these traits, ensuring that their offspring are always polled and have long legs.
A key benefit of true-breeding populations is predictability in offspring traits, which is useful in agriculture and breeding programs aiming to maintain specific characteristics.

• True-breeding organisms are homozygous for certain traits.
• Their offspring consistently display these traits.
• This predictability is valuable in selective breeding situations.

Dominant and Recessive Alleles

In genetics, dominant and recessive alleles play crucial roles in determining the traits of offspring. An allele is a version of a gene, and individuals inherit two alleles for each gene, one from each parent.
Dominant alleles are expressed in an organism, even if only one copy is present. For instance, in the case of the cattle, the polled (hornless) trait is dominant. Kerrys, having dominant alleles for this trait, will consistently pass this trait onto their offspring, making them true-breeding.
Recessive alleles, on the other hand, require both alleles to be recessive for this trait to be expressed. Horned Dexters, for example, have recessive alleles that result in the horned trait only showing up when both alleles are recessive.

• Dominant alleles are expressed with only one copy.
• Recessive alleles require two copies to be visible in the phenotype.
• Kerrys have dominant alleles for polled trait, ensuring they are hornless.
• Dexters have recessive alleles for horns, only visible when two recessive alleles are present.

F1 and F2 Generations

The study and understanding of the F1 and F2 generations are foundational in genetics to observe how traits are transmitted across generations. The F1 generation represents the first set of offspring from a cross between two distinct parental genotypes.
From the given exercise, the cross between polled Kerrys and horned Dexters results in the F1 generation containing both polled Dexters and polled Kerrys. These F1 individuals are heterozygous, having inherited alleles from both parental types.
The F2 generation is derived from the interbreeding of F1 individuals. As a result, the genetic combinations in the F2 generation lead to a variety of genotypic and phenotypic expressions, as seen in the exercise data: 3/8 polled Dexters, 1/8 horned Dexters, 3/8 polled Kerrys, and 1/8 horned Kerrys. These ratios reveal the segregation and recombination of parental traits according to Mendelian principles.

• F1 generation comes from the initial parental cross.
• F2 generation results from interbreeding F1 individuals.
• F2 shows variation in traits, reflecting genetic recombination.
• The ratios in the F2 generation provide insight into allele segregation.