The Length Of A Dog’s Tail Is An Inherited Trait

The Length Of A Dogs Tail Is An Inherited Trait The Allele For Short

The length of a dog's tail is an inherited trait, where the allele for short tails (T) is dominant over the allele for long tails (t). A dog breeder mates two short-tailed dogs, with one being homozygous (TT) and the other heterozygous (Tt). All twelve puppies in the first litter, produced from this mating, have short tails. Subsequently, one of these first-generation dogs is bred with another short-tailed dog from a different litter, resulting in a second litter of ten puppies, where unexpectedly, one puppy has a long tail. Explain the genetic makeup of the parents and puppies involved in this inheritance pattern.

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The inheritance of tail length in dogs follows a classic Mendelian dominance pattern, with the allele for short tails (T) being dominant over the allele for long tails (t). In this scenario, two short-tailed dogs are bred: one is homozygous dominant (TT), and the other is heterozygous (Tt). The genetic makeup of each parent influences the genotypes and phenotypes of the resulting puppies, and understanding these genetics is key to explaining the observed inheritance patterns.

Genetic makeup of initial parental dogs

The first parent, being homozygous for short tail, has the genotype TT. This dog can only pass on the T allele to its offspring. The second parent, being heterozygous, has the genotype Tt, and can pass either a T or a t allele, with equal probability. When these two dogs are mated, the Punnett square demonstrates the expected genotypic ratios:

  • TT (homozygous dominant) – 50%
  • Tt (heterozygous) – 50%

Because T is dominant, all the puppies with at least one T allele display the short tail phenotype. Hence, all twelve puppies in the first litter exhibit short tails, which aligns with expectations based on Mendelian inheritance.

Genetic explanation for the first litter

Given the genetic composition of the parent dogs, the first litter's outcome — all puppies having short tails — is expected. The presence of only TT and Tt genotypes among the puppies means they all express the dominant short tail trait. This ratio adheres to the Mendelian inheritance pattern where the dominant phenotype can be expressed even when a single dominant allele is present.

The second breeding and the unexpected long-tailed puppy

In the second mating, one of the first-generation puppies—whose exact genotype we need to determine—is bred to another short-tailed dog from a different litter. The second litter produces ten puppies, among which one exhibits a long tail phenotype, unexpectedly. The appearance of a long-tailed puppy indicates that the genotypic combinations of the parents include the recessive allele at a homozygous state.

Genetic makeup of the second-generation parents

The first-generation puppy bred in the second mating might be heterozygous (Tt) or potentially homozygous (TT), but the disclosure of a long tail in some puppies suggests that this puppy must carry the recessive allele t—in other words, it must be heterozygous (Tt). The other parent, from another litter and known to be short-tailed, could be either heterozygous (Tt) or homozygous dominant (TT). To produce a long-tailed puppy, both parents must contribute a recessive t allele.

Possible genotypic combinations and explanations

  • If the first-generation puppy (second-generation parent) is heterozygous (Tt), and the other parent from the other litter is also heterozygous (Tt), their offspring can be:
    • TT — 25% (short tail)
    • Tt — 50% (short tail)
    • tt — 25% (long tail)
  • Therefore, among ten puppies, approximately two could have long tails if the genes segregate according to Mendelian ratios, aligning with the observed occurrence of a single long-tailed puppy.

This scenario supports the conclusion that the heterozygous (Tt) genotype among the parents can produce both short and long tails, with the long tail phenotype manifesting only when a puppy inherits the recessive t allele from both parents.

Inheritance pattern and conclusion

The appearance of a long-tailed puppy amidst predominantly short-tailed siblings illustrates classical autosomal dominance of the T allele, with the recessive t allele only expressed phenotypically when inherited from both parents. The unexpected presence of the long tail phenotype underscores the importance of genotypic variation within breeding populations and the potential for recessive traits to re-emerge in subsequent generations. Proper understanding and genetic testing can help breeders predict and manage such inheritance patterns, ensuring desired traits are maintained or avoided.

References

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