5 BIO - EOC - Heredity: Inheritance & Variation of Traits

A shortened tail, called a bobtail, is a phenotype that can be preferred for some hunting and working dog breeds such as the corgi. It lowers the risk of a tail injury when working around livestock, fences, and machinery. Achieving the bobtail length used to be done by physically shortening corgis’ tails; however, now this shortened tail can occur naturally due to selective breeding.

Figure 1: Long-Tailed and Bobtailed Corgis

Long-Tailed Corgi (left) and Bobtailed Corgi (right)

The bobtail phenotype can be caused by a natural mutation in the T-gene, which can cause dogs to develop fewer vertebrae while in the embryonic state. A comparison of the amino acid sequences in both the long-tail and bobtail T-gene is shown.

Amino Acid Sequences

This mutation can be seen when studying gel electrophoresis data. Long-tailed dogs show two DNA fragments at 183 and 134 base pairs. Bobtailed dogs show 3 DNA fragments at 183, 150, and 134 base pairs. Figure 2 shows an electrophoresis DNA marker comparison of 2 parent corgis and their 8 pups.

When studying the genetics of 315 corgis, all the long-tailed corgis (182) were found to be homozygous dominant $(TT)$, and all the bobtailed corgis (133) were found to be heterozygous $(Tt)$. No corgis were born to the litters that were homozygous recessive $(tt)$. Following DNA testing of various corgi litters, no individual pups were homozygous recessive $(tt)$. During these studies, scientists have observed that any time heterozygous parents breed, the average litter size is smaller.

Figure 3 compares litter sizes when both parents are homozygous (long-tailed) versus when both parents are heterozygous (bobtailed).