Even though the mother is heterozygous for this trait yet her off springs will show dominant trait because her husband is homozygous dominant for this trait. The mother has one recessive allele but it will not be able to express in the presence of dominant allele even after their mating because father will contribute dominant allele each time.
The cross is depicted as under:
Mother Father
Parental Tt TT
F1 TT TT Tt Tt
So all their offsprings will pehnotypically be long tailed because dominant allele (T) will suppress expression of recessive allele (t). 50% progenies will be heterozygous for long tail while 50% will be homozygous dominant for this trait.
