The phenotype 'brindle' exhibits irregular vertical stripes in their hair coat texture along the neck, back, hindquarters, and upper legs.
This specific form of brindle is called brindle 1 (BR1) in which the stripes form a vertical pattern from the back to the sides of horses that is characterized by changes in hair colour and texture. The hairs from the stripes are less straight and unrulier when compared to the normal hair areas of the coat.
The BR1 mutation is located on the 'X' chromosome. Males have only one 'X' chromosome whereas females have two 'X' chromosomes. This means that Make can be X(N)/Y or X(BR1) / Y and females can be 'X'(N) / 'X' (N), 'X'(N) / 'X' (BR1), or 'X'(BR1) / 'X' (BR1).
Mode of inheritance is semi dominant.
The typical BR1 phenotype can only be seen in females with one copy of the mutation X'(N) / 'X' (BR1).
The BR1 phenotype is variable in that in some females, the predominant feature is the coat texture change while in others it is the striped pigmentation.
Males with the BR1 mutation X(BR1) / Y and females with two copies of the BR1 mutation 'X'(BR1) / 'X' (BR1) will not exhibit the BR1 pattern but will have sparse hair on the mane and the tail.
Since this mutation is located on the X chromoses, males only have one X chromosome and females have two, therefore possible genotypes will differ by sex.
- Female horses with N/N genotype and male horses with N genotype (males only have one X chromosome) will not exhibit the brindle coat pattern and cannot pass the brindle variant to their offspring.
- Female horses with BR1/N genotype will display the brindle coat texture and will pass on this brindle variant their offspring with a probability of 50%.
- Female horses with the BR1/BR1 genotype will have a sparse mane and tail, but will not display the brindle coat texture. They will pass on this brindle variant their entire offspring.
- Male horses with the BR1 genotype will have a sparse mane and tail, but will not display the brindle coat texture. They will transmit this brindle variant to all of their female offspring.
