In horses, black is dependent upon a dominant gene, b, and chestnut upon it’s recessive allele, t. if a homozygous black pacer is mated to a homozygous chestnut trotter, what will be the appearance of the f1 generation?

The phenotype of the F1 generation offsprings will be black. They will be heterozygous in nature.

The genes for the fur colour in horses are completely dominant and the dominant colour is black and is represented by B. The recessive colour is chestnut and is represented by t.

The genotype of the homozygous black horse is BB. and the genotype of the homozygous chestnut coloured parent is tt.

The gametes from the black parent =B.

The gametes from the chestnut parent = t.

So the genotype of the F1 offspring = Bt.

It is a heterozygous black horse because the black gene is completely dominant over chestnut gene.

you and your brother should be dizygotic twins (fraternal) twins since identical twins can only be one gender: boy and boy   or   girl and girl.