Explain the inheritance of coat colour in cattle, including codominance, using a cross between a pure breeding black bull and a pure breeding white cow.

The black bull produces gametes with allele B.
The white cow produces gametes with allele W.

When BB is crossed with WW, all offspring will inherit one B allele and one W allele.

The genotype of the offspring is $\boxed{\text{BW}}$.

ii) The offspring have a roan coat colour, which is a mixture of black and white. This occurs because the alleles for black (B) and white (W) coat colour are codominant. In codominance, both alleles are expressed equally and simultaneously in the heterozygote (BW), resulting in a phenotype that is distinct from either homozygous parent. Neither the black nor the white allele is dominant, so neither parent's coat colour is fully expressed in the offspring.

Genotypes: 50% BB, 50% BW
Phenotypes: 50% Black, 50% Roan

From the Punnett square, two out of four possible offspring genotypes are BW, which corresponds to the roan phenotype.
The chances of the offspring having a roan coat colour are $\boxed{50\% \text{ or } \frac{1}{2}}$.