When a male pig from a line of true-breeding (homozygous) black solid-hooved pigs was crossed to a female from a breed (homozygous) of red cloven-hooved pigs their several progeny all looked alike?

The progeny of the cross between the homozygous black solid-hooved male pig and the homozygous red cloven-hooved female pig all looked alike due to the principle of dominance in genetics. The black solid-hooved trait is likely dominant over the red cloven-hooved trait, resulting in all offspring displaying the black solid-hooved phenotype. Since both parents are homozygous, all offspring inherit the dominant allele from the black parent, leading to uniformity in appearance.