Okay let me break this down, so you have a pure red and pure white feathered chicken meaning that the chickens have homologous alleles. Overtime, as you breed the two you may get multicolored chickens but they can carry the recessive white feathers alleles (FWFW) meaning that they can still produce all white chickens and they can continuously do that over generations. Selective breeding may be negative because you may not get the desired result, you could breed for the wrong traits, other characteristics are lost and the resulting population is very similar. Less genetic variation can cause the chickens to not be resistant to problems like diseases and could fall ill.
substance formed by the chemical combination of two or more elements in definite proportions (a thing that is composed of two or more separate elements; a mixture.)
