Final answer:
Breeding to emphasize certain features can lead to inbreeding, inbreeding depression, and a variety of genetic disorders due to a reduced gene pool and increased prevalence of recessive mutations. Extreme physical traits in pets like those in the English bulldog can result in severe health issues and necessitate medical interventions like cesarean sections. Chromosomal abnormalities from selective breeding can cause lethal outcomes or disorders such as Down syndrome.
Step-by-step explanation:
Breeding to emphasize certain physical features has become so extreme that it can cause inbreeding and inbreeding depression, disorders, and abnormal behavior. The selective breeding, such as that seen in the English bulldog, often leads to narrowed and less diverse gene pools, increasing the propensity for genetic disorders like cancer, hip dysplasia, and hereditary epilepsy. Socially desirable traits like neotony, which is the preservation of juvenile features in adults, and morphological characteristics like larger head size can lead to significant health issues. For example, 86 percent of English bulldog litters require cesarean delivery due to large heads and narrow pelvises. These breeding practices can also lead to developmental delays and distinctive facial appearances. In extreme cases, breeding can result in chromosomal abnormalities such as nondisjunction, causing aneuploidy which can be lethal or lead to disorders like Down syndrome. Breeders therefore should avoid mating close relatives to prevent the accumulation of deleterious mutations and ensure the health of the offspring.