Founder effect occurs when a small number of individuals move out from a larger population and then develop a new population in an isolated place such that it leads to a decreased genetic diversity in the population established by a fewer individuals.
As a result of the founder effect the genetic diversity is reduced in the descendants of the founder population which leads to an increase in the probability of having a genetic disorder.
Therefore, since the mice being blind due to a genetic defect escape from the laboratory and breeds then the subsequent generations will most likely be blind from birth as a result of founder effect.
The two alleles representing the trait are different (e.g. Pp for purple color). Although the pea plant appears purple in color (its phenotype is purple), its genotype (genetic make-up) is a mixture of a dominant “P” and a recessive 'p” trait.