Good question. The answer to this is both yes and no. We need to think of the genetics behind all this.
An allele is one of two or more forms of a gene. Most multicellular organisms, ie. Us, have two sets of chromosomes, that is, we are “diploid”. Diploid organisms have one copy of each gene (and therefore one allele) on each chromosome. If both alleles are the same, they are called homozygotes. If the alleles are different, they are heterozygotes.
In the case of green fluorescent protein (GFP) mice:
– If the offspring are homozygous (express copies of the gene on both alleles), they do not survive as too much GFP is expressed/made and it is too toxic to the embryo.
– If the offspring are heterozygous (express the gene on one allele only), the mice fluoresce under a specific wavelength of ultra violet light (488nm). They survive and are quite healthy and happy and lead a normal life. These are the mice we use in research.
– If the offspring don’t have any functional copies on either allele, they do not glow at all, but are still healthy and happy.
Litters of GFP mice can be checked using the torch described in my earlier answer. If they glow, then they are heterozygous and are used in certain experiments. If they don’t glow, they are GFP “null” and are not used in research.
I hope this helps. I am happy to discuss further if necessary.