How do you maintain embryonic lethal mutations in genes?
Diploid organisms sometime get by with only one working copy of a gene to express functional proteins. One allele produces protein sufficient to generate the same phenotype as the homozygote with two working copies. Blood type AO example. Codominant genes are haplosufficient. One working allele essentially reduces us to the haploid condition. Sometimes this is fine but sometimes one working allele is insufficient ie haploinsufficient. One allele cannot generate enough product for full phenotype with normal function. There are genes that are dosage critical, they are haplolethal and always required in two (functional) copies for normal embryogenesis. Any variation from that number will be prenatally lethal. The way this is demonstrated is that live births are never hemizygous (only one copy inherited) or heterozygous with a null (loss of function) allele. Deficient alleles cannot be kept in the genome. There are other genes that are haploinsufficient or dosage sensitive and conditional
