In mammals, identical twins arise when an embryo derived from a single fertilized egg splits into two independent embryos, producing two genetically identical individuals.
The converse phenotype, fusion of two genetically distinct embryos into a single individual, is also known. What are the genetic implications of such an event?

The bicoid gene is a coordinate maternal–effect gene. A female that is homozygous for a loss-of-function bicoid allele is mated to a wild-type male. What are the phenotypes of their progeny?

In gain-of-function let-23 and let-60 C. elegans mutants, all of the vulval precursor cells differentiate with 1° or 2° fates. Do you expect adjacent cells to differentiate with 1° fates or with 2° fates? Explain.

The bicoid gene is a coordinate maternal–effect gene. If loss of bicoid function in the egg leads to lethality during embryogenesis, how are females homozygous for bicoid produced? What is the phenotype of a male homozygous for bicoid loss-of-function alleles?

Ablation of the anchor cell in wild-type C. elegans results in a vulva-less phenotype.

What about if the anchor cell is ablated in a let-23 gain-of-function mutant?

The bicoid gene is a coordinate maternal-effect gene.

A female Drosophila heterozygous for a loss-of-function bicoid allele is mated to a male that is heterozygous for the same allele. What are the phenotypes of their progeny?

In mammals, identical twins arise when an embryo derived from a single fertilized egg splits into two independent embryos, producing two genetically identical individuals.

What limits might there be, from a developmental genetic viewpoint, as to when this can occur?