From a fertilized egg to a grown up individual, every step is controlled by genes. Developmental genetics is a branch to study control of cell growth, differentiation and morphogenesis, which is the process that gives rise to tissues, organs and anatomy. In more details, it is the study of cell fate, cell determination and differentiation.
The Mosaic Theory was proposed by Wilhelm Roux and August Weismann in 1890s. It has a few points: Determinants specify the various differentiation pathways; Unequal cell division leads to determination. More recently, The Theory of Differential Gene Expression takes a leading position. This theory believes that differentiation occurs as a result of differential gene expression, i.e., in a particular cell, only a subset of the total genes are expressed.
Potency of stem cells
The ability to divide and produce all the differentiated cells in an organism, including extraembryonic tissues, is called totipotent. To a less extent, a pluripotent and mulipotent stem cell only give rise to certain types of organs and tissues, a unipotent stem cell gives rise to a family of related cells. From ES cells to pluripotent, multipotent, unipotent cells to differentiated cells, it forms a cell lineage. For each cell lineage, there is a master control gene which activates a group of downstream genes and define the lineage differentiation.
Due to the changes of gene expression patterns, a differentiated cell can undergo nuclear reprogramming and de-differentiates. It is the reason why it is possible to clone animals.
Immune cell fate
All the immune cells are derived from bone marrow stem cells. Each Step of differentiation follow the general rule of differential gene expression. B cells are antibody-producing cells, each development step of B cells occurs through several stages representing a change in the genome content at the antibody loci. Via recombination, millions of different types of B cells are produced in vivo and thus the antibody diversity is achieved.