The variable gene activity hypothesis states that cell differentiation and determination is controlled through differences in gene activation or inactivation.

How would you conduct a screen to identify recessive mutations in Drosophila that result in embryo lethality? How would you propagate the recessive mutant alleles?

During early development, human cells determine their differentiated fates based on the environment. What term describes this phenomenon?

If an embryonic cell is damaged in an organism that undergoes mosaic determination, will the offspring produced develop normally or abnormally?

Explain why many developmental genes encode either transcription factors or signaling molecules.

Bird beaks develop from an embryonic group of cells called neural crest cells that are part of the neural tube, which gives rise to the spinal column and related structures. Amazingly, neural crest cells can be surgically transplanted from one embryo to another, even between embryos of different species. When quail neural crest cells were transplanted into duck embryos, the beak of the host embryo developed into a shape similar to that found in quails, creating the 'quck.' Duck cells were recruited in addition to the quail cells to form part of the quck beak. Conversely, when duck neural crest cells were transplanted into quail embryos, the beak of the embryo resembled that of a duck, creating a 'duail,' and quail cells were recruited to form part of the beak. What do these experiments tell you about the autonomy or nonautonomy of the transplanted and host cells during beak development?