Chromosome Evolution

Fissions and fusion is the most common way for the number of chromosomes to change.

Humans have 23 pairs of chromosomes, but all of the other great apes have 24 pairs. At some point since our lineage split from that of chimpanzees, two chromosomes fused

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#Unit 5. Developmental Biology

Morphogen is a diffusable biochemical molecule that can determine the fate of a cell by its concentration.

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#Unit 5. Developmental Biology

Morphogens can be:

Transcription factors

Paracrine factors

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#Unit 5. Developmental Biology

A morphogen specifies cells in a quantitative (“more or less”) manner.

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#Unit 5. Developmental Biology

A morphogenetic determinant specifies cells in a qualitative (“present or absent”) way.

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#Unit 5. Developmental Biology

In the frog (Xenopus laevis)

A gradient of the paracrine factor activin (a morphogen), causes concentration-dependent expression differences of two genes in unspecified cells.

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#Unit 5. Developmental Biology

At high concentrations (about 300 molecules/cell), activin induced expression of the goosecoid gene and instructed the cell to become the frog’s dorsal-most structures.