Genetics as it applies to evolution, molecular biology, and medical aspects.
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Can anyone give a relatively general account on how DNA and gene expression determine an organism's physical shape, i.e., size, skeletal structure, body shape, etc. I would presume the explanations for size and shape are distinct or merely dependent on different genes/expression; "Ligers" (Lion-Tiger hybrids) display abnormally large SIZE possibly due to genomic imprinting. How is it that their physical shape comes about? How does a single cell with a set sequence "know" what shape to create. In theory, what aspects can be manipulated with genetic engineering?
Hypothetically, what kind of organism can be created from a blank slate? Any?
I understand my question is vague.
Any insight would be appreciated.
Organism's shape is determined through cell to cell interactions leading to cell differentiation and/or cell death. I will try giving the most simplistic embryo example. You have 3 layers of cells (precursors that can become anything). Cells in layer 1 and 3 sense another cell layer on one side only and "environment" on the other side. Hence, they become precursors of the outer cell types - skin/mucosa. The middle layer senses cells on both sides and, therefore, becomes precursor for everything on the inside... I oversimplified it, but that's the general scheme. Any defects in cell to cell signaling (sensing neighboring cells) results in birth defects.
As to the other aspect of your question - genetic regulation of organism's size:
Gene1 = produces growth hormone
Gene 1 allele A (which is a variant of this gene) - can make 100 units of hormone per hour.
Gene 1 allele B - can make only 50 units of hormone per hour.
Gene 1 allele C - makes defective hormone = no hormone.
Now, someone with 2 alleles A (AA) will produce 200 units of hormone/hour = tall.
One A and one B allele (AB) will produce 150 units = medium height,
BB and AC = 100 units = dwarf,
and CC = no hormone = incompatible with life.
This example is just one of many ways of regulation. It's oversimplified, since in nature many genes can be involved in regulation of a single phenotypic trait...
I think this would make more sense to you if you look into embryonic patterning. There is a good basic and accessible book, "Coming to Life: How Genes Drive Development" by Christine Nusslein-Volhard. I see Amazon has it for about $16. It is intended for a lay audience, but it would be better to have some basic bio background before tackling this one.
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