DNA is like a musical score. It is a written program but it is open to interpretation and variations in the performance. With identical musical scores two performances will differ because they are using different instruments, conductors and musicians. The same is true of two organisms using identical DNA but receiving different nutrients and experiencing different stresses so grow differently in response to these conditions. Extreme stressors such as high heat, starvation, toxic chemicals or drastic hormonal imbalances can produce these shifts in growth patterns during embryogenesis.
Development is not a rigid program. The embryo gets information and adjusts as it grows. This does not change the DNA sequences just the regulation or timing of genetic expression. Imagine if the embryo was following a road map with no alternate choices. If a snap was hit like a missing minor nutrient then everything would be stuck. Instead the body has some route options that find alternate paths to growing an adult body. This is called phenotype plasticity due to factors in addition to the encoded genetics. Epigenetic factors include nutrition, environmental qualities like sunlight intensity, and parenting while the brain is still developing.
http://en.wikipedia.org/wiki/Phenotypic_plasticityhttp://en.wikipedia.org/wiki/Epigeneticshttp://science.howstuffworks.com/genetic-science/epigenetics3.htm
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