Explain that gene expression is regulated so different cells use different genes, and that traits result from inherited genes interacting with the environment (Ohio's Learning Standards for Science, Biology, B.H.3).

What this topic is asking

Ohio standard B.H.3 states that "the characteristics of an organism are a result of inherited traits received from parent(s) and the interaction of those traits with the environment." Ohio's Biology EOC turns this into two connected ideas: that cells with the same DNA specialize because they express different genes (regulation), and that the environment interacts with genes to shape the phenotype. The crosscutting idea is cause and effect: genes are one cause of a trait, and the environment is another.

Same DNA, different cells

A surprising fact underlies this topic: almost every cell in your body carries the same complete set of DNA, the same one that was in the fertilized egg. So why is a muscle cell different from a brain cell? Because each cell type uses a different part of that shared DNA. This switching of genes on and off is called gene regulation, and it controls gene expression (whether a gene's protein is actually made).

• A gene that is expressed (switched on) is transcribed and translated, so its protein is made.
• A gene that is not expressed (switched off) makes no protein in that cell.

Because a nerve cell switches on nerve genes and a skin cell switches on skin genes, the two cells make different proteins and take on different structures and jobs. This is the molecular basis of differentiation, the specialization introduced in the cell cycle and mitosis.

Why regulation matters

Regulation lets one genome build a whole body of specialized tissues, and it also lets cells respond to conditions. Cells switch genes on or off in response to signals, so they make a protein only when it is needed. This efficient, responsive control is part of how cells and organisms maintain homeostasis and adapt to change.

Genes and the environment together

The second half of B.H.3 is that the environment shapes traits alongside genes. A genotype sets a range of possibilities, but the environment helps decide where in that range an organism ends up.

• Hydrangea flowers change color (blue or pink) depending on soil acidity, even when the plants are genetically identical.
• A person's height depends on inherited genes and on nutrition during growth.
• Skin color can darken with sun exposure, an environmental effect on a genetically set trait.
• The fur color of some animals depends on temperature during development.

So the phenotype is the product of genotype interacting with environment. The same genotype can give different phenotypes in different conditions, and the EOC often illustrates this with identical organisms raised in different environments.

The take-home for the EOC

When a question shows organisms with the same DNA looking different, ask which idea applies:

• different cell types in one organism, the cause is differential gene expression;
• genetically identical organisms in different environments, the cause is gene-environment interaction.

Both come straight from Ohio standard B.H.3.

Try this

Q1. Explain why a liver cell and a nerve cell in the same body look different even though they have the same DNA. [2]

• Cue. Each cell type expresses (switches on) a different subset of the shared genes, so it makes different proteins and specializes differently.

Q2. State two environmental factors that can affect an organism's phenotype. [2]

• Cue. Any two of: nutrition, temperature, light or sun exposure, soil chemistry, water availability.