Question

What is the relationship between phenotype and genotype? • What are SNPs? • How can restriction enzymes and electrophoresis be used to identify SNPs and determine genotype?

Answer 1

Short answers first, then a practical explanation.

1) Relationship between phenotype and genotype
- Genotype = the organism’s genetic makeup (the alleles at one or many loci).
- Phenotype = the observable characteristics or traits (morphology, behavior, biochemical traits, disease presence, etc.).
- Relationship: genotype provides the information that can produce a phenotype, but phenotype is the result of genotype interacting with environment, developmental history and other genes. Factors that modify the simple genotype→phenotype mapping include dominance/recessiveness, incomplete dominance, epistasis (gene–gene interactions), variable penetrance and expressivity, and environmental effects. Example: a person may carry a disease-causing allele (genotype) but not show symptoms (phenotype) if penetrance is incomplete or environment suppresses expression.

2) What are SNPs?
- SNP = Single Nucleotide Polymorphism, a position in the genome where a single base (A, T, C or G) differs among individuals in a population.
- Common features: usually biallelic (two common alleles), widespread across the genome, used as genetic markers. By convention an SNP is often defined as a variant present at >1% frequency in the population (rare single-base changes are called variants or mutations).
- Effects: many SNPs are neutral (in noncoding or synonymous positions), but some change amino acids (nonsynonymous), alter splicing, affect regulatory sequences, or serve as markers linked to functional variants. SNPs are widely used for association studies, population genetics and genotyping.

3) How restriction enzymes + electrophoresis can identify SNPs and determine genotype
- Principle: Some SNPs alter a restriction enzyme recognition site (create or abolish it). If you PCR-amplify the DNA region containing the SNP and then digest that PCR product with the appropriate restriction enzyme, the presence or absence of the cut site produces different fragment sizes. Running the digested DNA on a size-separating gel (electrophoresis) reveals band patterns that correspond to each genotype.

- Typical workflow (PCR-RFLP or CAPS/dCAPS approach):
1. Design primers to PCR-amplify a fragment that includes the SNP.
2. Choose a restriction enzyme whose recognition sequence overlaps the SNP so one allele is cut and the other is not. (If there is no natural enzyme site, introduce a mismatch in a primer to create a site for one allele — called dCAPS.)
3. PCR-amplify the target from individual DNA samples.
4. Digest the PCR product with the chosen restriction enzyme.
5. Run the digest on an agarose (or polyacrylamide for small differences) gel and stain/visualize bands.
6. Interpret band pattern:
- Homozygote for allele that keeps the site: product is cut into the expected smaller fragments only.
- Homozygote for allele that abolishes the site: product remains as the uncut full-length fragment only.
- Heterozygote: both cut fragments and the uncut fragment are present (mixed pattern).

- Example (illustrative): a 400 bp PCR product contains an EcoRI site when allele G is present but the SNP G→A abolishes EcoRI. After EcoRI digest:
- G/G homozygote → two bands (150 bp + 250 bp).
- A/A homozygote → one band (400 bp).
- G/A heterozygote → three bands (400, 250, 150 bp).

- Practical considerations and limitations:
- Not all SNPs affect restriction sites; dCAPS can be used to force a difference.
- Choose enzymes and fragment sizes so the gel can resolve the bands reliably.
- Incomplete digestion or PCR artifacts can confuse interpretation; include controls and a DNA size ladder.
- For higher throughput, sequencing or SNP-array/qPCR-based assays are commonly used instead.

If you want, I can walk through designing primers and choosing an enzyme for a specific SNP sequence or show a visual example of expected gel bands for a given SNP.