How to predict inheritance from a cross
Start with how many traits you are tracking. For one gene, the Punnett Square Calculator lays out the four gamete combinations and gives the classic 3:1 phenotype ratio from two heterozygous parents. For two independently assorting genes, the Dihybrid Cross Calculator produces the 9:3:3:1 ratio from a sixteen-cell grid.
Both use the Punnett square grid method and assume simple dominance and independent assortment, so they do not describe linked genes, incomplete dominance or polygenic traits. The ratios are also probabilities per offspring, not guarantees across a small litter — four offspring will rarely split exactly 3:1.
When to use Hardy-Weinberg and allele frequencies
Hardy-Weinberg describes a population that is not evolving, which makes it a null model rather than a prediction. The Hardy-Weinberg Calculator converts allele frequencies into expected genotype frequencies using p² + 2pq + q² = 1, and the interesting result is usually the deviation, not the match.
To go the other way, the Allele Frequency Calculator derives p and q from observed genotype counts. The five assumptions — no selection, no mutation, no migration, random mating and infinite population size — are never all true, so a departure from the frequencies given by the Hardy-Weinberg Calculator points at which one is being violated.
How to track growth in cells and populations
Growth compounds, which means small differences in rate produce large differences in outcome. The Doubling Time Calculator converts a growth rate into the time needed to double, and the rule of 70 gives a quick mental version — divide 70 by the percentage growth rate.
For cultures, the Cell Doubling Calculator projects cell count from a starting number and generation time, though real cultures leave exponential phase once nutrients run short. The Population Growth Calculator does the equivalent for organisms, and the Surface Area to Volume Ratio Calculator shows why cells cannot simply keep growing.
Frequently asked questions
- How do you read a Punnett square?
- Each parent's alleles go along one axis, and each interior cell shows one possible offspring genotype from combining them. Count the cells matching each phenotype to get the ratio — two heterozygous parents give 3:1 dominant to recessive.
- What are the Hardy-Weinberg assumptions?
- No selection, no mutation, no migration, random mating and an infinitely large population. If all hold, allele frequencies stay constant across generations.
- Why does GC content matter in DNA?
- Guanine-cytosine pairs are held by three hydrogen bonds versus two for adenine-thymine, so higher GC content raises the melting temperature of a sequence. This directly affects PCR primer design and annealing temperature.
- How do I calculate total microscope magnification?
- Multiply the eyepiece magnification by the objective lens magnification — a 10× eyepiece with a 40× objective gives 400×. Resolution, not magnification, sets the real limit on what you can see.
- Why can't cells grow indefinitely large?
- Because volume grows with the cube of radius while surface area grows only with the square, so the surface area to volume ratio falls as a cell enlarges. Eventually the membrane cannot exchange nutrients and waste fast enough for the interior.