A Punnett square is a simple grid for predicting offspring of a cross. One parent's possible gametes go across the top, the other's down the side, and each cell shows the genotype from that combination. The proportions of cells are the expected proportions of offspring.
In practice
For one gene, Aa × Aa gives a 2 × 2 square: one AA, two Aa, one aa. If A is dominant, three of four plants look the same. For two independent genes the square is 4 × 4 and gives the 9:3:3:1 ratio.
Breeders use it to plan how many seeds to pop. If you want a plant homozygous for two recessive traits from an F2, only 1 in 16 is expected; to be about 95% sure of getting at least one, you need roughly 47 plants. For three recessives it is 1 in 64 and a few hundred seeds.
Chance of at least one hit in n plants = 1 - (1 - p)^n, where p is the expected fraction from the square. Solve for n to size a grow-out.
Don't confuse with
- Actual results: the square gives expected ratios; small grows deviate by chance.
- Quantitative traits: yield or potency do not fit a Punnett square; they need heritability and population thinking.
- Linked genes: genes near each other on a chromosome do not assort freely, so the 9:3:3:1 square overstates recombinants.
The genotype is the set of alleles a plant actually carries, either at one locus or across its whole genome, as opposed to the traits you can see.A gene is a stretch of DNA that carries the instructions for one functional product, usually a protein such as an enzyme, and is the basic unit of inheritance.A plant is homozygous at a locus when it carries two identical alleles there (AA or aa), so it passes the same allele to every offspring.A recessive allele only shows its effect when a plant carries two copies of it; in a single copy it is masked by a dominant allele and carried silently.F2 is the second filial generation, made by crossing or selfing F1 plants; it shows the widest segregation, bringing back hidden recessives and new combinations.A quantitative trait varies continuously (yield, height, flowering time, cannabinoid content) because it is controlled by many genes of small effect plus the environment.Heritability is the share of a trait's variation within a population that is due to genetic differences rather than environment, on a scale from 0 to 1.Linkage is the tendency of genes that sit close together on the same chromosome to be inherited together, rather than independently.Mendelian inheritance is the pattern in which single genes pass from parents to offspring in predictable ratios, following Mendel's laws of segregation and independent assortment.Segregation is the separation of a plant's two alleles at each locus into different gametes, so offspring of a heterozygous parent vary in which allele they get.A dominant allele is one whose effect shows in the phenotype even when only one copy is present, masking the other (recessive) allele at that locus.A test cross mates a plant of unknown genotype with one that is homozygous recessive for a trait, so the offspring ratios reveal what the first plant carries.
