Choose the parents, and the Punnett square shows every possible child. Try one-gene and two-gene crosses, ABO blood groups and sex-linked conditions, then make real offspring at random to see how chance works.
Loading the lab...
| Gene and allele | A gene is a section of DNA that controls a trait. An allele is one version of a gene, such as T (tall) or t (short). |
|---|---|
| Dominant / recessive | A dominant allele (capital letter) shows even when only one copy is present. A recessive allele (small letter) shows only when two copies are present. |
| Genotype | The alleles an organism has, written as letters: TT, Tt or tt. |
| Phenotype | What you can see: tall or short. |
| Homozygous / heterozygous | Homozygous means two identical alleles (TT or tt). Heterozygous means two different alleles (Tt). A heterozygous person who does not show a recessive condition is a carrier. |
| Gamete | A sex cell (sperm or egg). It has only one allele of each gene, so a Tt parent makes half T gametes and half t gametes. |
| Incomplete dominance | The heterozygote is in between (red x white snapdragon gives pink). |
| Codominance | Both alleles show fully in the heterozygote (roan cattle, blood group AB). |
| Cross | Offspring phenotypes | Name |
|---|---|---|
| TT x tt | All tall (all Tt) | First generation (F1) |
| Tt x Tt | 3 tall : 1 short | Second generation (F2), monohybrid ratio |
| Tt x tt | 1 tall : 1 short | Test cross (shows if a tall plant is TT or Tt) |
| RrYy x RrYy | 9 : 3 : 3 : 1 | Dihybrid ratio (law of independent assortment) |
| RW x RW (incomplete dominance) | 1 red : 2 pink : 1 white | 1 : 2 : 1 |
Test yourself with chapter-wise MCQs and explanations:
Punnett squares give probabilities, not guarantees: each child is an independent chance event. Real families are small, so the ratios are only approximate. Works offline: after your first visit the lab is saved in your browser.