A monohybrid cross follows one gene from parents to offspring in four rows: parents, gametes, offspring genotypes and phenotype ratio. Each row depends on the one before it, so a slip early on spoils the rest.
This lesson belongs to the cluster on inheritance. If words such as allele or genotype feel shaky, read using allele, genotype and phenotype terminology first.
What is the four-row layout?
- Parents: write each parent’s genotype.
- Gametes: list the alleles each parent can pass on, one per gamete.
- Offspring: combine the gametes in a Punnett square.
- Ratio: state the genotype ratio, then the phenotype ratio.
Keep each gamete to one allele. A parent has two alleles but each gamete carries only one.
Worked example with an invented plant
An invented plant has tall (T) dominant over short (t).
Cross 1: Tt × Tt.
- Parents: Tt and Tt.
- Gametes: T or t from each parent.
- Offspring: TT, Tt, Tt, tt.
- Ratio: genotypes 1 TT : 2 Tt : 1 tt. Phenotypes 3 tall : 1 short.
Cross 2: Tt × tt (a test cross).
- Parents: Tt and tt.
- Gametes: T or t from the first, only t from the second.
- Offspring: Tt, Tt, tt, tt.
- Ratio: genotypes 1 Tt : 1 tt. Phenotypes 1 tall : 1 short.
Written answer for Cross 1: “The parents are both Tt and make T and t gametes in equal numbers. The offspring genotypes are 1 TT : 2 Tt : 1 tt, so the phenotype ratio is 3 tall to 1 short.”
The mistake that loses marks
The common slip is to write whole genotypes as gametes, as in “Gametes: Tt and Tt.” A gamete carries only one allele, because meiosis separates the pair.
| Row | Wrong | Right |
|---|---|---|
| Gametes of Tt | Tt | T and t |
| Gametes of tt | tt | t |
| Result | Wrong square | Correct square |
The fix is to ask “how many alleles per gamete?” The answer is always one.
A routine for any cross
- Write the key: which allele is dominant and what it codes for.
- Write the parents’ genotypes.
- Split each parent into gametes with one allele each.
- Fill the square, then count genotypes before phenotypes.
The simple inheritance cross explorer lets you check your squares.
Check yourself
Invented plant: red flowers (R) are dominant over white (r). Cross two heterozygous plants and state the phenotype ratio and the probability that one offspring is white.
Answer
Parents Rr × Rr. Gametes R or r from each. Offspring RR, Rr, Rr, rr. Phenotype ratio 3 red : 1 white, so the probability that one offspring is white is 1 in 4, or 25%.
The 25% is the chance for each offspring. It does not guarantee one white plant in every four.
What to study next
Move on to interpreting sex-linked inheritance and then test yourself with the inheritance practice set.
If you want a teacher to watch you set out a cross, see online one-to-one Biology tuition.