These questions ask you to build or read a one-gene cross, choose the right way to group its outcomes, and interpret the resulting ratio as a chance. The main decision is whether you are counting allele pairs, visible characteristics, or repeated real results.
Work from alleles to groups to chances
Write the allele each gamete can carry. Put one parent's gametes across the top of a Punnett square and the other parent's down the side, then combine one row allele with one column allele in each box. Keep repeated combinations: if the same mixed genotype appears by two routes, both boxes count. Only after filling the square should you decide how to group the boxes.
| What the question asks for | How to count |
|---|---|
| Genotype ratio | Keep every distinct allele pair as its own group: two upper-case copies, one of each form, and two lower-case copies. |
| Phenotype ratio | Combine every genotype that produces the same visible characteristic. With complete dominance, a dominant allele in either position gives the dominant phenotype. |
| Probability from a ratio | Add all parts to get the total, then place the relevant part over that total. |
| A later offspring | Use the original cross again. Each fertilisation is independent, so earlier outcomes do not remove boxes from the square. |
| Observed results | Expect chance variation in a small group. A larger sample usually lies closer to the predicted proportions. |
For example, if both parents can make two gamete types, the square has four equally likely boxes. A matching pair may occur once while a mixed pair occurs twice because either parent can contribute the dominant allele. Count routes, not just the names of the genotypes you notice.
Where ratio reasoning goes wrong
| Tempting shortcut | How to reject it |
|---|---|
| Using the three-group genotype pattern when appearance is requested | Ask whether mixed-pair offspring and offspring with two dominant copies look different. If they share a phenotype, those boxes belong in one visible group. |
| Reading a three-to-one ratio as three chances out of one or three out of three | The two parts make a total of four. A ratio part becomes a fraction only after the parts are added. |
| Counting one favourable result among three genotypes | Punnett boxes, not genotype names, are the equally likely units. A genotype reached by two routes must be counted twice. |
| Treating a predicted ratio as a quota for every small litter | A probability applies afresh to each offspring. It does not force a group of four to contain one of each expected share. |
| Counting only one route to a heterozygous offspring | Check both directions of inheritance. The dominant allele can come from either parent when both carry both alleles. |
Try it
In tomato plants the allele T for tall stems is dominant to the allele t for short stems. A plant carrying one T allele and one t allele is crossed with a plant carrying two t alleles. What phenotype ratio is expected?
- 1 tall to 1 short
- 3 tall to 1 short
- All the offspring tall
- All the offspring short
Two heterozygous rabbits have already produced three offspring, and all three show the dominant characteristic. How does this affect the chance that the next offspring shows the recessive characteristic?
- It is unchanged, because each offspring gets its own independent pair of alleles
- It rises, because the recessive characteristic is now overdue
- It falls, because this pair evidently passes on the dominant allele
- It becomes impossible, because the earlier offspring used up the recessive outcomes
A class crosses many heterozygous pea plants and compares the seedlings they count with the predicted 3 to 1 ratio. Which statements about that comparison are true? Select all that apply.
- Counting a large number of seedlings usually gives a closer match to the prediction
- Counting more seedlings pushes the results further from the prediction
- Small groups of seedlings vary noticeably from one group to the next
- A group of seedlings that does not match exactly shows the prediction is wrong