An inheritance diagram shows how alleles pass from two parents to their offspring. The four steps are: write the parents, split each into gametes, combine the gametes in a grid, then read the ratio.
This lesson is part of SPM Science genetics. If the word allele is unfamiliar, read distinguishing genes, chromosomes and DNA first.
What are the four steps?
- Parents: write the genotype of each parent, using one capital letter for the dominant allele and its small letter for the recessive one.
- Gametes: split each parent into gametes, one allele per gamete.
- Grid: place one parent’s gametes across the top and the other’s down the side, then fill each box.
- Read: count each genotype, then state the phenotype ratio.
Do the steps in this order every time, so that a wrong answer can be traced to one step.
Worked example: two heterozygous parents
In a pea plant, tall (T) is dominant to short (t). Two tall plants, each with genotype Tt, are crossed.
Step 2 gametes. Each parent makes two kinds of gamete: T and t.
Step 3 grid.
| T | t | |
|---|---|---|
| T | TT | Tt |
| t | Tt | tt |
Step 4 read. The genotypes are 1 TT : 2 Tt : 1 tt. TT and Tt are both tall, so the phenotype ratio is 3 tall : 1 short.
If the cross gave 80 seeds, the expected counts are 80 ÷ 4 = 20 for each box, so 60 tall and 20 short. The actual count may differ slightly, and the word expected in your answer shows you know that.
The mistake that costs marks
The usual slip is writing a gamete as a pair, such as Tt, because the parent is written as Tt. The grid then fills with four-letter boxes or repeated pairs, and the final ratio is wrong.
| Step | Wrong | Right |
|---|---|---|
| Gametes from Tt | Tt and Tt | T and t |
| Grid box | TtTt | Tt |
| Ratio statement | Exactly 3 of 4 offspring | Expected 3 : 1 |
The fix is to ask one question before writing each gamete: how many alleles of this gene does a gamete carry? The answer is always one.
A second cross to show the method holds
Now cross a tall plant Tt with a short plant tt. The gametes are T and t from the first parent, and only t from the second.
| t | t | |
|---|---|---|
| T | Tt | Tt |
| t | tt | tt |
The genotypes are 2 Tt : 2 tt, which simplifies to 1 Tt : 1 tt. The phenotype ratio is 1 tall : 1 short. You can test your own pairs with the simple inheritance cross explorer.
Check yourself
In a cross between two parents, Tt and tt, 40 seeds are produced. State the expected number of tall and short seeds. Then write the genotype of the short parent’s gametes.
Answer
The expected ratio is 1 tall : 1 short, so 20 tall and 20 short from 40 seeds.
The short parent is tt, so every gamete is t, written as a single letter.
What to study next
Variation is the next idea, because different gametes meeting explains why offspring differ. Continue with explaining variation with examples, then the genetics practice set.
If you want a teacher to watch you build diagrams from new crosses, see online one-to-one Science tuition.