An inheritance grid shows every way two parents’ gametes can combine. You write the gametes of one parent along the first row, the gametes of the other down the first column, and each box shows one possible offspring genotype. Questions then ask for genotypes, phenotypes, a ratio or a probability.
This lesson uses the words from distinguishing gene, allele and chromosome and feeds directly into separating genotype from phenotype.
Why does the grid work?
Each parent passes on one allele of the gene in each gamete, and fertilisation is random. If a parent has two different alleles, half the gametes carry one and half carry the other. The grid lists every pairing with equal weight, so the boxes show the chance of each genotype.
How to build a grid, step by step
- Choose letters and state the meaning. Use a capital letter for the dominant allele and the same letter in lower case for the recessive one, for example R (purple) and r (white).
- Write the parents’ genotypes. For example, Rr × Rr.
- Write each parent’s gametes. A parent makes one gamete type for each allele. Rr gives R and r. RR gives only R, and rr gives only r.
- Draw the grid with one parent’s gametes along the first row and the other’s down the first column.
- Fill each box with the two alleles that meet, writing the capital letter first (Rr, not rR).
- Read the result. Count the genotypes, then state the phenotypes.
Worked example
A fictional plant has purple flowers (allele R, dominant) or white flowers (allele r, recessive). Two purple plants, both with the genotype Rr, are crossed. Find the genotype and phenotype ratios.
Parents: Rr × Rr.
Gametes: each parent makes R and r.
| R | r | |
|---|---|---|
| R | RR | Rr |
| r | Rr | rr |
Genotypes: 1 RR : 2 Rr : 1 rr.
Phenotypes: RR and Rr are purple, so there are 3 purple to 1 white.
Probability of a white offspring: 1 box out of 4, which is 1/4 or 25%.
Check: 1 + 2 + 1 = 4 boxes, and 3 + 1 = 4. The fractions add up to 4/4.
Now cross a heterozygous plant with a white plant, Rr × rr. The gametes are R and r for the first parent and only r for the second. The boxes are Rr, rr, Rr and rr, so the ratio is 1 Rr : 1 rr, or 1 purple to 1 white, with a probability of 1/2 for each.
The mistake to watch for
A frequent error is to write a parent’s whole genotype in the gamete labels, for example Rr as a single column label.
Mistaken grid: first row labelled “Rr”, first column labelled “Rr”, box filled with RrRr.
Each box now holds four alleles, which no offspring has, and the count of outcomes is wrong.
The correction is to split the genotype first. A gamete holds one allele of the gene, so Rr becomes two labels, R and r. Each box then holds exactly two letters, one from each parent.
Check yourself
1. Two parents are both Rr. State the probability that an offspring is Rr.
Show answer
The grid has RR, Rr, Rr and rr. Two of four boxes are Rr, so the probability is 2/4 = 1/2.
2. Cross RR with rr. State the genotype and phenotype of all offspring.
Show answer
Gametes are R only and r only, so every box is Rr. All offspring are Rr and all are purple.
3. A purple plant crossed with a white plant gives some white offspring. What must the genotype of the purple parent be?
Show answer
A white offspring is rr, so it received an r from each parent. The purple parent must carry r as well as R, so its genotype is Rr.
Where this leads next
Next, learn to separate genotype from phenotype so that the labels you write under each grid are correct. The inheritance model board lets you rebuild a cross in a fictional model, and the probability tree and counting board shows the same outcomes as paths.
A teacher in online one-to-one Biology tuition can give you fresh crosses to set up until the first step is automatic.