2.4.4Unit 2 — Multicellular Organisms
Monohybrid Inheritance
5 min read 6 parts Lesson 18 of 39 Virtual lab
Part 1 of 6Monohybrid Crosses
Pea plant height is a classic example: the tall allele T is dominant and the short allele t is recessive.
Monohybrid Crosses
Single gene
- Monohybrid inheritance follows one characteristic controlled by one gene that has a dominant and a recessive allele.
True-breeding
- A pure-breeding parent is homozygous, so it always passes on the same allele to every offspring it produces.
Gametes
- Each parent makes gametes carrying only one allele of the gene, so a heterozygous parent produces two types of gamete.
Prediction
- A genetic cross shows the probability of each outcome, not a guarantee of what will happen in any particular family.
Part 2 of 6Constructing a Punnett Square
1
Write down the genotypes of the two parents, using a letter whose capital and lowercase forms are easy to tell apart.
2
Split each parent genotype into its two alleles and write one parent’s alleles across the top and the other’s down the side.
3
Fill each of the four boxes by combining the allele from the top with the allele from the side.
4
Read off the offspring genotypes, then work out the phenotype of each using the rule that one dominant allele gives the dominant phenotype.
5
State the ratio or percentage chance asked for, simplifying the ratio where possible.
Part 3 of 6Worked Crosses
TT × tt
- All F₁ offspring are Tt and tall, because each receives one T from one parent and one t from the other.
Tt × Tt
- The F₂ genotypes are 1 TT : 2 Tt : 1 tt, giving a phenotype ratio of 3 tall to 1 short.
Tt × tt
- Half the offspring are Tt (tall) and half are tt (short), giving a 1:1 ratio, which shows the parent was heterozygous.
Probability
- In a cross of two heterozygotes each offspring has a 25% chance of being short, whatever the previous offspring were.
| Cross | Offspring genotypes | Phenotype ratio |
|---|---|---|
| TT × tt | All Tt | All tall |
| Tt × Tt | 1 TT : 2 Tt : 1 tt | 3 tall : 1 short |
| Tt × tt | 1 Tt : 1 tt | 1 tall : 1 short |
Part 4 of 6Why Results Differ From Predictions
Why Results Differ From Predictions
Chance
- Fertilisation is random, so which gamete meets which is a matter of chance and actual numbers need not match the ratio.
Small samples
- With few offspring the observed ratio can be far from 3:1, whereas large numbers usually come closer to the prediction.
Environment
- Environmental factors can alter how a gene is expressed, so the phenotype may not match the genotype exactly.
Theoretical ratio
- A predicted ratio is a theoretical probability, so it describes what is expected on average over many offspring.
Part 5 of 6Worked Example
Question
- Two heterozygous tall pea plants (Tt) are crossed.
- Calculate the percentage chance that an offspring is short.
- Gametes from each parent are T or t.
- Combine them in a Punnett squareTT, Tt, Tt and tt.
- Only tt produces a short plant, which is 1 box out of 4.
- Convert to a percentage1 ÷ 4 × 100.
Answer There is a 25% chance that an offspring is short (ratio 3 tall : 1 short).
Part 6 of 6Exam Checklist & Tips
2.4.4 – Key Points for the Exam
- Monohybrid cross A genetic cross that follows the inheritance of one characteristic controlled by a single gene.
- Punnett square A grid showing all possible combinations of parental gametes and the resulting offspring genotypes.
- Pure-breeding Homozygous for the characteristic, so always producing offspring with the same phenotype when crossed together.
- Phenotype ratio The proportion of each phenotype expected among offspring, for example 3 : 1.
- F₂ generation Offspring produced by crossing two F₁ individuals.
Common Mistakes
- Writing a gamete as containing two alleles; each gamete carries only one allele of the gene.
- Giving the ratio of genotypes when the question asks for the ratio of phenotypes.
- Thinking that four offspring from a Tt × Tt cross must be exactly three tall and one short.
- Forgetting to state which letter stands for which allele before drawing the cross.