Mendel’s garden
Between 1856 and 1863, Gregor Mendel, a monk in what is now the Czech Republic, grew and crossed about 28 000 pea plants. He chose peas because they have clear contrasting traits (tall/short, round/wrinkled, yellow/green), grow quickly and can be cross-pollinated by hand. His careful counting revealed the basic rules of inheritance, decades before anyone knew about DNA.
Key vocabulary
| Term | Meaning | Example |
|---|---|---|
| Allele | A version of a gene | T (tall), t (short) |
| Homozygous | Two identical alleles | TT or tt |
| Heterozygous | Two different alleles | Tt |
| Genotype | The alleles present | Tt |
| Phenotype | The observable trait | Tall |
| P generation | The original parents | |
| F₁ generation | The first offspring | |
| F₂ generation | Offspring of the F₁ |
Mendel’s laws
The law of segregation
The law of segregation says that each organism has two alleles for a trait, and these separate during gamete formation (meiosis). Each gamete gets only one allele.
The law of independent assortment
The law of independent assortment says that alleles for different genes separate independently of each other, as long as the genes are on different chromosomes (or far apart on the same one).
The law of dominance
When two different alleles are present, one (dominant) can mask the other (recessive).
Monohybrid crosses
A monohybrid cross follows one trait.
Mendel crossed true-breeding tall (TT) and short (tt) plants. All F₁ were Tt (tall). Then he crossed the F₁ plants:
| T | t | |
|---|---|---|
| T | TT | Tt |
| t | Tt | tt |
- Genotypic ratio: 1 TT : 2 Tt : 1 tt
- Phenotypic ratio: 3 tall : 1 short
Mendel’s actual F₂ count for this trait was 787 tall to 277 short, a ratio of 2.84 : 1, very close to 3 : 1.
Test cross
How can you tell if a tall plant is TT or Tt? Use a test cross: cross it with a homozygous recessive (tt) plant.
- If all offspring are tall → the unknown is TT.
- If about half are short → the unknown is Tt.
Dihybrid crosses
A dihybrid cross follows two traits. In peas, round seeds (R) are dominant to wrinkled (r), and yellow (Y) is dominant to green (y).
Crossing two RrYy plants: each parent can make four types of gametes: RY, Ry, rY, ry. That gives a 4 × 4 Punnett square with 16 boxes.
| Phenotype | Fraction |
|---|---|
| Round, yellow | 9/16 |
| Round, green | 3/16 |
| Wrinkled, yellow | 3/16 |
| Wrinkled, green | 1/16 |
This 9 : 3 : 3 : 1 ratio is the signature of independent assortment.
Using probability
Instead of drawing big squares, multiply probabilities for each gene.
What is the chance an RrYy × RrYy cross gives a wrinkled, green plant (rryy)?
- P(rr) = 1/4
- P(yy) = 1/4
- P(rryy) = 1/4 × 1/4 = 1/16
Pedigrees
A pedigree is a family tree showing a trait across generations. Squares are males, circles are females, shaded shapes show the trait. Pedigrees help genetic counsellors determine whether a trait is dominant or recessive and whether it is sex-linked.
- Autosomal recessive traits can skip generations (cystic fibrosis).
- Autosomal dominant traits appear in every generation (Huntington’s disease).
- X-linked recessive traits are more common in males (red–green colour blindness, hemophilia).
Exceptions to Mendel
- Linked genes, close together on one chromosome, tend to be inherited together.
- Incomplete dominance, codominance, multiple alleles and polygenic traits all produce different ratios.
Common mistakes
- Putting both alleles of a gene into one gamete. Each gamete gets one allele per gene.
- Confusing ratios with exact counts.
- Forgetting to list all four gamete combinations in a dihybrid cross.
Try it yourself
In rabbits, black fur (B) is dominant to brown (b), and short fur (S) is dominant to long (s).
- What gametes can a BbSs rabbit make?
- Predict the phenotype ratio from BbSs × BbSs.
- Use probability to find the chance of a brown, long-furred rabbit from that cross.
- A black rabbit is crossed with a brown one, and half the babies are brown. What was the black rabbit’s genotype for colour?
(Answers: BS, Bs, bS, bs; 9 : 3 : 3 : 1; 1/16; Bb)
Quick recap
- Mendel discovered inheritance rules by counting pea plant offspring.
- Segregation: allele pairs separate into gametes.
- Independent assortment: different genes sort independently.
- Monohybrid crosses give 3 : 1; dihybrid crosses give 9 : 3 : 3 : 1.
- Test crosses and pedigrees reveal hidden genotypes.