Mendel and the Rules of Inheritance
Inheritance is the passing of characteristics from parents to offspring through genes. The monk Gregor Mendel discovered the basic rules by cross-breeding pea plants in the 1860s. He found that traits are controlled by pairs of factors, which we now call alleles.
Dominant and Recessive Alleles
When two different alleles are present, one may hide the effect of the other.
- A dominant allele is expressed even when only one copy is present. It is written with a capital letter, e.g. T.
- A recessive allele is expressed only when two copies are present. It is written with a small letter, e.g. t.
So a plant with the genotype Tt shows the dominant trait, while only tt shows the recessive trait. The genes an organism carries is its genotype; the trait we actually see is its phenotype.
Key idea
Dominant hides recessive. A recessive characteristic only appears when the organism is homozygous recessive (two small letters).
The Punnett Square
A Punnett square is a simple grid used to predict the possible genotypes of offspring. If we cross two heterozygous parents (Tt × Tt), the square gives offspring in the ratio 1 TT : 2 Tt : 1 tt. This means the phenotype ratio is 3 dominant : 1 recessive.
Example
Cross Tt × Tt for tall (T) and short (t) pea plants. Three out of four offspring are tall and one is short — a 3 : 1 ratio. In humans, a similar cross can predict the chance of a child inheriting an attached earlobe.
A cross that follows just one characteristic, like the one above, is called a monohybrid cross. Breeders and geneticists use these predictions to work out the chance of an inherited trait appearing, but remember that the ratio describes probability over many offspring, not a guarantee for any single individual.
Remember
Capital letter = dominant; small letter = recessive. A monohybrid cross of two heterozygotes always gives a 3 : 1 phenotype ratio.